Difference between revisions of "Testing for Weak SSL/TSL Ciphers, Insufficient Transport Layer Protection (OWASP-EN-002)"

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== Brief Summary ==
 
== Brief Summary ==
<br>
+
Sensitive data must be protected when it is transmitted through the network. These data includes credentials and credit cards. As a rule of thumb if data must be protected when it is stored, it must be protected also during transmission.
..here: we describe in "natural language" what we want to test.
+
 
<br>
+
HTTP is a clear-text protocol and it is normally secured via an SSL/TLS tunnel, resulting in HTTPS traffic [1]. Use of these protocols ensure not only confidentiality but also authentication. Servers are authenticated using digital certificates, and it is also possibile to use client certificate for mutual authentication.
 +
 
 +
Even if high grade ciphers are today supported and normally used, some misconfiguration in server can be used to force the use of a weak cipher - or at worst no encryption - permitting to an attacker to gain access to the supposed secure communication channel. Other misconfiguration can be used for a Denial of Service attack.
 +
 
 
== Description of the Issue ==  
 
== Description of the Issue ==  
<br>
+
If control is missed and HTTP protocol is used to transmit sensitive information is a vulnerability [2] (e.g. credentials transmitted over HTTP [3]) and there are a specific OWASP Testing Guide v4’s test.
...here: Short Description of the Issue: Topic and Explanation
+
 
<br>
+
If SSL/TLS service is present it is good but it increments the attack surface and some vulnerabilities insist on it, such as:
 +
* SSL/TLS protocols, ciphers, keys and renegotiation must be properly configured.
 +
* Certificate validity must be ensured.
 +
Other vulnerabilities linked to this is:
 +
* Software exposed must be updated due to possibility of known vulnerabilities [4].
 +
* Usage of Secure flag for Session Cookies [5].
 +
* Usage of HTTP Strict Transport Security (HSTS) [6].
 +
* The presence of HTTP and HTTPS both, which can be used to intercept traffic [7], [8].
 +
* The presence of mixed HTTP and HTTP content in the same page, which can be used to Leak information.
 +
 
 +
===Sensitive data transmitted in clear-text===
 +
If the application transmits sensitive information via unencrypted channes - e.g. HTTP - it is a vulnerability. Typically it is possible to find BASIC authentication over HTTP, input password sent via HTTP and, in general, other information considered by regulations, laws or organization policy.
 +
 
 +
===Weak SSL/TSL Ciphers/Protocols/Keys===
 +
Historically, there have been limitations set in place by the U.S. government to allow cryptosystems to be exported only for key sizes of at most 40 bits, a key length which could be broken and would allow the decryption of communications. Since then cryptographic export regulations have been relaxed the maximum key size is 128 bits.
 +
It is important to check the SSL configuration being used to avoid putting in place cryptographic support which could be easily defeated. To reach this goal SSL-based services should not offer the possibility to choose weak cipher suite. A cipher suite is specified by an encryption protocol (e.g. DES, RC4, AES), the encryption key length (e.g. 40, 56, or 128 bits), and a hash algorithm (e.g. SHA, MD5) used for integrity checking.
 +
Briefly, the key points for the cipher suite determination are the following:  
 +
# The client sends to the server a ClientHello message specifying, among other information, the protocol and the cipher suites that it is able to handle. Note that a client is usually a web browser (most popular SSL client nowadays), but not necessarily, since it can be any SSL-enabled application; the same holds for the server, which needs not to be a web server, though this is the most common case [9].
 +
#The server responds with a ServerHello message, containing the chosen protocol and cipher suite that will be used for that session (in general the server selects the strongest protocol and cipher suite supported by both the client and server).
 +
 
 +
It is possible (for example, by means of configuration directives) to specify which cipher suites the server will honor. In this way you may control, for example, whether or not conversations with clients will support 40-bit encryption only.
 +
 
 +
#The server sends its Certificate message and, if client authentication is required, also sends a CertificateRequest message to the client.
 +
#The server sends a ServerHelloDone message and waits for a client response.
 +
#Upon receipt of the ServerHelloDone message, the client verifies the validity of the server's digital certificate.
 +
 
 +
===SSL certificate validity – client and server===
 +
 
 +
When accessing a web application via the HTTPS protocol, a secure channel is established between the client and the server. The identity of one (the server) or both parties (client and server) is then established by means of digital certificates. So, once the cipher suite is determined, the “SSL Handshake” continues with the exchange of the certificates, like follow:
 +
# The server sends its Certificate message and, if client authentication is required, also sends a CertificateRequest message to the client.
 +
# The server sends a ServerHelloDone message and waits for a client response.
 +
# Upon receipt of the ServerHelloDone message, the client verifies the validity of the server's digital certificate.
 +
 
 +
In order for the communication to be set up, a number of checks on the certificates must be passed. While discussing SSL and certificate based authentication is beyond the scope of this Guide, we will focus on the main criteria involved in ascertaining certificate validity:
 +
 
 +
* Checking if the Certificate Authority (CA) is a known one (meaning one considered trusted);
 +
* Checking that the certificate is currently valid;
 +
* Checking that the name of the site and the name reported in the certificate match.
 +
 
 +
Let is examine each check more in detail.
 +
 
 +
* Each browser comes with a preloaded list of trusted CAs, against which the certificate signing CA is compared (this list can be customized and expanded at will). During the initial negotiations with an HTTPS server, if the server certificate relates to a CA unknown to the browser, a warning is usually raised. This happens most often because a web application relies on a certificate signed by a self-established CA. Whether this is to be considered a concern depends on several factors. For example, this may be fine for an Intranet environment (think of corporate web email being provided via HTTPS; here, obviously all users recognize the internal CA as a trusted CA). When a service is provided to the general public via the Internet, however (i.e. when it is important to positively verify the identity of the server we are talking to), it is usually imperative to rely on a trusted CA, one which is recognized by all the user base (and here we stop with our considerations; we won’t delve deeper in the implications of the trust model being used by digital certificates).
 +
 
 +
* Certificates have an associated period of validity, therefore they may expire. Again, we are warned by the browser about this. A public service needs a temporally valid certificate; otherwise, it means we are talking with a server whose certificate was issued by someone we trust, but has expired without being renewed.
 +
 
 +
* What if the name on the certificate and the name of the server do not match? If this happens, it might sound suspicious. For a number of reasons, this is not so rare to see. A system may host a number of name-based virtual hosts, which share the same IP address and are identified by means of the HTTP 1.1 Host: header information. In this case, since the SSL handshake checks the server certificate before the HTTP request is processed, it is not possible to assign different certificates to each virtual server. Therefore, if the name of the site and the name reported in the certificate do not match, we have a condition which is typically signaled by the browser. To avoid this, IP-based virtual servers must be used. [33] and [34] describe techniques to deal with this problem and allow name-based virtual hosts to be correctly referenced.
 +
 
 +
===Other vulnerabilities===
 +
The presence of a new service, listening in a separate tcp port may introduce vulnerabilities such as Infrastructure vulnerability if software is not up to date [4]. Futhermore for a correct protection of data during transmission Session Cookie must use the Secure flag [5] and some directives should be sent to the browser to accept only secure traffic (e.g. HSTS [6], CSP [9]).
 +
 
 +
Also there are some attacks can be used to intercept traffic if the web server exposes the application on both HTTP and HTTPS [6], [7] or in case of mixed HTTP and HTTPS resources in the same page.
 +
 
 
== Black Box testing and example ==
 
== Black Box testing and example ==
'''Testing for Topic X vulnerabilities:''' <br>
+
 
...<br>
+
===Testing for sensitive data transmitted in clear-text===
'''Result Expected:'''<br>
+
Various typologies of information which must be protected can be also transmitted in clear text. It is possible to check if these information is transmitted over HTTP instead of HTTPS.
...<br><br>
+
 
== References ==
+
Please refer to specific Tests for full details, for credentials [3] and other kind of data [2]. 
'''Whitepapers'''<br>
+
 
...<br>
+
=====Example 1. Basic Authentication over HTTP=====
'''Tools'''<br>
+
A typical example is the usage of Basic Autentication over HTTP. Also because with Basic Autentication, after login, credentials are encoded - and not encrypted - into HTTP Headers.
...<br>
+
<pre>
 +
$ curl -kis http://example.com/restricted/
 +
HTTP/1.1 401 Authorization Required
 +
Date: Fri, 01 Aug 2013 00:00:00 GMT
 +
WWW-Authenticate: Basic realm="Restricted Area"
 +
Accept-Ranges: bytes
 +
Vary: Accept-Encoding
 +
Content-Length: 162
 +
Content-Type: text/html
 +
 
 +
<html><head><title>401 Authorization Required</title></head>
 +
<body bgcolor=white>
 +
<h1>401 Authorization Required</h1>
 +
 
