SQL Injection question

[Sasa Jusic <sjusic () pamela zesoi fer hr>]

Hi group,

I am conducting a Pen test for a customer, and last few days I have been
struggling with their Web application running on Apache/mod_ssl Web Server
using CGI interface. During the initial assessment I found several Web forms
using POST method, so I began searching for SQL Injection Vulnerabilities. 

The problem is that forms are well protected, and they are only accepting
numeric values, so I can't insert any malicious characters to test for SQL
vulnerabilities. Then I discovered that the form input validation is done
with JavaScript code on the client side, so I used the Paros proxy tool for
intercepting and modification of submitted form values. In this way I
managed to submit the arbitrary data to the server, and the server response
was "500 Internal Server Error" without any useful information about the
error reason or underlying database structure. I tried various combinations
typical for SQL Injection assessment, but the response was always the same.

On several places I have red that this type of error is one of the possible
indicators of SQL Injection problems, so I would like to examine this
problem more carefully. 

How can I know if this is really a SQL Injection problem or some other
error? Is there any way I can elicit some more information about the
structure of the database or any other useful information I can use for
further testing?

I don't have much practical experience with SQL Injection so I would really
appreciate any help. 

Best regards,

Sasa.

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