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Test HTTP Methods (OTG-CONFIG-006)

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OWASP Testing Guide v2 Table of Contents

Brief Summary

In this test we check that the web server is not configured to allow potentially dangerous HTTP commands (methods) and that Cross Site Tracing (XST) is not possible

Short Description of the Issue (Topic and Explanation)

While GET and POST are by far the most common methods that are used to access information provided by a web server, the Hypertext Transfer Protocol (HTTP) allows several other (and somewhat less known) methods. RFC 2616 (which describes HTTP version 1.1 which is the today standard) defines the following eight methods:

  • HEAD
  • GET
  • POST
  • PUT
  • DELETE
  • TRACE
  • OPTIONS
  • CONNECT

Some of these methods can potentially pose a security risk for a web application, as they allow an attacker to modify the files stored on the web server and, in some scenarios, steal the credentials of legitimate users. More specifically, the methods that should be disabled are the following:

  • PUT: This method allows a client to upload new files on the web server. An attacker can exploit it by uploading malicious files (e.g.: an asp file that executes commands by invoking cmd.exe), or by simply using the victim server as a file repository
  • DELETE: This method allows a client to delete a file on the web server. An attacker can exploit it as a very simple and direct way to deface a web site or to mount a DoS attack
  • CONNECT: This method could allow a client to use the web server as a proxy
  • TRACE: This method simply echoes back to the client whatever string has been sent to the server, and it is used mainly for debugging purposes. This method, apparently harmless, can be used to mount an attack known as Cross Site Tracing, which has been discovered by Jeremiah Grossman (see links at the bottom of the page)

If an application needs one or more of these methods, it is important to check that their use is properly limited to trusted users and safe conditions.

Black Box testing and example

Discover the Supported Methods
To perform this test, we need some way to figure out which HTTP methods are supported by the web server we are examining. The OPTIONS HTTP method provides us with the most direct and effective way to do that. RFC 2616 states that “The OPTIONS method represents a request for information about the communication options available on the request/response chain identified by the Request-URI”.

The testing method is extremely straightforward and we only need to fire up netcat (or telnet): 

icesurfer@nightblade ~ $ nc www.victim.com 80 
OPTIONS / HTTP/1.1
Host: www.victim.com

HTTP/1.1 200 OK
Server: Microsoft-IIS/5.0
Date: Tue, 31 Oct 2006 08:00:29 GMT
Connection: close
Allow: GET, HEAD, POST, TRACE, OPTIONS
Content-Length: 0

icesurfer@nightblade ~ $

As we can see in the example, OPTIONS provides a list of the methods that are supported by the web server, and in this case we can see, for instance, that TRACE method is enabled. The danger that is posed by this method is illustrated in the following section

Test XST Potential
Note: in order to understand the logic and the goals of this attack you need to be familiar with Cross Site Scripting attacks.

The TRACE method, while apparently harmless, can be successfully leveraged in some scenarios to steal legitimate users' credentials. This attack technique was discovered by Jeremiah Grossman in 2003, in an attempt to bypass the httpOnly tag that Microsoft introduced in Internet Explorer 6 sp1 to protect cookies from being accessed by JavaScript. As a matter of fact, one of the most recurring attack patterns in Cross Site Scripting is to access the document.cookie object and send it to a web server controlled by the attacker so that he/she can hijack the victim's session. Tagging a cookie as httpOnly forbids JavaScript to access it, protecting it from being sent to a third party. However, the TRACE method can be used to bypass this protection and access the cookie even in this scenario.

As mentioned before, TRACE simply returns any string that is sent to the web server. In order to verify its presence (or to double-check the results of the OPTIONS request shown above), we can proceed as shown in the following example: 

icesurfer@nightblade ~ $ nc www.victim.com 80
TRACE / HTTP/1.1
Host: www.victim.com

HTTP/1.1 200 OK
Server: Microsoft-IIS/5.0
Date: Tue, 31 Oct 2006 08:01:48 GMT
Connection: close
Content-Type: message/http
Content-Length: 39

TRACE / HTTP/1.1
Host: www.victim.com

As we can see, the response body is exactly a copy of our original request, meaning that our target allows this method. Now, where is the danger lurking? If we instruct a browser to issue a TRACE request to the web server, and this browser has a cookie for that domain, the cookie will be automatically included in the request headers, and will therefore echoed back in the resulting response. At that point, the cookie string will be accessible by JavaScript and it will be finally possible to send it to a third party even when the cookie is tagged as httpOnly.

There are multiple ways to make a browser issue a TRACE request, as the XMLHTTP ActiveX control in Internet Explorer and XMLDOM in Mozilla and Netscape. However, for security reasons the browser is allowed to start a connection only to the domain where the hostile script resides. This is a mitigating factor, as the attacker needs to combine the TRACE method with another vulnerability in order to mount the attack. Basically, an attacker has two ways to successfully launch a Cross Site Tracing attack:

  • 1.Leveraging another server-side vulnerability: the attacker injects the hostile JavaScript snippet, that contains the TRACE request, in the vulnerable application, as in a normal Cross Site Scripting attack
  • 2.Leveraging a client-side vulnerability: the attacker creates a malicious website that contains the hostile JavaScript snippet and exploits some cross-domain vulnerability of the browser of the victim, in order to make the JavaScript code successfully perform a connection to the site that supports the TRACE method and that originated the cookie that the attacker is trying to steal.

More detailed information, together with code samples, can be found in the original whitepaper written by Jeremiah Grossman.

Gray Box testing and example

The testing in a Gray Box scenario follows the same steps of a Black Box scenario

References

Whitepapers

Tools


OWASP Testing Guide v2

Here is the OWASP Testing Guide v2 Table of Contents

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