Testing for Weak SSL/TSL Ciphers, Insufficient Transport Layer Protection (OWASP-EN-002)

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

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Contenido


Brief Summary

Sensitive data must be protected when it is transmitted through the network. Such data can include user 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]. The use of this protocol ensures not only confidentiality, but also authentication. Servers are authenticated using digital certificates and it is also possible to use client certificate for mutual authentication.


Even if high grade ciphers are today supported and normally used, some misconfiguration in the 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

A vulnerability occurs if the HTTP protocol is used to transmit sensitive information [2] (e.g. credentials transmitted over HTTP [3]).

When the SSL/TLS service is present it is good but it increments the attack surface and the following vulnerabilities exist:

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

Other vulnerabilities linked to this are:

  • 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 HTTPS and HTTP content in the same page, which can be used to Leak information.


Sensitive data transmitted in clear-text

The application should not transmit sensitive information via unencrypted channels. Typically it is possible to find basic authentication over HTTP, input password or session cookie sent via HTTP and, in general, other information considered by regulations, laws or organization policy.


Weak SSL/TLS 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 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:

  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, this section 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's examine each check more in detail.

  • Each browser comes with a pre-loaded 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 vulnerabilities if the software is not up to date [4]. Furthermore, for the correct protection of data during transmission the 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 that 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

Testing for sensitive data transmitted in clear-text

Various types of information which must be protected can be also transmitted in clear text. It is possible to check if this 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 Authentication over HTTP because with Basic Authentication, after log in, 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/TLS Ciphers/Protocols/Keys vulnerabilities

The large number of available cipher suites and quick progress in cryptanalysis makes testing an 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 more complete checklist includes:

  • Secure Renegotiation should be enabled.
  • MD5 should not be used, due to known collision attacks. [35]
  • 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].


The 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 free (e.g. SSLAudit [28] or SSLScan [29]) and commercial (e.g. Tenable Nessus [27]), can be used to assess SSL/TLS vulnerabilities. But due to evolution of these vulnerabilities a good way to test is to check them manually with openssl [30] or use the tool’s output as an input for manual evaluation using the references.


Sometimes the SSL/TLS enabled service is not directly accessible and the tester can access it only via a HTTP proxy using CONNECT method [36]. Most of the tools will try to connect to desired tcp port to start SSL/TLS handshake. This will not work since desired port is accessible only via HTTP proxy. The tester can easily circumvent this by using relaying software such as socat [37].


Example 2. SSL service recognition via nmap

The 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 to 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 the tester tries to initiate a renegotiation by client [m] connecting to server with openssl. The tester then writes the fist line of an HTTP request and types “R” in a new line. He then waits for renegotiaion and completion of the HTTP request and checks if secure renegotiaion is supported by looking at the server output. Using manual requests it is also possible to see if Compression is enabled for TLS and to check for CRIME [13], for ciphers and for 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 the tester 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 the tester 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 the tester to check the cipher suite and also for 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 should be disabled. What is interesting is that the first fix for BEAST was the use of RC4, but this is now 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 mass scanning and XML output. The following is an example of a regular scan. It is one of the most complete and versatile tools 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
 ------------------------


Example 7. Testing SSL/TLS with testssl.sh

Testssl.sh [38] is a Linux shell script which provides clear output to facilitate good decision making. It can not only check web servers but also services on other ports, supports STARTTLS, SNI, SPDY and does a few check on the HTTP header as well.


It's a very easy to use tool. Here's some sample output (without colors):

user@myhost: % testssl.sh owasp.org      

########################################################
testssl.sh v2.0rc3  (https://testssl.sh)
($Id: testssl.sh,v 1.97 2014/04/15 21:54:29 dirkw Exp $)

   This program is free software. Redistribution + 
   modification under GPLv2 is permitted. 
   USAGE w/o ANY WARRANTY. USE IT AT YOUR OWN RISK!

 Note you can only check the server against what is
 available (ciphers/protocols) locally on your machine
########################################################

Using "OpenSSL 1.0.2-beta1 24 Feb 2014" on
      "myhost:/<mypath>/bin/openssl64"


Testing now (2014-04-17 15:06) ---> owasp.org:443 <---
("owasp.org" resolves to "192.237.166.62 / 2001:4801:7821:77:cd2c:d9de:ff10:170e") 


--> Testing Protocols
 
 SSLv2     NOT offered (ok) 
 SSLv3     offered 
 TLSv1     offered (ok) 
 TLSv1.1   offered (ok) 
 TLSv1.2   offered (ok) 

 SPDY/NPN  not offered

--> Testing standard cipher lists
 
 Null Cipher              NOT offered (ok) 
 Anonymous NULL Cipher    NOT offered (ok) 
 Anonymous DH Cipher      NOT offered (ok) 
 40 Bit encryption        NOT offered (ok) 
 56 Bit encryption        NOT offered (ok) 
 Export Cipher (general)  NOT offered (ok) 
 Low (<=64 Bit)           NOT offered (ok) 
 DES Cipher               NOT offered (ok) 
 Triple DES Cipher        offered
 Medium grade encryption  offered
 High grade encryption    offered (ok) 

