Difference between revisions of "Testing for SSI Injection (OWASP-DV-009)"

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[[http://www.owasp.org/index.php/Web_Application_Penetration_Testing_AoC Up]]<br>
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{{Template:OWASP Testing Guide v4}}
{{Template:OWASP Testing Guide v2}}
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== Brief Summary ==
 
== Brief Summary ==
  
Web servers usually give to the developer the possibility to add small pieces of dynamic code inside static html pages, without having to play with full-fledged server-side or client-side languages. This feature is incarnated by the '''Server-Side Includes''' ('''SSI'''), a very simple extensions that can enable an attacker to inject code into html pages, or even perform remote code execution.<br>
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Web servers usually give developers the ability to add small pieces of dynamic code inside static HTML pages, without having to deal with full-fledged server-side or client-side languages. This feature is incarnated by the '''[[Server-Side Includes %28SSI%29 Injection|Server-Side Includes]]''' ('''SSI'''). In SSI injection testing, we test if it is possible to inject into the application data that will be interpreted by SSI mechanisms. A successful exploitation of this vulnerability allows an attacker to inject code into HTML pages or even perform remote code execution.
  
 
== Description of the Issue ==
 
== Description of the Issue ==
  
Server-Side Includes are directives that the web server parses before serving the page to the user. They represent an alternative to writing CGI program or embedding code using server-side scripting languages, when there's only need to perform very simple tasks. Common SSI implementations provide commands to include external files, to set and print web server CGI environment variables and to execute external CGI scripts or system commands.<br>
+
Server-Side Includes are directives that the web server parses before serving the page to the user. They represent an alternative to writing CGI programs or embedding code using server-side scripting languages, when there's only need to perform very simple tasks. Common SSI implementations provide commands to include external files, to set and print web server CGI environment variables, and to execute external CGI scripts or system commands.<br>
  
Putting an SSI directive into a static html document is as easy as writing a piece of code like the following:<br>
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Putting an SSI directive into a static HTML document is as easy as writing a piece of code like the following:<br>
  
 
<pre>
 
<pre>
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to include the output of a system command.<br>
 
to include the output of a system command.<br>
  
Then, if the web server's SSI support is enabled, the server will parse these directives, both in the body or inside the headers. In the default configuration, usually, most web servers don't allow the use of the '''''exec''''' directive to execute system commands.<br>
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Then, if the web server's SSI support is enabled, the server will parse these directives. In the default configuration, usually, most web servers don't allow the use of the '''''exec''''' directive to execute system commands.<br>
  
As in every bad input validation situation, problems arise when the user of a web application is allowed to provide data that's going to make the application or the web server itself behave in an unforseen manner. Talking about SSI injection, the attacker could be able to provide an input that, if inserted by the application (or maybe directly by te server) into a dynamically generated page would be parsed as SSI directives.<br>
+
As in every bad input validation situation, problems arise when the user of a web application is allowed to provide data that makes the application or the web server behave in an unforeseen manner. With regard to SSI injection, the attacker could provide input that, if inserted by the application (or maybe directly by the server) into a dynamically generated page, would be parsed as one or more SSI directives.<br>
  
We are talking about an issue very similar to a classical scripting language injection problem; maybe less dangerous, as the SSI directive are not comparable to a real scripting language and because the web server needs to be configured to allow SSI; but also simpler to exploit, as SSI directives easy to understand and powerful enough to output the content of files and to execute system commands.<br>
+
This is a vulnerability very similar to a classical scripting language injection vulnerability. One mitigation is that the web server needs to be configured to allow SSI. On the other hand, SSI injection vulnerabilities are often simpler to exploit, since SSI directives are easy to understand and, at the same time, quite powerful, e.g., they can output the content of files and execute system commands.<br>
  
 
== Black Box testing ==
 
== Black Box testing ==
  
The first thing to do when testing in a Black Box fashion is finding if the web server actually support SSI directives. The answer is almost certainly a yes, as SSI support is quite common. To find out we just need to discover which kind of web server is running on our target, using classical information gathering techniques.<br>
+
The first thing to do when testing in a Black Box fashion is finding if the web server actually supports SSI directives. Often, the answer is yes, as SSI support is quite common. To find out we just need to discover which kind of web server is running on our target, using classic information gathering techniques.<br>
  
Whether we succeded or not in discovering this piece of information, we could guess if SSI are supported just looking at the content of the target web site we are testing: if it makes use of '''''.shtml''''' file then SSI are probably supported, as this extension is used to identify pages containing these directives. Unfortunately, the use of the '''''shtml''''' extension is not mandatory, so not having found any '''''shtml''''' files doesn't necessarily mean that the target is not prone to SSI injection attacks.<br>
+
Whether we succeed or not in discovering this piece of information, we could guess if SSI are supported just by looking at the content of the target web site. If it contains '''''.shtml''''' files, then SSI are probably supported, as this extension is used to identify pages containing these directives. Unfortunately, the use of the '''''shtml''''' extension is not mandatory, so not having found any '''''shtml''''' files doesn't necessarily mean that the target is not prone to SSI injection attacks.<br>
  