 +
Invalid login credentials!
 +
 
 +
</body></html>
 +
</pre>
 +
 
 +
===Testing for Weak SSL/TSL Ciphers/Protocols/Keys vulnerabilities===
 +
Large number of available cipher suites and quick progress in cryptanalysis makes judging a SSL server a non-trivial task. At the time of writing these criteria are widely recognized as minimum checklist:
 +
* Weak ciphers must not be used (e.g. less than 128 bits [10]; no NULL ciphers suite, due to no encryption used; no Anonymous Diffie-Hellmann, due to not provides authentication).
 +
* Weak protocols must be disabled (e.g. SSLv2 must be disabled, due to known weaknesses in protocol design [11]).
 +
* Renegotiation must be properly configured (e.g. Insecure Renegotiation must be disabled, due to MiTM attacks [12] and Client-initiated Renegotiation must be disabled, due to Denial of Service vulnerability [13]).
 +
* No Export (EXP) level cipher suites, due to can be easly broken [10].
 +
* X.509 certificates key length must be strong (e.g. if RSA or DSA is used the key must be at least 1024 bits).
 +
* X.509 certificates must be signed only with secure hashing algoritms (e.g. not signed using MD5 hash, due to known collision attacks on this hash).
 +
* Keys must be generated with proper entropy (e.g, Weak Key Generated with Debian) [14].
 +
A most complete checklist includes:
 +
* Secure Renegotiation should be enabled.
 +
* MD5 should not be used, due to known collision attacks, but it is ok the use with at least 128 bit key.
 +
* RC4 should not be used, due to crypto-analytical attacks [15].
 +
* Server should be protected from BEAST Attack [16].
 +
* Server should be protected from CRIME attack, TLS compression must be disabled [17].
 +
* Server should support Forward Secrecy [18].
 +
 
 +
Following standards can be used as reference while assessing SSL servers:
 +
* PCI-DSS v2.0 in point 4.1 requires compliant parties to use "strong cryptography" without precisely defining key lengths and algorithms. Common interpretation, partially based on previous versions of the standard, is that at least 128 bit key cipher, no export strength algorithms and no SSLv2 should be used [19].
 +
* Qualys SSL Labs Server Rating Guide [14], Depoloyment best practice [10] and SSL Threat Model [20] has been proposed to standardize SSL server assessment and configuration. But is less updated than the SSL Server tool [21].
 +
* OWASP has a lot of resources about SSL/TLS Security [22], [23], [24], [25]. [26].
 +
 
 +
Some tools and scanners both commercial - e.g. Tenable Nessus [27] - and free - e.g. SSLAudit [28] or SSLScan [29], and other used into examples - can be used to assess SSL/TLS vulnerabilities. But due to evolution of these vulnerabilities a good way is also to check them manually with openssl [30] or using tool’s output as an input for manual evaluation using the references on the bottom on the Test to stay updated.
 +
 
 +
====Example 2. SSL service recognition via nmap====
 +
First step is to identify ports which have SSL/TLS wrapped services. Typically tcp ports with SSL for web and mail services are -  but not limited to - 443 (https), 465 (ssmtp), 585 (imap4-ssl), 993 (imaps), 995 (ssl-pop).
 +
In this example we search for SSL services using nmap with “-sV” option, used for identify services and it is also able to identify SSL services [31]. Other options are for this particular example and must be customized. Often in a Web Application Penetration Test scope is limited to port 80 and 443.
 +
 
 +
<pre>
 +
$ nmap -sV --reason -PN -n --top-ports 100 www.example.com
 +
Starting Nmap 6.25 ( http://nmap.org ) at 2013-01-01 00:00 CEST
 +
Nmap scan report for www.example.com (127.0.0.1)
 +
Host is up, received user-set (0.20s latency).
 +
Not shown: 89 filtered ports
 +
Reason: 89 no-responses
 +
PORT    STATE SERVICE  REASON  VERSION
 +
21/tcp  open  ftp      syn-ack Pure-FTPd
 +
22/tcp  open  ssh      syn-ack OpenSSH 5.3 (protocol 2.0)
 +
25/tcp  open  smtp    syn-ack Exim smtpd 4.80
 +
26/tcp  open  smtp    syn-ack Exim smtpd 4.80
 +
80/tcp  open  http    syn-ack
 +
110/tcp open  pop3    syn-ack Dovecot pop3d
 +
143/tcp open  imap    syn-ack Dovecot imapd
 +
443/tcp open  ssl/http syn-ack Apache
 +
465/tcp open  ssl/smtp syn-ack Exim smtpd 4.80
 +
993/tcp open  ssl/imap syn-ack Dovecot imapd
 +
995/tcp open  ssl/pop3 syn-ack Dovecot pop3d
 +
Service Info: Hosts: example.com
 +
Service detection performed. Please report any incorrect results at http://nmap.org/submit/ .
 +
Nmap done: 1 IP address (1 host up) scanned in 131.38 seconds
 +
</pre>
 +
 
 +
====Example 3. Checking for Certificate information, Weak Ciphers and SSLv2 via nmap====
 +
nmap has two scripts for checking Certificate information, Weak Ciphers and SSLv2 [31].
 +
 
 +
<pre>
 +
$ nmap --script ssl-cert,ssl-enum-ciphers -p 443,465,993,995 www.example.com
 +
Starting Nmap 6.25 ( http://nmap.org ) at 2013-01-01 00:00 CEST
 +
Nmap scan report for www.example.com (127.0.0.1)
 +
Host is up (0.090s latency).
 +
rDNS record for 127.0.0.1: www.example.com
 +
PORT    STATE SERVICE
 +
443/tcp open  https
 +
| ssl-cert: Subject: commonName=www.example.org
 +
| Issuer: commonName=*******
 +
| Public Key type: rsa
 +
| Public Key bits: 1024
 +
| Not valid before: 2010-01-23T00:00:00+00:00
 +
| Not valid after:  2020-02-28T23:59:59+00:00
 +
| MD5:  *******
 +
|_SHA-1: *******
 +
| ssl-enum-ciphers:
 +
|  SSLv3:
 +
|    ciphers:
 +
|      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
 +
|      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
 +
|      TLS_RSA_WITH_RC4_128_SHA - strong
 +
|    compressors:
 +
|      NULL
 +
|  TLSv1.0:
 +
|    ciphers:
 +
|      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
 +
|      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
 +
|      TLS_RSA_WITH_RC4_128_SHA - strong
 +
|    compressors:
 +
|      NULL
 +
|_  least strength: strong
 +
465/tcp open  smtps
 +
| ssl-cert: Subject: commonName=*.exapmple.com
 +
| Issuer: commonName=*******
 +
| Public Key type: rsa
 +
| Public Key bits: 2048
 +
| Not valid before: 2010-01-23T00:00:00+00:00
 +
| Not valid after:  2020-02-28T23:59:59+00:00
 +
| MD5:  *******
 +
|_SHA-1: *******
 +
| ssl-enum-ciphers:
 +
|  SSLv3:
 +
|    ciphers:
 +
|      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
 +
|      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
 +
|      TLS_RSA_WITH_RC4_128_SHA - strong
 +
|    compressors:
 +
|      NULL
 +
|  TLSv1.0:
 +
|    ciphers:
 +
|      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
 +
|      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
 +
|      TLS_RSA_WITH_RC4_128_SHA - strong
 +
|    compressors:
 +
|      NULL
 +
|_  least strength: strong
 +
993/tcp open  imaps
 +
| ssl-cert: Subject: commonName=*.exapmple.com
 +
| Issuer: commonName=*******
 +
| Public Key type: rsa
 +
| Public Key bits: 2048
 +
| Not valid before: 2010-01-23T00:00:00+00:00
 +
| Not valid after:  2020-02-28T23:59:59+00:00
 +
| MD5:  *******
 +
|_SHA-1: *******
 +
| ssl-enum-ciphers:
 +
|  SSLv3:
 +
|    ciphers:
 +
|      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
 +
|      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
 +
|      TLS_RSA_WITH_RC4_128_SHA - strong
 +
|    compressors:
 +
|      NULL
 +
|  TLSv1.0:
 +
|    ciphers:
 +
|      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
 +
|      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
 +
|      TLS_RSA_WITH_RC4_128_SHA - strong
 +
|    compressors:
 +
|      NULL
 +
|_  least strength: strong
 +
995/tcp open  pop3s
 +
| ssl-cert: Subject: commonName=*.exapmple.com
 +
| Issuer: commonName=*******
 +
| Public Key type: rsa
 +
| Public Key bits: 2048
 +
| Not valid before: 2010-01-23T00:00:00+00:00
 +
| Not valid after:  2020-02-28T23:59:59+00:00
 +
| MD5:  *******
 +
|_SHA-1: *******
 +
| ssl-enum-ciphers:
 +
|  SSLv3:
 +
|    ciphers:
 +
|      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
 +
|      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
 +
|      TLS_RSA_WITH_RC4_128_SHA - strong
 +
|    compressors:
 +
|      NULL
 +
|  TLSv1.0:
 +
|    ciphers:
 +
|      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
 +
|      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
 +
|      TLS_RSA_WITH_RC4_128_SHA - strong
 +
|    compressors:
 +
|      NULL
 +
|_  least strength: strong
 +
Nmap done: 1 IP address (1 host up) scanned in 8.64 seconds
 +
</pre>
 +
 