--> Testing server defaults (Server Hello)
 
 Negotiated protocol       TLSv1.2 
 Negotiated cipher         AES128-GCM-SHA256 
 
 Server key size           2048 bit
 TLS server extensions:    server name, renegotiation info, session ticket, heartbeat
 Session Tickets RFC 5077  300 seconds

--> Testing specific vulnerabilities
 
 Heartbleed (CVE-2014-0160), experimental  NOT vulnerable (ok) 
 Renegotiation (CVE 2009-3555)             NOT vulnerable (ok) 
 CRIME, TLS (CVE-2012-4929)                NOT vulnerable (ok)  

--> Checking RC4 Ciphers 

RC4 seems generally available. Now testing specific ciphers...
 
 Hexcode    Cipher Name                   KeyExch.  Encryption Bits
--------------------------------------------------------------------
 [0x05]     RC4-SHA                       RSA         RC4      128

RC4 is kind of broken, for e.g. IE6 consider 0x13 or 0x0a

--> Testing HTTP Header response 
 
 HSTS        no 
 Server      Apache
 Application (None)
 
--> Testing (Perfect) Forward Secrecy  (P)FS) 

no PFS available 

Done now (2014-04-17 15:07) ---> owasp.org:443 <---

user@myhost: %    



STARTTLS would be tested via testssl.sh -t smtp.gmail.com:587 smtp, each ciphers with testssl -e <target>, each ciphers per protocol with testssl -E <target>. To just display what local ciphers that are installed for openssl see testssl -V. For a thorough check it is best to dump the supplied OpenSSL binaries in the path or the one of testssl.sh.


The interesting thing is if a tester looks at the sources they learn how features are tested, see e.g. Example 4. What is even better is that it does the whole handshake for heartbleed in pure /bin/bash with /dev/tcp sockets -- no piggyback perl/python/you name it.


Additionally it provides a prototype (via "testssl.sh -V") of mapping to RFC cipher suite names to OpenSSL ones. The tester needs the file mapping-rfc.txt in same directory.


Testing SSL certificate validity – client and server

Firstly upgrade the browser because CA certs expire and in every release of the browser these are 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 name wise with the site to which they should refer.


By clicking on the padlock that appears in the browser window when visiting an HTTPS site, testers can look at information related to the certificate – including the issuer, period of validity, encryption characteristics, etc. If the application requires a client certificate, that tester has probably 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.


Example 7. Testing for certificate validity (manually)

Rather than providing a fictitious example, this guide includes 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 a .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 pre-loaded 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. Apart from other vulnerabilities discussed in other parts of this guide, a vulnerability exists when the server provides the website both with the HTTP and HTTPS protocols, and permits an attacker to force a victim into using a non-secure channel instead of a secure one.


Surf Jacking

The 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:

  • 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 the cookie in the HTTP header in clear text
  • The attacker sees this traffic and logs the cookie for later use.


To test if a website is vulnerable carry out the following tests:

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


SSL Strip

Some applications supports both HTTP and HTTPS, either for usability or so users can type both addresses and get to the site. Often users go into an HTTPS website from link or a redirect. Typically personal banking sites have a similar configuration with an iframed log in 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 intercept traffic when the user is in the http site and manipulate it to get a Man-In-The-Middle attack under HTTPS. An application is vulnerable if it supports both HTTP and HTTPS.


Testing via HTTP proxy

Inside corporate environments testers can see services that are not directly accessible and they can access them only via HTTP proxy using the CONNECT method [36]. Most of the tools will not work in this scenario because they try to connect to the desired tcp port to start the SSL/TLS handshake. With the help of relaying software such as socat [37] testers can enable those tools for use with services behind an HTTP proxy.


Example 8. Testing via HTTP proxy

To connect to destined.application.lan:443 via proxy 10.13.37.100:3128 run socat as follows:

$ socat TCP-LISTEN:9999,reuseaddr,fork PROXY:10.13.37.100:destined.application.lan:443,proxyport=3128


Then the tester can target all other tools to localhost:9999:

$ openssl s_client -connect localhost:9999


All connections to localhost:9999 will be effectively relayed by socat via proxy to destined.application.lan:443.


Gray Box testing and example

Testing for Weak SSL/TLS Cipher Suites

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


Example 9. 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\

that has some sub-keys including Ciphers, Protocols and KeyExchangeAlgorithms.


Example 10: Apache

To check the cipher suites and protocols supported by the 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). Testers should check the application architecture to identify all SSL protected channels.


References

OWASP Resources


Whitepapers


Tools