Testing for Error Code (OWASP-IG-006)
Often during a penetration test on web applications we come up against many error codes generated from applications or web servers. It's possible to cause these errors to be displayed by using a particular request, either specially crafted with tools or created manually. These codes are very useful to penetration testers during their activities because they reveal a lot of information about databases, bugs, and other technological components directly linked with web applications. Within this section we'll analyse the more common codes (error messages) and bring into focus the steps of vulnerability assessment. The most important aspect for this activity is to focus one's attention on these errors, seeing them as a collection of information that will aid in the next steps of our analysis. A good collection can facilitate assessment efficiency by decreasing the overall time taken to perform the penetration test.
Description of the Issue
A common error that we can see during our search is the HTTP 404 Not Found. Often this error code provides useful details about the underlying web server and associated components. For example:
Not Found The requested URL /page.html was not found on this server. Apache/2.2.3 (Unix) mod_ssl/2.2.3 OpenSSL/0.9.7g DAV/2 PHP/5.1.2 Server at localhost Port 80
This error message can be generated by requesting a non-existant URL. After the common message that shows a page not found, there is information about web server version, OS, modules and other products used. This information can be very important from an OS and application type and version identification point of view.
Web server errors aren't the only useful output returned requiring security analysis. Consider the next example error message:
Microsoft OLE DB Provider for ODBC Drivers (0x80004005) [DBNETLIB][ConnectionOpen(Connect())] - SQL server does not exist or access denied
What happened? We will explain step-by-step below.
In this example, the 80004005 is a generic IIS error code which indicates that it could not establish a connection to its associated database.
In many cases, the error message will detail the type of the database. This will often indicate the underlying operating system by association. With this information, the penetration tester can plan an appropriate strategy for the security test.
By manipulating the variables that are passed to the database connect string, we can invoke more detailed errors.
Microsoft OLE DB Provider for ODBC Drivers error '80004005' [Microsoft][ODBC Access 97 ODBC driver Driver]General error Unable to open registry key 'DriverId'
In this example, we can see a generic error in the same situation which reveals the type and version of the associated database system and a dependence on Windows operating system registry key values.
Now we will look at a practical example with a security test against a web application that loses its link to its database server and does not handle the exception in a controlled manner. This could be caused by a database name resolution issue, processing of unexpected variable values, or other network problems.
Consider the scenario where we have a database administration web portal which can be used as a front end GUI to issue database queries, create tables and modify database fields. During POST of the logon credentials, the following error message is presented to the penetration tester that which indicates the presence of a MySQL database server:
Microsoft OLE DB Provider for ODBC Drivers (0x80004005) [MySQL][ODBC 3.51 Driver]Unknown MySQL server host
If we see in the HTML code of the logon page the presence of a hidden field with a database IP, we can try to change this value in the URL with the address of database server under the penetration tester's control in an attempt to fool the application into thinking that logon was successful.
Another example: knowing the database server that services a web application, we can take advantage of this information to carry out a SQL Injection for that kind of database or a persistent XSS test.
Information Gathering on web applications with server-side technology is quite difficult, but the information discovered can be useful for the correct execution of an attempted exploit (for example, SQL injection or Cross Site Scripting (XSS) attacks)and can reduce false positives.
Black Box testing and example
telnet <host target> 80 GET /<wrong page> HTTP/1.1 <CRLF><CRLF>
HTTP/1.1 404 Not Found Date: Sat, 04 Nov 2006 15:26:48 GMT Server: Apache/2.2.3 (Unix) mod_ssl/2.2.3 OpenSSL/0.9.7g Content-Length: 310 Connection: close Content-Type: text/html; charset=iso-8859-1
1. network problems 2. bad configuration about host database address
Microsoft OLE DB Provider for ODBC Drivers (0x80004005) ' [MySQL][ODBC 3.51 Driver]Unknown MySQL server host
1. Authentication Failed 2. Credentials not inserted
Firewall version used for authentication
Error 407 FW-1 at <firewall>: Unauthorized to access the document. • Authorization is needed for FW-1. • The authentication required by FW-1 is: unknown. • Reason for failure of last attempt: no user
Gray Box testing and example
Enumeration of the directories with access denied.
Directory Listing Denied This Virtual Directory does not allow contents to be listed.
Forbidden You don't have permission to access /<dir> on this server.
-  [RFC2616] Hypertext Transfer Protocol -- HTTP/1.1
OWASP Testing Guide v2
Here is the OWASP Testing Guide v2 Table of Contents