 +
====Example 4 Checking for Client-initiated Renegotiation and Secure Renegotiation via openssl (manually)====
 +
openssl [30] can be used for testing manually SSL/TLS. In this example we try to initiate a renegotiation by client [m] connecting to server with openssl - writing the fist line of an HTTP request, in a new line typing “R”, waiting for renegotiaion and completing the HTTP request - and check if secure renegotiaion is supperted looking server output. Using manual request it is also possible to see if Compression is enabled for TLS in order to check for CRIME [13], check for ciphers and other vulnerabilities.
 +
 
 +
<pre>
 +
$ openssl s_client -connect www2.example.com:443
 +
CONNECTED(00000003)
 +
depth=2 ******
 +
verify error:num=20:unable to get local issuer certificate
 +
verify return:0
 +
---
 +
Certificate chain
 +
0 s:******
 +
  i:******
 +
1 s:******
 +
  i:******
 +
2 s:******
 +
  i:******
 +
---
 +
Server certificate
 +
-----BEGIN CERTIFICATE-----
 +
******
 +
-----END CERTIFICATE-----
 +
subject=******
 +
issuer=******
 +
---
 +
No client certificate CA names sent
 +
---
 +
SSL handshake has read 3558 bytes and written 640 bytes
 +
---
 +
New, TLSv1/SSLv3, Cipher is DES-CBC3-SHA
 +
Server public key is 2048 bit
 +
Secure Renegotiation IS NOT supported
 +
Compression: NONE
 +
Expansion: NONE
 +
SSL-Session:
 +
    Protocol  : TLSv1
 +
    Cipher    : DES-CBC3-SHA
 +
    Session-ID: ******
 +
    Session-ID-ctx:
 +
    Master-Key: ******
 +
    Key-Arg  : None
 +
    PSK identity: None
 +
    PSK identity hint: None
 +
    SRP username: None
 +
    Start Time: ******
 +
    Timeout  : 300 (sec)
 +
    Verify return code: 20 (unable to get local issuer certificate)
 +
---
 +
</pre>
 +
Now we can write the first line of an HTTP request and then R in a new line.
 +
<pre>
 +
HEAD / HTTP/1.1
 +
R
 +
</pre>
 +
Server is renegotiating
 +
<pre>
 +
RENEGOTIATING
 +
depth=2 C******
 +
verify error:num=20:unable to get local issuer certificate
 +
verify return:0
 +
</pre>
 +
And we can complete our request, checking for response.
 +
<pre>
 +
HEAD / HTTP/1.1
 +
 
 +
HTTP/1.1 403 Forbidden ( The server denies the specified Uniform Resource Locator (URL). Contact the server administrator.  )
 +
Connection: close
 +
Pragma: no-cache
 +
Cache-Control: no-cache
 +
Content-Type: text/html
 +
Content-Length: 1792 
 +
 
 +
read:errno=0
 +
</pre>
 +
Even if the HEAD is not permitted, Client-intiated renegotiaion is permitted.
 +
 
 +
====Example 5. Testing supported Cipher Suites, BEAST and CRIME attacks via TestSSLServer====
 +
TestSSLServer [32] is a script which permits to check cipher suite and also BEAST and CRIME attacks. BEAST (Browser Exploit Against SSL/TLS)  exploits a vulnerability of CBC in TLS 1.0. CRIME (Compression Ratio Info-leak Made Easy) exploits a vulnerability of TLS Compression, that sould be disabled. It is really interesting a first fix for BEAST was the usage of RC4, but this is discouraged due to a crypto-analytical attack to RC4 [15].
 +
 
 +
An online tool to check for these attacks is SSL Labs, but can be used only for internet facing servers. Also consider that target data will be stored on SSL Labs server and also will result some connection from SSL Labs server [21].
 +
 
 +
<pre>
 +
$ java -jar TestSSLServer.jar www3.example.com 443
 +
Supported versions: SSLv3 TLSv1.0 TLSv1.1 TLSv1.2
 +
Deflate compression: no
 +
Supported cipher suites (ORDER IS NOT SIGNIFICANT):
 +
  SSLv3
 +
    RSA_WITH_RC4_128_SHA
 +
    RSA_WITH_3DES_EDE_CBC_SHA
 +
    DHE_RSA_WITH_3DES_EDE_CBC_SHA
 +
    RSA_WITH_AES_128_CBC_SHA
 +
    DHE_RSA_WITH_AES_128_CBC_SHA
 +
    RSA_WITH_AES_256_CBC_SHA
 +
    DHE_RSA_WITH_AES_256_CBC_SHA
 +
    RSA_WITH_CAMELLIA_128_CBC_SHA
 +
    DHE_RSA_WITH_CAMELLIA_128_CBC_SHA
 +
    RSA_WITH_CAMELLIA_256_CBC_SHA
 +
    DHE_RSA_WITH_CAMELLIA_256_CBC_SHA
 +
    TLS_RSA_WITH_SEED_CBC_SHA
 +
    TLS_DHE_RSA_WITH_SEED_CBC_SHA
 +
  (TLSv1.0: idem)
 +
  (TLSv1.1: idem)
 +
  TLSv1.2
 +
    RSA_WITH_RC4_128_SHA
 +
    RSA_WITH_3DES_EDE_CBC_SHA
 +
    DHE_RSA_WITH_3DES_EDE_CBC_SHA
 +
    RSA_WITH_AES_128_CBC_SHA
 +
    DHE_RSA_WITH_AES_128_CBC_SHA
 +
    RSA_WITH_AES_256_CBC_SHA
 +
    DHE_RSA_WITH_AES_256_CBC_SHA
 +
    RSA_WITH_AES_128_CBC_SHA256
 +
    RSA_WITH_AES_256_CBC_SHA256
 +
    RSA_WITH_CAMELLIA_128_CBC_SHA
 +
    DHE_RSA_WITH_CAMELLIA_128_CBC_SHA
 +
    DHE_RSA_WITH_AES_128_CBC_SHA256
 +
    DHE_RSA_WITH_AES_256_CBC_SHA256
 +
    RSA_WITH_CAMELLIA_256_CBC_SHA
 +
    DHE_RSA_WITH_CAMELLIA_256_CBC_SHA
 +
    TLS_RSA_WITH_SEED_CBC_SHA
 +
    TLS_DHE_RSA_WITH_SEED_CBC_SHA
 +
    TLS_RSA_WITH_AES_128_GCM_SHA256
 +
    TLS_RSA_WITH_AES_256_GCM_SHA384
 +
    TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
 +
    TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
 +
----------------------
 +
Server certificate(s):
 +
  ******
 +
----------------------
 +
Minimal encryption strength:    strong encryption (96-bit or more)
 +
Achievable encryption strength:  strong encryption (96-bit or more)
 +
BEAST status: vulnerable
 +
CRIME status: protected
 +
 
 +
</pre>
 +
 
 +
====Example 6.  Testing SSL/TLS vulnerabilities with sslyze====
 +
sslyze [33] is a python script which permits also mass scan and XML output. Follows an example of a regular scan. Is one of the most complete and versatile tool for SSL/TLS testing.
 +
 
 +
<pre>
 +
./sslyze.py --regular example.com:443
 +
 
 +
REGISTERING AVAILABLE PLUGINS
 +
-----------------------------
 +
 
 +
  PluginHSTS
 +
  PluginSessionRenegotiation
 +
  PluginCertInfo
 +
  PluginSessionResumption
 +
  PluginOpenSSLCipherSuites
 +
  PluginCompression
 +
 
 +
 
 +
 
 +
CHECKING HOST(S) AVAILABILITY
 +
-----------------------------
 +
 
 +
  example.com:443                      => 127.0.0.1:443
 +
 
 +
 
 +
 
 +
SCAN RESULTS FOR EXAMPLE.COM:443 - 127.0.0.1:443
 +
---------------------------------------------------
 +
 
 +
  * Compression :
 +
        Compression Support:      Disabled
 +
 
 +
  * Session Renegotiation :
 +
      Client-initiated Renegotiations:    Rejected
 +
      Secure Renegotiation:              Supported
 +
 
 +
  * Certificate :
 +
      Validation w/ Mozilla's CA Store:  Certificate is NOT Trusted: unable to get local issuer certificate
 +
      Hostname Validation:              MISMATCH                         
 +
      SHA1 Fingerprint:                  ******
 +
 
 +
      Common Name:                      www.example.com                   
 +
      Issuer:                            ******
 +
      Serial Number:                    ****                             
 +
      Not Before:                        Sep 26 00:00:00 2010 GMT         
 +
      Not After:                        Sep 26 23:59:59 2020 GMT 
 +
     
 +
      Signature Algorithm:              sha1WithRSAEncryption             
 +
      Key Size:                          1024 bit                         
 +
      X509v3 Subject Alternative Name:  {'othername': ['<unsupported>'], 'DNS': ['www.example.com']}
 +
 
 +
  * OCSP Stapling :
 +
      Server did not send back an OCSP response.                                  
 +
 
 +
  * Session Resumption :
 +
      With Session IDs:          Supported (5 successful, 0 failed, 0 errors, 5 total attempts).
 +
      With TLS Session Tickets:  Supported
 +
 