Let's go to the next step, which is needed not only to find out if an SSI injection attack is really plausible, but also to identify the input points we can use to inject our malicious code.<br>
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The next step consists of determining if an SSI injection attack is actually possible and, if so, what are the input points that we can use to inject our malicious code.<br>
 +
The testing activity required to do this is exactly the same used to test for other code injection vulnerabilities. In particular, we need to find every page where the user is allowed to submit some kind of input, and verify whether the application is correctly validating the submitted input. If sanitization is insufficient, we need to test if we can provide data that is going to be displayed unmodified (for example, in an error message or forum post). Besides common user-supplied data, input vectors that should always be considered are HTTP request headers and cookies content, since they can be easily forged.<br>
  
In this step the testing activity is exactly the same needed to test for other code injection vulnerabilities. We need to find every page where the user is allowed to submit some kind of input and verify whether the application is correctly validating the submitted input or, otherwise, if we could provide data that is going to be displayed unmodified (as error message, forum post, etc.). Beside common user supplied data, input vectors that are always to be considered are HTTP request headers and cookies content, that can be easily forged.<br>
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Once we have a list of potential injection points, we can check if the input is correctly validated and then find out where the provided input is stored. We need to make sure that we can inject characters used in SSI directives:<br>
 
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Once we have a list of potential injection points, we can check if the input is correctly validated and then find out where in the web site the data we provided are going to be displayed. We need to make sure that we are going to be able to make characters like that used in SSI directives:<br>
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<pre>
 
<pre>
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</pre>
 
</pre>
  
go through the application and be parsed by the server at some point.<br>
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To test if validation is insufficient, we can input, for example, a string like the following in an input form:<br>
 
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Exploiting the lack of validation, is as easy as submitting, for example, a string like the following:<br>
+
  
 
<pre>
 
<pre>
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</pre>
 
</pre>
  
in a input form, instead of the classical:<br>
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This is similar to testing for XSS vulnerabilities using<br>
  
 
<pre>
 
<pre>
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</pre>
 
</pre>
  
The directive would be then parsed by the server next time it needs to serve the given page, thus including the content of the Unix standard password file.<br>
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If the application is vulnerable, the directive is injected and it would be interpreted by the server the next time the page is served, thus including the content of the Unix standard password file.<br>
  
 
The injection can be performed also in HTTP headers, if the web application is going to use that data to build a dynamically generated page:<br>
 
The injection can be performed also in HTTP headers, if the web application is going to use that data to build a dynamically generated page:<br>
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GET / HTTP/1.0
 
GET / HTTP/1.0
 
Referer: <!--#exec cmd="/bin/ps ax"-->
 
Referer: <!--#exec cmd="/bin/ps ax"-->
User-Agent: <!--#virtual include="/proc/version"-->
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User-Agent: <!--#include virtual="/proc/version"-->
 
</pre>
 
</pre>
  
 
== Gray Box testing and example ==  
 
== Gray Box testing and example ==  
  
Being able to review the application source code we can quite easily find out:<br>
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If we have access to the application source code, we can quite easily find out:<br>
# If SSI directives are used; if they are, then the web server is going to have SSI support enabled, making SSI injection at least a potential issue to investigate;<br>
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# If SSI directives are used. If they are, then the web server is going to have SSI support enabled, making SSI injection at least a potential issue to investigate.<br>
# Where user input, cookie content and http headers are handled; the complete input vectors list is then quickly built;<br>
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# Where user input, cookie content and HTTP headers are handled. The complete list of input vectors is then quickly determined.<br>
# How the input is handled, what kind of filtering is performed, what characters the application is not letting through and how many type of encoding are taken into account.<br>
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# How the input is handled, what kind of filtering is performed, what characters the application is not letting through, and how many types of encoding are taken into account.<br>
  
Performing these steps is mostly a matter of using grep, to find the right keywords inside the source code (SSI directives, CGI environment variables, variables assignment involving user input, filtering functions and so on).<br>
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Performing these steps is mostly a matter of using grep to find the right keywords inside the source code (SSI directives, CGI environment variables, variables assignment involving user input, filtering functions and so on).
  
 
== References ==
 
== References ==
 
'''Whitepapers'''<br>
 
'''Whitepapers'''<br>
* IIS: "Notes on Server-Side Includes (SSI) syntax" - http://support.microsoft.com/kb/203064<br>
 
 
* Apache Tutorial: "Introduction to Server Side Includes" - http://httpd.apache.org/docs/1.3/howto/ssi.html<br>
 
* Apache Tutorial: "Introduction to Server Side Includes" - http://httpd.apache.org/docs/1.3/howto/ssi.html<br>
 
* Apache: "Module mod_include" - http://httpd.apache.org/docs/1.3/mod/mod_include.html<br>
 
* Apache: "Module mod_include" - http://httpd.apache.org/docs/1.3/mod/mod_include.html<br>
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* Web Proxy Burp Suite - http://portswigger.net
 
