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136 Web Application Penetration Testing foo> ]>&xxe; ]>&xxe; ]>&xxe; Tag Injection Once the first step is accomplished, the tester will have some information about the structure of the XML document. Then, it is possible to try to inject XML data and tags. We will show an example of how this can lead to a privilege escalation attack. Let’s considering the previous application. By inserting the following values: Username: tony Password: Un6R34kb!e E-mail: s4tan@hell.com0s4tan@hell.com the application will build a new node and append it to the XML database: gandalf !c3 0 gandalf@middleearth.com Stefan0 w1s3c 500 Stefan0@whysec.hmm tony Un6R34kb!e 500 s4tan@hell.com0s4tan@hell.com The resulting XML file is well formed. Furthermore, it is likely that, for the user tony, the value associated with the userid tag is the one appearing last, i.e., 0 (the admin ID). In other words, we have injected a user with administrative privileges. The only problem is that the userid tag appears twice in the last user node. Often, XML documents are associated with a schema or a DTD and will be rejected if they don’t comply with it. Let’s suppose that the XML document is specified by the following DTD: ]> Note that the userid node is defined with cardinality 1. In this case, the attack we have shown before (and other simple attacks) will not work, if the XML document is validated against its DTD before any processing occurs. However, this problem can be solved, if the tester controls the value of some nodes preceding the offending node (userid, in this example). In fact, the tester can comment out such node, by injecting a comment start/end sequence: Username: tony Password: Un6R34kb!e0s4tan@hell.com In this case, the final XML database is: gandalf !c3 0 gandalf@middleearth.com Stefan0 w1s3c 500 Stefan0@whysec.hmm tony Un6R34kb!e0s4tan@hell.com
137 Web Application Penetration Testing The original userid node has been commented out, leaving only the injected one. The document now complies with its DTD rules. Tools • XML Injection Fuzz Strings (from wfuzz tool) - https: /wfuzz.googlecode.com/svn/trunk/wordlist/Injections/ XML.txt References Whitepapers • Alex Stamos: “Attacking Web Services” - http: /www.owasp.org/images/d/d1/AppSec2005DC-Alex_Stamos-Attacking_Web_Services.ppt • Gregory Steuck, “XXE (Xml eXternal Entity) attack”, http: /www.securityfocus.com/archive/1/297714 Testing for SSI Injection (OTG-INPVAL-009) 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. 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: to print out the current time. to include the output of a CGI script. to include the content of a file or list files in a directory. 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. How to Test 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:
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