 +
  * SSLV2 Cipher Suites :
 +
 
 +
      Rejected Cipher Suite(s): Hidden
 +
 
 +
      Preferred Cipher Suite: None   
 +
 
 +
      Accepted Cipher Suite(s): None 
 +
 
 +
      Undefined - An unexpected error happened: None
 +
 
 +
  * SSLV3 Cipher Suites :
 +
 
 +
      Rejected Cipher Suite(s): Hidden
 +
 
 +
      Preferred Cipher Suite:         
 +
        RC4-SHA                      128 bits      HTTP 200 OK                       
 +
 
 +
      Accepted Cipher Suite(s):       
 +
        CAMELLIA256-SHA              256 bits      HTTP 200 OK                       
 +
        RC4-SHA                      128 bits      HTTP 200 OK                       
 +
        CAMELLIA128-SHA              128 bits      HTTP 200 OK                       
 +
 
 +
      Undefined - An unexpected error happened: None
 +
 
 +
  * TLSV1_1 Cipher Suites :
 +
 
 +
      Rejected Cipher Suite(s): Hidden
 +
 
 +
      Preferred Cipher Suite: None   
 +
 
 +
      Accepted Cipher Suite(s): None 
 +
 
 +
      Undefined - An unexpected error happened:
 +
        ECDH-RSA-AES256-SHA            socket.timeout - timed out       
 +
        ECDH-ECDSA-AES256-SHA          socket.timeout - timed out       
 +
 
 +
  * TLSV1_2 Cipher Suites :
 +
 
 +
      Rejected Cipher Suite(s): Hidden
 +
 
 +
      Preferred Cipher Suite: None   
 +
 
 +
      Accepted Cipher Suite(s): None 
 +
 
 +
      Undefined - An unexpected error happened:
 +
        ECDH-RSA-AES256-GCM-SHA384      socket.timeout - timed out       
 +
        ECDH-ECDSA-AES256-GCM-SHA384    socket.timeout - timed out       
 +
 
 +
  * TLSV1 Cipher Suites :
 +
 
 +
      Rejected Cipher Suite(s): Hidden
 +
 
 +
      Preferred Cipher Suite:         
 +
        RC4-SHA                      128 bits      Timeout on HTTP GET               
 +
 
 +
      Accepted Cipher Suite(s):       
 +
        CAMELLIA256-SHA              256 bits      HTTP 200 OK                       
 +
        RC4-SHA                      128 bits      HTTP 200 OK                       
 +
        CAMELLIA128-SHA              128 bits      HTTP 200 OK                       
 +
 
 +
      Undefined - An unexpected error happened:
 +
        ADH-CAMELLIA256-SHA            socket.timeout - timed out       
 +
 
 +
 
 +
 
 +
SCAN COMPLETED IN 9.68 S
 +
------------------------
 +
</pre>
 +
 
 +
===Testing SSL certificate validity – client and server===
 +
Firstly upgrade your browser because also CA certs expire and, in every release of the browser, these are been renewed.
 +
Examine the validity of the certificates used by the application. Browsers will issue a warning when encountering expired certificates, certificates issued by untrusted CAs, and certificates which do not match namewise with the site to which they should refer. By clicking on the padlock which appears in the browser window when visiting an HTTPS site, you can look at information related to the certificate – including the issuer, period of validity, encryption characteristics, etc. If the application requires a client certificate, you probably have installed one to access it. Certificate information is available in the browser by inspecting the relevant certificate(s) in the list of the installed certificates.
 +
These checks must be applied to all visible SSL-wrapped communication channels used by the application. Though this is the usual https service running on port 443, there may be additional services involved depending on the web application architecture and on deployment issues (an HTTPS administrative port left open, HTTPS services on non-standard ports, etc.). Therefore, apply these checks to all SSL-wrapped ports which have been discovered. For example, the nmap scanner features a scanning mode (enabled by the –sV command line switch) which identifies SSL-wrapped services. The Nessus vulnerability scanner has the capability of performing SSL checks on all SSL/TLS-wrapped services.
 +
 
 +
Some tools, as in previous examples, check also for certificate validity.
 +
 
 +
====Example 7. Testing for certificate validity (manually)====
 +
Rather than providing a fictitious example, we have inserted an anonymized real-life example to stress how frequently one stumbles on https sites whose certificates are inaccurate with respect to naming.
 +
The following screenshots refer to a regional site of a high-profile IT company.
 +
 
 +
We are visiting an .it site and the certificate was issued to a .com site! Internet Explorer warns that the name on the certificate does not match the name of the site.
 +
 
 +
[[Image:SSL Certificate Validity Testing IE Warning.gif]]
 +
''Warning issued by Microsoft Internet Explorer''
 +
 
 +
The message issued by Firefox is different – Firefox complains because it cannot ascertain the identity of the .com site the certificate refers to because it does not know the CA which signed the certificate. In fact, Internet Explorer and Firefox do not come preloaded with the same list of CAs. Therefore, the behavior experienced with various browsers may differ.
 +
 
 +
[[Image:SSL Certificate Validity Testing Firefox Warning.gif]]
 +
''Warning issued by Mozilla Firefox''
 +
 
 +
===Testing for other vulnerabilities===
 +
As mentioned previously there are other types of vulnerabilities that are not related with the SSL/TLS protocol used, the cipher suites or Certificates. A part from others discussed in other parts of the Guide, the another one is possible when the server provide the website both with the HTTP and HTTPS protocols, and permit to an attacker to force a victim into using a non-secure channel instead of a secure one.
 +
 
 +
====Surf Jacking====
 +
Surf Jacking attack [7] was first presented by Sandro Gauci and permits to an attacker to hijack an HTTP session even when the victim’s connection is encrypted using SSL or TLS.
 +
The following is a scenario of how the attack can take place:
 +
 
 +
The following is a scenario of how the attack can take place:
 +
* Victim logs into the secure website at https://somesecuresite/.
 +
* The secure site issues a session cookie as the client logs in.
 +
* While logged in, the victim opens a new browser window and goes to http:// examplesite/
 +
* An attacker sitting on the same network is able to see the clear text traffic to http://examplesite.
 +
* The attacker sends back a "301 Moved Permanently" in response to the clear text traffic to http://examplesite. The response contains the header “Location: http://somesecuresite /”, which makes it appear that examplesite is sending the web browser to somesecuresite. Notice that the URL scheme is HTTP not HTTPS.
 +
* The victim's browser starts a new clear text connection to http://somesecuresite/ and sends an HTTP request containing cookie in the HTTP header in clear text
 +
* The attacker sees this traffic and logs the cookie for later (ab)use.
 +
To test if a website is vulnerable is sufficient to proceed like follow:
 +
# Check if website supports both HTTP and HTTPS protocol
 +
# Check if cookies do not have the “Secure” flag
 +
 
 +
====SSL Strip====
 +
Often applications supports both HTTP and HTTPS. As for usability or because users do not use to type “https://www.example.com”. Often users go into an HTTPS website from link or a redirect. Typically also home banking site have a similar configuration with an iframed login or a form with action attribute over HTTPS but the page under HTTP.
 +
An attacker in a privileged position - as described in SSL strip [8] - can incercept traffic when user is into HTTP and manipulate it to get a Man-In-The-Middle attack under HTTPS.
 +
To test if application is vulnerable is sufficient the website supports both HTTP and HTTPS.
 +
 
 +
== Gray Box testing and example ==
 +
 
 +
===Testing for Weak SSL/TSL Cipher Suites===
 +
Check the configuration of the web servers which provide https services. If the web application provides other SSL/TLS wrapped services, these should be checked as well.
 +
 
 +
====Example 8. Windows Server====
 +
Check the configuration on a Microsoft Windows Server (2000, 2003 and 2008) using the registry key:
 +
<pre>HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\</pre>
 +
which has some sub-keys like Ciphers, Protocols and KeyExchangeAlgorithms.
 +
 
 +
====Example 9: Apache====
 +
To check the cipher suites and protocols supported by Apache2 web server open the ssl.conf file and search for the SSLCipherSuite, SSLProtocol, SSLHonorCipherOrder,SSLInsecureRenegotiation and SSLCompression directives.
 +
 
 +
===Testing SSL certificate validity – client and server===
 +
Examine the validity of the certificates used by the application at both server and client levels. The usage of certificates is primarily at the web server level; however, there may be additional communication paths protected by SSL (for example, towards the DBMS). You should check the application architecture to identify all SSL protected channels.
 +
 