* Web Proxy Burp Suite - http://portswigger.net
 
* Paros - http://www.parosproxy.org/index.shtml
 
* Paros - http://www.parosproxy.org/index.shtml
* WebScarab - http://www.owasp.org/index.php/OWASP_WebScarab_Project
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* [[OWASP WebScarab Project|WebScarab]]
 
* String searcher: grep - http://www.gnu.org/software/grep, your favorite text editor
 
* String searcher: grep - http://www.gnu.org/software/grep, your favorite text editor
  
  
{{Category:OWASP Testing Project AoC}}
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 +
[[Category:FIXME| link doesn't work
 +
 
 +
* IIS: "Notes on Server-Side Includes (SSI) syntax" - http://support.microsoft.com/kb/203064<br>
 +
 
 +
 
 +
 
 +
]]

Revision as of 09:52, 9 October 2012

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

Web servers usually give developers the ability to add small pieces of dynamic code inside static HTML pages, without having to deal with full-fledged server-side or client-side languages. This feature is incarnated by the Server-Side Includes (SSI). In SSI injection testing, we test if it is possible to inject into the application data that will be interpreted by SSI mechanisms. A successful exploitation of this vulnerability allows an attacker to inject code into HTML pages or even perform remote code execution.

Description of the Issue

Server-Side Includes are directives that the web server parses before serving the page to the user. They represent an alternative to writing CGI programs or embedding code using server-side scripting languages, when there's only need to perform very simple tasks. Common SSI implementations provide commands to include external files, to set and print web server CGI environment variables, and to execute external CGI scripts or system commands.

Putting an SSI directive into a static HTML document is as easy as writing a piece of code like the following:

<!--#echo var="DATE_LOCAL" -->

to print out the current time.

<!--#include virtual="/cgi-bin/counter.pl" -->

to include the output of a CGI script.

<!--#include virtual="/footer.html" -->

to include the content of a file.

<!--#exec cmd="ls" -->

to include the output of a system command.

Then, if the web server's SSI support is enabled, the server will parse these directives. In the default configuration, usually, most web servers don't allow the use of the exec directive to execute system commands.

As in every bad input validation situation, problems arise when the user of a web application is allowed to provide data that makes the application or the web server behave in an unforeseen manner. With regard to SSI injection, the attacker could provide input that, if inserted by the application (or maybe directly by the server) into a dynamically generated page, would be parsed as one or more SSI directives.

This is a vulnerability very similar to a classical scripting language injection vulnerability. One mitigation is that the web server needs to be configured to allow SSI. On the other hand, SSI injection vulnerabilities are often simpler to exploit, since SSI directives are easy to understand and, at the same time, quite powerful, e.g., they can output the content of files and execute system commands.

Black Box testing

The first thing to do when testing in a Black Box fashion is finding if the web server actually supports SSI directives. Often, the answer is yes, as SSI support is quite common. To find out we just need to discover which kind of web server is running on our target, using classic information gathering techniques.

Whether we succeed or not in discovering this piece of information, we could guess if SSI are supported just by looking at the content of the target web site. If it contains .shtml files, then SSI are probably supported, as this extension is used to identify pages containing these directives. Unfortunately, the use of the shtml extension is not mandatory, so not having found any shtml files doesn't necessarily mean that the target is not prone to SSI injection attacks.

The next step consists of determining if an SSI injection attack is actually possible and, if so, what are the input points that we can use to inject our malicious code.
The testing activity required to do this is exactly the same used to test for other code injection vulnerabilities. In particular, we need to find every page where the user is allowed to submit some kind of input, and verify whether the application is correctly validating the submitted input. If sanitization is insufficient, we need to test if we can provide data that is going to be displayed unmodified (for example, in an error message or forum post). Besides common user-supplied data, input vectors that should always be considered are HTTP request headers and cookies content, since they can be easily forged.

Once we have a list of potential injection points, we can check if the input is correctly validated and then find out where the provided input is stored. We need to make sure that we can inject characters used in SSI directives:

< ! # = / . " - > and [a-zA-Z0-9]

To test if validation is insufficient, we can input, for example, a string like the following in an input form:

<!--#include virtual="/etc/passwd" -->

This is similar to testing for XSS vulnerabilities using

<script>alert("XSS")</script>

If the application is vulnerable, the directive is injected and it would be interpreted by the server the next time the page is served, thus including the content of the Unix standard password file.

The injection can be performed also in HTTP headers, if the web application is going to use that data to build a dynamically generated page:

GET / HTTP/1.0
Referer: <!--#exec cmd="/bin/ps ax"-->
User-Agent: <!--#include virtual="/proc/version"-->

Gray Box testing and example

If we have access to the application source code, we can quite easily find out:

  1. If SSI directives are used. If they are, then the web server is going to have SSI support enabled, making SSI injection at least a potential issue to investigate.
  2. Where user input, cookie content and HTTP headers are handled. The complete list of input vectors is then quickly determined.
  3. How the input is handled, what kind of filtering is performed, what characters the application is not letting through, and how many types of encoding are taken into account.

Performing these steps is mostly a matter of using grep to find the right keywords inside the source code (SSI directives, CGI environment variables, variables assignment involving user input, filtering functions and so on).

References

Whitepapers

Tools