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Glossary

HTTP Verb Tampering

HTTP Verb Tampering is an attack that exploits vulnerabilities in HTTP verb (also known as HTTP method) authentication and access control mechanisms. Many authentication mechanisms only limit access to the most common HTTP methods, thus allowing unauthorized access to restricted resources by other HTTP methods.

Detailed Description

Many Web server authentication mechanisms use verb-based authentication and access controls. Such security mechanisms include access control rules for requests with specific HTTP methods. For example, an administrator can configure a Web server to allow unrestricted access to a Web page using HTTP GET requests, but restrict POSTs to administrators only. However, many implementations of verb-based security mechanisms enforce the security rules in an unsecure manner, allowing access to restricted resources by using alternative HTTP methods (such as HEAD) or even arbitrary character strings.

For example, Java Platform Enterprise Edition (Java EE) supports verb-based authentication and access control through the web.xml configuration file. In Java EE, one can limit access to the admin/ directories for “admin” users by adding the following to web.xml:

<security-constraint>
  <web-resource-collection>
    <url-pattern>/admin/*</url-pattern>
    <http-method>GET</http-method>
    <http-method>POST</http-method>
  </web-resource-collection>
  <auth-constraint>
    <role-name>admin</role-name>
  </auth-constraint>
</security-constraint>

These security rules ensure that GET or POST requests to admin/ directories from non admin users will be blocked. However, HTTP requests to admin/ directories other than GET or POST will not be blocked. While a GET request from a non admin user will be blocked, a HEAD request from the same user will not. Unless the administrator explicitly configures the Web server to deny all methods other than GET and POST, the access control mechanism can be bypassed simply by using different methods that are supported by the server. Other examples of Web servers that are affected by this issue include IIS 6.0, Apache 2.2.8, and TomCat 6.0.

In some Web servers–for example, Apache 2.2/PHP–it is even possible to bypass the access control mechanism by using arbitrary character strings for HTTP methods. Such Web servers implement default handlers for requests that are not bound to a specific HTTP method. Unlike an HTTP Servlet where a GET request is only handled if a doGet() is defined, some Web servers attempt to process any and all methods including unknown methods. Thus, by replacing a legitimate method with an arbitrary one (MPRV instead of GET) the attacker can exploit vulnerabilities in the internal processing logic and bypass the access control mechanism.

HTTP Verb Tampering Prevention

Verb tampering attacks exploit either configuration flaws in the access control mechanism or vulnerabilities in the request handlers’ code. As presented in the example above, blocking requests that use non-standard HTTP methods is not enough because in many cases an attacker can use a legitimate HTTP method like HEAD.

Imperva SecureSphere combines two mitigation techniques to detect and stop verb tampering attacks. In the first, SecureSphere learns which methods are allowed for each URL. Any attempt to use HTTP methods that are not part of the application’s normal usage will be detected and blocked. The second technique detects non-standard HTTP methods and blocks requests using such methods. In cases where the application uses non-standard methods normally, this mechanism can be easily updated with the allowed methods.

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