 +
==References==
 +
'''OWASP Resources'''
 +
* [5] [OWASP Testing Guide - Testing for cookie attributes (OTG-SESS-002)|https://www.owasp.org/index.php/Testing_for_cookies_attributes_(OWASP-SM-002)]
 +
* [4][OWASP Testing Guide - Test Network/Infrastructure Configuration (OTG-CONFIG-001)|https://www.owasp.org/index.php/Testing_for_infrastructure_configuration_management_(OWASP-CM-003)]
 +
* [6] [OWASP Testing |https://www.owasp.org/index.php/Testing_for_cookies_attributes_(OWASP-SM-002)][Guide - Testing for Missing HSTS header (OTG-CONFIG-009)|https://www.owasp.org/index.php/Testing_for_Missing_HSTS_header]
 +
* [2] [OWASP Testing Guide - Testing for Sensitive information sent via unencrypted channels (OTG-CRYPST-007)|https://www.owasp.org/index.php?title=Testing_for_Sensitive_information_sent_via_unencrypted_channels_(OTG-CRYPST-007)&action=edit&redlink=1]
 +
* [3] [OWASP Testing Guide - Testing for Credentials Transported over an Encrypted Channel (OWASP-AT-001)|https://www.owasp.org/index.php/Testing_for_Credentials_Transported_over_an_Encrypted_Channel_(OWASP-AT-001)]
 +
* [9] [OWASP Testing Guide - Test Content Security Policy (OTG-CONFIG-008)|https://www.owasp.org/index.php/Testing_for_Content_Security_Policy_weakness]
 +
* [22] [OWASP Cheat sheet - Transport Layer Protection|https://www.owasp.org/index.php/Transport_Layer_Protection_Cheat_Sheet]
 +
* [23] [OWASP TOP 10 2013 - A6 Sensitive Data Exposure|https://www.owasp.org/index.php/Top_10_2013-A6-Sensitive_Data_Exposure]
 +
* [24] [OWASP TOP 10 2010 - A9 Insufficient Transport Layer Protection|https://www.owasp.org/index.php/Top_10_2010-A9-Insufficient_Transport_Layer_Protection]
 +
* [25] [OWASP ASVS 2009 - Verification 10|https://code.google.com/p/owasp-asvs/wiki/Verification_V10]
 +
* [26] [OWASP Application Security FAQ - Cryptography/SSL|https://www.owasp.org/index.php/OWASP_Application_Security_FAQ#Cryptography.2FSSL]
 +
 
 +
'''Whitepapers'''
 +
* [1] [RFC5246 - The Transport Layer Security (TLS) Protocol Version 1.2 (Updated by RFC 5746, RFC 5878, RFC 6176)|http://www.ietf.org/rfc/rfc5246.txt]
 +
* [33] [RFC2817 - Upgrading to TLS Within HTTP/1.1|]
 +
* [34] [RFC6066 - Transport Layer Security (TLS) Extensions: Extension Definitions|http://www.ietf.org/rfc/rfc6066.txt]
 +
* [11] [SSLv2 Protocol Multiple Weaknesses |http://osvdb.org/56387]
 +
* [12] [Mitre - TLS Renegotiation MiTM|http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2009-3555]
 +
* [13] [Qualys SSL Labs - TLS Renegotiation DoS|https://community.qualys.com/blogs/securitylabs/2011/10/31/tls-renegotiation-and-denial-of-service-attacks]
 +
* [10] [Qualys SSL Labs - SSL/TLS Deployment Best Practices|https://www.ssllabs.com/projects/best-practices/index.html]
 +
* [14] [Qualys SSL Labs - SSL Server Rating Guide|https://www.ssllabs.com/projects/rating-guide/index.html]
 +
* [20] [Qualys SSL Labs - SSL Threat Model|https://www.ssllabs.com/projects/ssl-threat-model/index.html]
 +
* [18] [Qualys SSL Labs - Forward Secrecy|https://community.qualys.com/blogs/securitylabs/2013/06/25/ssl-labs-deploying-forward-secrecy]
 +
* [15] [Qualys SSL Labs - RC4 Usage|https://community.qualys.com/blogs/securitylabs/2013/03/19/rc4-in-tls-is-broken-now-what]
 +
* [16] [Qualys SSL Labs - BEAST|https://community.qualys.com/blogs/securitylabs/2011/10/17/mitigating-the-beast-attack-on-tls]
 +
* [17] [Qualys SSL Labs - CRIME|https://community.qualys.com/blogs/securitylabs/2012/09/14/crime-information-leakage-attack-against-ssltls]
 +
* [7] [SurfJacking attack|https://resources.enablesecurity.com/resources/Surf%20Jacking.pdf]
 +
* [8] [SSLStrip attack|http://www.thoughtcrime.org/software/sslstrip/]
 +
* [19] [PCI-DSS v2.0|https://www.pcisecuritystandards.org/security_standards/documents.php]
 +
 
 +
'''Tools'''
 +
* [21][Qualys SSL Labs - SSL Server Test|https://www.ssllabs.com/ssltest/index.html]: internet facing scanner
 +
* [27] [Tenable - Nessus Vulnerability Scanner|http://www.tenable.com/products/nessus]: includes some plugins to test different SSL related vulnerabilities, Certificates and the presence of HTTP Basic authentication without SSL.
 +
* [32] [TestSSLServer|http://www.bolet.org/TestSSLServer/]: a java scanner - and also windows executable - includes tests for cipher suites, CRIME and BEAST
 +
* [33] [sslyze|https://github.com/iSECPartners/sslyze]: is a python script to check vulnerabilities in SSL/TLS.
 +
* [28] [SSLAudit|https://code.google.com/p/sslaudit/]: a perl script/windows executable scanner which follows Qualys SSL Labs Rating Guide.
 +
* [29] [SSLScan|http://sourceforge.net/projects/sslscan/] with [SSL Tests|http://www.pentesterscripting.com/discovery/ssl_tests]: a SSL Scanner and a wrapper in order to enumerate SSL vulnerabilities.
 +
* [31] [nmap|http://nmap.org/]: can be used primary to identify SSL-based services and then to check Certificate and SSL/TLS vulnerabilities. In particular it has some scripts to check [Certificate and SSLv2|http://nmap.org/nsedoc/scripts/ssl-cert.html] and supported [SSL/TLS protocols/ciphers|http://nmap.org/nsedoc/scripts/ssl-enum-ciphers.html] with an internal rating.
 +
* [30] [curl|http://curl.haxx.se/] and [openssl|http://www.openssl.org/]: can be used to query manually SSL/TLS services
 +
* [9] [Stunnel|http://www.stunnel.org]: a noteworthy class of SSL clients is that of SSL proxies such as stunnel available at which can be used to allow non-SSL enabled tools to talk to SSL services)
 +
 
 +
[[Category:Cryptographic Vulnerability]]
 +
[[Category:SSL]]

Revision as of 04:51, 24 August 2013

This article is part of the new OWASP Testing Guide v4. 
At the moment the project is in the REVIEW phase.

Back to the OWASP Testing Guide v4 ToC: https://www.owasp.org/index.php/OWASP_Testing_Guide_v4_Table_of_Contents Back to the OWASP Testing Guide Project: http://www.owasp.org/index.php/OWASP_Testing_Project

Contents


Brief Summary

Sensitive data must be protected when it is transmitted through the network. These data includes credentials and credit cards. As a rule of thumb if data must be protected when it is stored, it must be protected also during transmission.

HTTP is a clear-text protocol and it is normally secured via an SSL/TLS tunnel, resulting in HTTPS traffic [1]. Use of these protocols ensure not only confidentiality but also authentication. Servers are authenticated using digital certificates, and it is also possibile to use client certificate for mutual authentication.

Even if high grade ciphers are today supported and normally used, some misconfiguration in server can be used to force the use of a weak cipher - or at worst no encryption - permitting to an attacker to gain access to the supposed secure communication channel. Other misconfiguration can be used for a Denial of Service attack.

Description of the Issue

If control is missed and HTTP protocol is used to transmit sensitive information is a vulnerability [2] (e.g. credentials transmitted over HTTP [3]) and there are a specific OWASP Testing Guide v4’s test.

If SSL/TLS service is present it is good but it increments the attack surface and some vulnerabilities insist on it, such as:

  • SSL/TLS protocols, ciphers, keys and renegotiation must be properly configured.
  • Certificate validity must be ensured.

Other vulnerabilities linked to this is:

  • Software exposed must be updated due to possibility of known vulnerabilities [4].
  • Usage of Secure flag for Session Cookies [5].
  • Usage of HTTP Strict Transport Security (HSTS) [6].
  • The presence of HTTP and HTTPS both, which can be used to intercept traffic [7], [8].
  • The presence of mixed HTTP and HTTP content in the same page, which can be used to Leak information.

Sensitive data transmitted in clear-text

If the application transmits sensitive information via unencrypted channes - e.g. HTTP - it is a vulnerability. Typically it is possible to find BASIC authentication over HTTP, input password sent via HTTP and, in general, other information considered by regulations, laws or organization policy.

Weak SSL/TSL Ciphers/Protocols/Keys

Historically, there have been limitations set in place by the U.S. government to allow cryptosystems to be exported only for key sizes of at most 40 bits, a key length which could be broken and would allow the decryption of communications. Since then cryptographic export regulations have been relaxed the maximum key size is 128 bits. It is important to check the SSL configuration being used to avoid putting in place cryptographic support which could be easily defeated. To reach this goal SSL-based services should not offer the possibility to choose weak cipher suite. A cipher suite is specified by an encryption protocol (e.g. DES, RC4, AES), the encryption key length (e.g. 40, 56, or 128 bits), and a hash algorithm (e.g. SHA, MD5) used for integrity checking. Briefly, the key points for the cipher suite determination are the following:

  1. The client sends to the server a ClientHello message specifying, among other information, the protocol and the cipher suites that it is able to handle. Note that a client is usually a web browser (most popular SSL client nowadays), but not necessarily, since it can be any SSL-enabled application; the same holds for the server, which needs not to be a web server, though this is the most common case [9].
  2. The server responds with a ServerHello message, containing the chosen protocol and cipher suite that will be used for that session (in general the server selects the strongest protocol and cipher suite supported by both the client and server).

It is possible (for example, by means of configuration directives) to specify which cipher suites the server will honor. In this way you may control, for example, whether or not conversations with clients will support 40-bit encryption only.

  1. The server sends its Certificate message and, if client authentication is required, also sends a CertificateRequest message to the client.
  2. The server sends a ServerHelloDone message and waits for a client response.
  3. Upon receipt of the ServerHelloDone message, the client verifies the validity of the server's digital certificate.

SSL certificate validity – client and server

When accessing a web application via the HTTPS protocol, a secure channel is established between the client and the server. The identity of one (the server) or both parties (client and server) is then established by means of digital certificates. So, once the cipher suite is determined, the “SSL Handshake” continues with the exchange of the certificates, like follow:

  1.  The server sends its Certificate message and, if client authentication is required, also sends a CertificateRequest message to the client.
  2.  The server sends a ServerHelloDone message and waits for a client response.
  3.  Upon receipt of the ServerHelloDone message, the client verifies the validity of the server's digital certificate.

In order for the communication to be set up, a number of checks on the certificates must be passed. While discussing SSL and certificate based authentication is beyond the scope of this Guide, we will focus on the main criteria involved in ascertaining certificate validity:

  • Checking if the Certificate Authority (CA) is a known one (meaning one considered trusted);
  • Checking that the certificate is currently valid;
  • Checking that the name of the site and the name reported in the certificate match.

Let is examine each check more in detail.

  • Each browser comes with a preloaded list of trusted CAs, against which the certificate signing CA is compared (this list can be customized and expanded at will). During the initial negotiations with an HTTPS server, if the server certificate relates to a CA unknown to the browser, a warning is usually raised. This happens most often because a web application relies on a certificate signed by a self-established CA. Whether this is to be considered a concern depends on several factors. For example, this may be fine for an Intranet environment (think of corporate web email being provided via HTTPS; here, obviously all users recognize the internal CA as a trusted CA). When a service is provided to the general public via the Internet, however (i.e. when it is important to positively verify the identity of the server we are talking to), it is usually imperative to rely on a trusted CA, one which is recognized by all the user base (and here we stop with our considerations; we won’t delve deeper in the implications of the trust model being used by digital certificates).
  • Certificates have an associated period of validity, therefore they may expire. Again, we are warned by the browser about this. A public service needs a temporally valid certificate; otherwise, it means we are talking with a server whose certificate was issued by someone we trust, but has expired without being renewed.
  • What if the name on the certificate and the name of the server do not match? If this happens, it might sound suspicious. For a number of reasons, this is not so rare to see. A system may host a number of name-based virtual hosts, which share the same IP address and are identified by means of the HTTP 1.1 Host: header information. In this case, since the SSL handshake checks the server certificate before the HTTP request is processed, it is not possible to assign different certificates to each virtual server. Therefore, if the name of the site and the name reported in the certificate do not match, we have a condition which is typically signaled by the browser. To avoid this, IP-based virtual servers must be used. [33] and [34] describe techniques to deal with this problem and allow name-based virtual hosts to be correctly referenced.

Other vulnerabilities

The presence of a new service, listening in a separate tcp port may introduce vulnerabilities such as Infrastructure vulnerability if software is not up to date [4]. Futhermore for a correct protection of data during transmission Session Cookie must use the Secure flag [5] and some directives should be sent to the browser to accept only secure traffic (e.g. HSTS [6], CSP [9]).

Also there are some attacks can be used to intercept traffic if the web server exposes the application on both HTTP and HTTPS [6], [7] or in case of mixed HTTP and HTTPS resources in the same page.

Black Box testing and example

Testing for sensitive data transmitted in clear-text

Various typologies of information which must be protected can be also transmitted in clear text. It is possible to check if these information is transmitted over HTTP instead of HTTPS.

Please refer to specific Tests for full details, for credentials [3] and other kind of data [2].

Example 1. Basic Authentication over HTTP

A typical example is the usage of Basic Autentication over HTTP. Also because with Basic Autentication, after login, credentials are encoded - and not encrypted - into HTTP Headers.

$ curl -kis http://example.com/restricted/
HTTP/1.1 401 Authorization Required
Date: Fri, 01 Aug 2013 00:00:00 GMT
WWW-Authenticate: Basic realm="Restricted Area"
Accept-Ranges: bytes
Vary: Accept-Encoding
Content-Length: 162
Content-Type: text/html

<html><head><title>401 Authorization Required</title></head>
<body bgcolor=white>
<h1>401 Authorization Required</h1>

Invalid login credentials!

</body></html>

Testing for Weak SSL/TSL Ciphers/Protocols/Keys vulnerabilities

Large number of available cipher suites and quick progress in cryptanalysis makes judging a SSL server a non-trivial task. At the time of writing these criteria are widely recognized as minimum checklist:

  • Weak ciphers must not be used (e.g. less than 128 bits [10]; no NULL ciphers suite, due to no encryption used; no Anonymous Diffie-Hellmann, due to not provides authentication).
  • Weak protocols must be disabled (e.g. SSLv2 must be disabled, due to known weaknesses in protocol design [11]).
  • Renegotiation must be properly configured (e.g. Insecure Renegotiation must be disabled, due to MiTM attacks [12] and Client-initiated Renegotiation must be disabled, due to Denial of Service vulnerability [13]).
  • No Export (EXP) level cipher suites, due to can be easly broken [10].
  • X.509 certificates key length must be strong (e.g. if RSA or DSA is used the key must be at least 1024 bits).
  • X.509 certificates must be signed only with secure hashing algoritms (e.g. not signed using MD5 hash, due to known collision attacks on this hash).
  • Keys must be generated with proper entropy (e.g, Weak Key Generated with Debian) [14].

A most complete checklist includes:

  • Secure Renegotiation should be enabled.
  • MD5 should not be used, due to known collision attacks, but it is ok the use with at least 128 bit key.
  • RC4 should not be used, due to crypto-analytical attacks [15].
  • Server should be protected from BEAST Attack [16].
  • Server should be protected from CRIME attack, TLS compression must be disabled [17].
  • Server should support Forward Secrecy [18].

Following standards can be used as reference while assessing SSL servers:

  • PCI-DSS v2.0 in point 4.1 requires compliant parties to use "strong cryptography" without precisely defining key lengths and algorithms. Common interpretation, partially based on previous versions of the standard, is that at least 128 bit key cipher, no export strength algorithms and no SSLv2 should be used [19].
  • Qualys SSL Labs Server Rating Guide [14], Depoloyment best practice [10] and SSL Threat Model [20] has been proposed to standardize SSL server assessment and configuration. But is less updated than the SSL Server tool [21].
  • OWASP has a lot of resources about SSL/TLS Security [22], [23], [24], [25]. [26].

Some tools and scanners both commercial - e.g. Tenable Nessus [27] - and free - e.g. SSLAudit [28] or SSLScan [29], and other used into examples - can be used to assess SSL/TLS vulnerabilities. But due to evolution of these vulnerabilities a good way is also to check them manually with openssl [30] or using tool’s output as an input for manual evaluation using the references on the bottom on the Test to stay updated.

Example 2. SSL service recognition via nmap

First step is to identify ports which have SSL/TLS wrapped services. Typically tcp ports with SSL for web and mail services are - but not limited to - 443 (https), 465 (ssmtp), 585 (imap4-ssl), 993 (imaps), 995 (ssl-pop). In this example we search for SSL services using nmap with “-sV” option, used for identify services and it is also able to identify SSL services [31]. Other options are for this particular example and must be customized. Often in a Web Application Penetration Test scope is limited to port 80 and 443.

$ nmap -sV --reason -PN -n --top-ports 100 www.example.com
Starting Nmap 6.25 ( http://nmap.org ) at 2013-01-01 00:00 CEST
Nmap scan report for www.example.com (127.0.0.1)
Host is up, received user-set (0.20s latency).
Not shown: 89 filtered ports
Reason: 89 no-responses
PORT    STATE SERVICE  REASON  VERSION
21/tcp  open  ftp      syn-ack Pure-FTPd
22/tcp  open  ssh      syn-ack OpenSSH 5.3 (protocol 2.0)
25/tcp  open  smtp     syn-ack Exim smtpd 4.80
26/tcp  open  smtp     syn-ack Exim smtpd 4.80
80/tcp  open  http     syn-ack
110/tcp open  pop3     syn-ack Dovecot pop3d
143/tcp open  imap     syn-ack Dovecot imapd
443/tcp open  ssl/http syn-ack Apache
465/tcp open  ssl/smtp syn-ack Exim smtpd 4.80
993/tcp open  ssl/imap syn-ack Dovecot imapd
995/tcp open  ssl/pop3 syn-ack Dovecot pop3d
Service Info: Hosts: example.com
Service detection performed. Please report any incorrect results at http://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 131.38 seconds

Example 3. Checking for Certificate information, Weak Ciphers and SSLv2 via nmap

nmap has two scripts for checking Certificate information, Weak Ciphers and SSLv2 [31].

$ nmap --script ssl-cert,ssl-enum-ciphers -p 443,465,993,995 www.example.com
Starting Nmap 6.25 ( http://nmap.org ) at 2013-01-01 00:00 CEST
Nmap scan report for www.example.com (127.0.0.1)
Host is up (0.090s latency).
rDNS record for 127.0.0.1: www.example.com
PORT    STATE SERVICE
443/tcp open  https
| ssl-cert: Subject: commonName=www.example.org
| Issuer: commonName=*******
| Public Key type: rsa
| Public Key bits: 1024
| Not valid before: 2010-01-23T00:00:00+00:00
| Not valid after:  2020-02-28T23:59:59+00:00
| MD5:   *******
|_SHA-1: *******
| ssl-enum-ciphers: 
|   SSLv3: 
|     ciphers: 
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
|       TLS_RSA_WITH_RC4_128_SHA - strong
|     compressors: 
|       NULL
|   TLSv1.0: 
|     ciphers: 
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
|       TLS_RSA_WITH_RC4_128_SHA - strong
|     compressors: 
|       NULL
|_  least strength: strong
465/tcp open  smtps
| ssl-cert: Subject: commonName=*.exapmple.com
| Issuer: commonName=*******
| Public Key type: rsa
| Public Key bits: 2048
| Not valid before: 2010-01-23T00:00:00+00:00
| Not valid after:  2020-02-28T23:59:59+00:00
| MD5:   *******
|_SHA-1: *******
| ssl-enum-ciphers: 
|   SSLv3: 
|     ciphers: 
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
|       TLS_RSA_WITH_RC4_128_SHA - strong
|     compressors: 
|       NULL
|   TLSv1.0: 
|     ciphers: 
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
|       TLS_RSA_WITH_RC4_128_SHA - strong
|     compressors: 
|       NULL
|_  least strength: strong
993/tcp open  imaps
| ssl-cert: Subject: commonName=*.exapmple.com
| Issuer: commonName=*******
| Public Key type: rsa
| Public Key bits: 2048
| Not valid before: 2010-01-23T00:00:00+00:00
| Not valid after:  2020-02-28T23:59:59+00:00
| MD5:   *******
|_SHA-1: *******
| ssl-enum-ciphers: 
|   SSLv3: 
|     ciphers: 
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
|       TLS_RSA_WITH_RC4_128_SHA - strong
|     compressors: 
|       NULL
|   TLSv1.0: 
|     ciphers: 
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
|       TLS_RSA_WITH_RC4_128_SHA - strong
|     compressors: 
|       NULL
|_  least strength: strong
995/tcp open  pop3s
| ssl-cert: Subject: commonName=*.exapmple.com
| Issuer: commonName=*******
| Public Key type: rsa
| Public Key bits: 2048
| Not valid before: 2010-01-23T00:00:00+00:00
| Not valid after:  2020-02-28T23:59:59+00:00
| MD5:   *******
|_SHA-1: *******
| ssl-enum-ciphers: 
|   SSLv3: 
|     ciphers: 
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
|       TLS_RSA_WITH_RC4_128_SHA - strong
|     compressors: 
|       NULL
|   TLSv1.0: 
|     ciphers: 
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA - strong
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA - strong
|       TLS_RSA_WITH_RC4_128_SHA - strong
|     compressors: 
|       NULL
|_  least strength: strong
Nmap done: 1 IP address (1 host up) scanned in 8.64 seconds

Example 4 Checking for Client-initiated Renegotiation and Secure Renegotiation via openssl (manually)

openssl [30] can be used for testing manually SSL/TLS. In this example we try to initiate a renegotiation by client [m] connecting to server with openssl - writing the fist line of an HTTP request, in a new line typing “R”, waiting for renegotiaion and completing the HTTP request - and check if secure renegotiaion is supperted looking server output. Using manual request it is also possible to see if Compression is enabled for TLS in order to check for CRIME [13], check for ciphers and other vulnerabilities.

$ openssl s_client -connect www2.example.com:443
CONNECTED(00000003)
depth=2 ******
verify error:num=20:unable to get local issuer certificate
verify return:0
---
Certificate chain
 0 s:******
   i:******
 1 s:******
   i:******
 2 s:******
   i:******
---
Server certificate
-----BEGIN CERTIFICATE-----
******
-----END CERTIFICATE-----
subject=******
issuer=******
---
No client certificate CA names sent
---
SSL handshake has read 3558 bytes and written 640 bytes
---
New, TLSv1/SSLv3, Cipher is DES-CBC3-SHA
Server public key is 2048 bit
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
SSL-Session:
    Protocol  : TLSv1
    Cipher    : DES-CBC3-SHA
    Session-ID: ******
    Session-ID-ctx: 
    Master-Key: ******
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    Start Time: ******
    Timeout   : 300 (sec)
    Verify return code: 20 (unable to get local issuer certificate)
---

Now we can write the first line of an HTTP request and then R in a new line.

HEAD / HTTP/1.1
R

Server is renegotiating

RENEGOTIATING
depth=2 C******
verify error:num=20:unable to get local issuer certificate
verify return:0

And we can complete our request, checking for response.

HEAD / HTTP/1.1

HTTP/1.1 403 Forbidden ( The server denies the specified Uniform Resource Locator (URL). Contact the server administrator.  )
Connection: close
Pragma: no-cache
Cache-Control: no-cache
Content-Type: text/html
Content-Length: 1792  

read:errno=0

Even if the HEAD is not permitted, Client-intiated renegotiaion is permitted.

Example 5. Testing supported Cipher Suites, BEAST and CRIME attacks via TestSSLServer

TestSSLServer [32] is a script which permits to check cipher suite and also BEAST and CRIME attacks. BEAST (Browser Exploit Against SSL/TLS) exploits a vulnerability of CBC in TLS 1.0. CRIME (Compression Ratio Info-leak Made Easy) exploits a vulnerability of TLS Compression, that sould be disabled. It is really interesting a first fix for BEAST was the usage of RC4, but this is discouraged due to a crypto-analytical attack to RC4 [15].

An online tool to check for these attacks is SSL Labs, but can be used only for internet facing servers. Also consider that target data will be stored on SSL Labs server and also will result some connection from SSL Labs server [21].

$ java -jar TestSSLServer.jar www3.example.com 443
Supported versions: SSLv3 TLSv1.0 TLSv1.1 TLSv1.2
Deflate compression: no
Supported cipher suites (ORDER IS NOT SIGNIFICANT):
  SSLv3
     RSA_WITH_RC4_128_SHA
     RSA_WITH_3DES_EDE_CBC_SHA
     DHE_RSA_WITH_3DES_EDE_CBC_SHA
     RSA_WITH_AES_128_CBC_SHA
     DHE_RSA_WITH_AES_128_CBC_SHA
     RSA_WITH_AES_256_CBC_SHA
     DHE_RSA_WITH_AES_256_CBC_SHA
     RSA_WITH_CAMELLIA_128_CBC_SHA
     DHE_RSA_WITH_CAMELLIA_128_CBC_SHA
     RSA_WITH_CAMELLIA_256_CBC_SHA
     DHE_RSA_WITH_CAMELLIA_256_CBC_SHA
     TLS_RSA_WITH_SEED_CBC_SHA
     TLS_DHE_RSA_WITH_SEED_CBC_SHA
  (TLSv1.0: idem)
  (TLSv1.1: idem)
  TLSv1.2
     RSA_WITH_RC4_128_SHA
     RSA_WITH_3DES_EDE_CBC_SHA
     DHE_RSA_WITH_3DES_EDE_CBC_SHA
     RSA_WITH_AES_128_CBC_SHA
     DHE_RSA_WITH_AES_128_CBC_SHA
     RSA_WITH_AES_256_CBC_SHA
     DHE_RSA_WITH_AES_256_CBC_SHA
     RSA_WITH_AES_128_CBC_SHA256
     RSA_WITH_AES_256_CBC_SHA256
     RSA_WITH_CAMELLIA_128_CBC_SHA
     DHE_RSA_WITH_CAMELLIA_128_CBC_SHA
     DHE_RSA_WITH_AES_128_CBC_SHA256
     DHE_RSA_WITH_AES_256_CBC_SHA256
     RSA_WITH_CAMELLIA_256_CBC_SHA
     DHE_RSA_WITH_CAMELLIA_256_CBC_SHA
     TLS_RSA_WITH_SEED_CBC_SHA
     TLS_DHE_RSA_WITH_SEED_CBC_SHA
     TLS_RSA_WITH_AES_128_GCM_SHA256
     TLS_RSA_WITH_AES_256_GCM_SHA384
     TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
     TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
----------------------
Server certificate(s):
  ******
----------------------
Minimal encryption strength:     strong encryption (96-bit or more)
Achievable encryption strength:  strong encryption (96-bit or more)
BEAST status: vulnerable
CRIME status: protected

Example 6. Testing SSL/TLS vulnerabilities with sslyze

sslyze [33] is a python script which permits also mass scan and XML output. Follows an example of a regular scan. Is one of the most complete and versatile tool for SSL/TLS testing.

./sslyze.py --regular example.com:443

 REGISTERING AVAILABLE PLUGINS
 -----------------------------

  PluginHSTS
  PluginSessionRenegotiation
  PluginCertInfo
  PluginSessionResumption
  PluginOpenSSLCipherSuites
  PluginCompression



 CHECKING HOST(S) AVAILABILITY
 -----------------------------

  example.com:443                      => 127.0.0.1:443



 SCAN RESULTS FOR EXAMPLE.COM:443 - 127.0.0.1:443
 ---------------------------------------------------

  * Compression :
        Compression Support:      Disabled

  * Session Renegotiation :
      Client-initiated Renegotiations:    Rejected
      Secure Renegotiation:               Supported

  * Certificate :
      Validation w/ Mozilla's CA Store:  Certificate is NOT Trusted: unable to get local issuer certificate
      Hostname Validation:               MISMATCH                           
      SHA1 Fingerprint:                  ******

      Common Name:                       www.example.com                     
      Issuer:                            ******
      Serial Number:                     ****                               
      Not Before:                        Sep 26 00:00:00 2010 GMT           
      Not After:                         Sep 26 23:59:59 2020 GMT   
       
      Signature Algorithm:               sha1WithRSAEncryption              
      Key Size:                          1024 bit                           
      X509v3 Subject Alternative Name:   {'othername': ['<unsupported>'], 'DNS': ['www.example.com']}

  * OCSP Stapling :
      Server did not send back an OCSP response.                                   

  * Session Resumption :
      With Session IDs:           Supported (5 successful, 0 failed, 0 errors, 5 total attempts).
      With TLS Session Tickets:   Supported

  * SSLV2 Cipher Suites :

      Rejected Cipher Suite(s): Hidden 

      Preferred Cipher Suite: None     

      Accepted Cipher Suite(s): None   

      Undefined - An unexpected error happened: None 

  * SSLV3 Cipher Suites :

      Rejected Cipher Suite(s): Hidden 

      Preferred Cipher Suite:          
        RC4-SHA                       128 bits      HTTP 200 OK                        

      Accepted Cipher Suite(s):        
        CAMELLIA256-SHA               256 bits      HTTP 200 OK                        
        RC4-SHA                       128 bits      HTTP 200 OK                        
        CAMELLIA128-SHA               128 bits      HTTP 200 OK                        

      Undefined - An unexpected error happened: None 

  * TLSV1_1 Cipher Suites :

      Rejected Cipher Suite(s): Hidden 

      Preferred Cipher Suite: None     

      Accepted Cipher Suite(s): None   

      Undefined - An unexpected error happened: 
        ECDH-RSA-AES256-SHA             socket.timeout - timed out         
        ECDH-ECDSA-AES256-SHA           socket.timeout - timed out         

  * TLSV1_2 Cipher Suites :

      Rejected Cipher Suite(s): Hidden 

      Preferred Cipher Suite: None     

      Accepted Cipher Suite(s): None   

      Undefined - An unexpected error happened: 
        ECDH-RSA-AES256-GCM-SHA384      socket.timeout - timed out         
        ECDH-ECDSA-AES256-GCM-SHA384    socket.timeout - timed out         

  * TLSV1 Cipher Suites :

      Rejected Cipher Suite(s): Hidden 

      Preferred Cipher Suite:          
        RC4-SHA                       128 bits      Timeout on HTTP GET                

      Accepted Cipher Suite(s):        
        CAMELLIA256-SHA               256 bits      HTTP 200 OK                        
        RC4-SHA                       128 bits      HTTP 200 OK                        
        CAMELLIA128-SHA               128 bits      HTTP 200 OK                        

      Undefined - An unexpected error happened: 
        ADH-CAMELLIA256-SHA             socket.timeout - timed out         



 SCAN COMPLETED IN 9.68 S
 ------------------------

Testing SSL certificate validity – client and server

Firstly upgrade your browser because also CA certs expire and, in every release of the browser, these are been renewed. Examine the validity of the certificates used by the application. Browsers will issue a warning when encountering expired certificates, certificates issued by untrusted CAs, and certificates which do not match namewise with the site to which they should refer. By clicking on the padlock which appears in the browser window when visiting an HTTPS site, you can look at information related to the certificate – including the issuer, period of validity, encryption characteristics, etc. If the application requires a client certificate, you probably have installed one to access it. Certificate information is available in the browser by inspecting the relevant certificate(s) in the list of the installed certificates. These checks must be applied to all visible SSL-wrapped communication channels used by the application. Though this is the usual https service running on port 443, there may be additional services involved depending on the web application architecture and on deployment issues (an HTTPS administrative port left open, HTTPS services on non-standard ports, etc.). Therefore, apply these checks to all SSL-wrapped ports which have been discovered. For example, the nmap scanner features a scanning mode (enabled by the –sV command line switch) which identifies SSL-wrapped services. The Nessus vulnerability scanner has the capability of performing SSL checks on all SSL/TLS-wrapped services.

Some tools, as in previous examples, check also for certificate validity.

Example 7. Testing for certificate validity (manually)

Rather than providing a fictitious example, we have inserted an anonymized real-life example to stress how frequently one stumbles on https sites whose certificates are inaccurate with respect to naming. The following screenshots refer to a regional site of a high-profile IT company.

We are visiting an .it site and the certificate was issued to a .com site! Internet Explorer warns that the name on the certificate does not match the name of the site.

SSL Certificate Validity Testing IE Warning.gif Warning issued by Microsoft Internet Explorer

The message issued by Firefox is different – Firefox complains because it cannot ascertain the identity of the .com site the certificate refers to because it does not know the CA which signed the certificate. In fact, Internet Explorer and Firefox do not come preloaded with the same list of CAs. Therefore, the behavior experienced with various browsers may differ.

SSL Certificate Validity Testing Firefox Warning.gif Warning issued by Mozilla Firefox

Testing for other vulnerabilities

As mentioned previously there are other types of vulnerabilities that are not related with the SSL/TLS protocol used, the cipher suites or Certificates. A part from others discussed in other parts of the Guide, the another one is possible when the server provide the website both with the HTTP and HTTPS protocols, and permit to an attacker to force a victim into using a non-secure channel instead of a secure one.

Surf Jacking

Surf Jacking attack [7] was first presented by Sandro Gauci and permits to an attacker to hijack an HTTP session even when the victim’s connection is encrypted using SSL or TLS. The following is a scenario of how the attack can take place:

The following is a scenario of how the attack can take place:

  • Victim logs into the secure website at https://somesecuresite/.
  • The secure site issues a session cookie as the client logs in.
  • While logged in, the victim opens a new browser window and goes to http:// examplesite/
  • An attacker sitting on the same network is able to see the clear text traffic to http://examplesite.
  • The attacker sends back a "301 Moved Permanently" in response to the clear text traffic to http://examplesite. The response contains the header “Location: http://somesecuresite /”, which makes it appear that examplesite is sending the web browser to somesecuresite. Notice that the URL scheme is HTTP not HTTPS.
  • The victim's browser starts a new clear text connection to http://somesecuresite/ and sends an HTTP request containing cookie in the HTTP header in clear text
  • The attacker sees this traffic and logs the cookie for later (ab)use.

To test if a website is vulnerable is sufficient to proceed like follow:

  1. Check if website supports both HTTP and HTTPS protocol
  2. Check if cookies do not have the “Secure” flag

SSL Strip

Often applications supports both HTTP and HTTPS. As for usability or because users do not use to type “https://www.example.com”. Often users go into an HTTPS website from link or a redirect. Typically also home banking site have a similar configuration with an iframed login or a form with action attribute over HTTPS but the page under HTTP. An attacker in a privileged position - as described in SSL strip [8] - can incercept traffic when user is into HTTP and manipulate it to get a Man-In-The-Middle attack under HTTPS. To test if application is vulnerable is sufficient the website supports both HTTP and HTTPS.

Gray Box testing and example

Testing for Weak SSL/TSL Cipher Suites

Check the configuration of the web servers which provide https services. If the web application provides other SSL/TLS wrapped services, these should be checked as well.

Example 8. Windows Server

Check the configuration on a Microsoft Windows Server (2000, 2003 and 2008) using the registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\

which has some sub-keys like Ciphers, Protocols and KeyExchangeAlgorithms.

Example 9: Apache

To check the cipher suites and protocols supported by Apache2 web server open the ssl.conf file and search for the SSLCipherSuite, SSLProtocol, SSLHonorCipherOrder,SSLInsecureRenegotiation and SSLCompression directives.

Testing SSL certificate validity – client and server

Examine the validity of the certificates used by the application at both server and client levels. The usage of certificates is primarily at the web server level; however, there may be additional communication paths protected by SSL (for example, towards the DBMS). You should check the application architecture to identify all SSL protected channels.

References

OWASP Resources

Whitepapers

Tools