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Test File Extensions Handling for Sensitive Information



Web servers commonly use file extensions to determine which technologies, languages, and plugins must be used to fulfill web requests. While this behavior is consistent with RFCs and Web Standards, using standard file extensions provides the penetration tester useful information about the underlying technologies used in a web appliance and greatly simplifies the task of determining the attack scenario to be used on particular technologies. In addition, mis-configuration of web servers could easily reveal confidential information about access credentials.

File extension checks are often done to validate files before uploading them to the server. Unrestricted file uploads can lead to unforeseen results because the content may not be what is expected, or due to unexpected OS filename handling.

Understanding how web servers handle requests for files with different extensions can clarify server behavior based on the types of files accessed. For example, it can help to understand which file extensions are returned as text or plain versus those that cause server-side execution. The latter are indicative of technologies, languages, or plugins used by web servers or application servers. This information may provide additional insight into how the web application is engineered. For example, while a “.pl” extension is typically associated with server-side Perl support, the file extension alone can be misleading and not entirely indicative of the underlying technology. Take, for instance, server-side resources written in Perl, which might be renamed to disguise the usage of Perl. See the next section on “web server components” for more on identifying server-side technologies and components.

Test Objectives

  • Brute force sensitive file extensions that might contain raw data such as scripts, credentials, etc.
  • Validate that no system framework bypasses exist for the rules that have been set

How to Test

Forced Browsing

Submit requests with different file extensions and verify how they are handled. The verification should be on a per web directory basis. Verify directories that allow script execution. Web server directories can be identified by scanning tools which look for the presence of well-known directories. Additionally, mirroring the site structure helps testers reconstruct the directory tree served by the application.

If the web application architecture is load-balanced, it is important to assess all of the web servers. The ease of this task depends on the configuration of the balancing infrastructure. In an infrastructure with redundant components, there may be slight variations in the configuration of individual web or application servers. This may happen if the web architecture employs heterogeneous technologies (think of a set of IIS and Apache web servers in a load-balancing configuration, which may introduce slight asymmetric behavior between them, and possibly different vulnerabilities).


The tester has identified the existence of a file named Trying to access it directly gives back its contents, which are:

    mysql_connect("", "root", "password")
        or die("Could not connect");

The tester determines the existence of a MySQL DBMS backend and the weak credentials used by the web application to access it.

The following file extensions should never be returned by a web server, as they pertain to files that could contain sensitive information or files that have no valid reason to be served.

  • .asa
  • .inc
  • .config

The following file extensions are related to files which, when accessed, are either displayed or downloaded by the browser. Therefore, files with these extensions must be checked to verify that they are indeed supposed to be served (and are not leftovers), and that they do not contain sensitive information.

  • .zip, .tar, .gz, .tgz, .rar, etc.: (Compressed) archive files
  • .java: No reason to provide access to Java source files
  • .txt: Text files
  • .pdf: PDF documents
  • .docx, .rtf, .xlsx, .pptx, etc.: Office documents
  • .bak, .old and other extensions indicative of backup files (for example: ~ for Emacs backup files)

The list given above details only a few examples, since file extensions are too many to be comprehensively treated here. Refer to FILExt for a more thorough database of extensions.

To identify files with a given extension, a mix of techniques can be employed. These techniques can include using vulnerability scanners, spidering and mirroring tools, and querying search engines (see Testing: Spidering and googling). Manual inspection of the application can also be beneficial, as it overcomes limitations in automatic spidering. See also Testing for Old, Backup and Unreferenced Files which deals with the security issues related to “forgotten” files.

File Upload

Windows 8.3 legacy file handling can sometimes be used to defeat file upload filters.

Usage examples:

  1. file.phtml gets processed as PHP code.
  2. FILE~1.PHT is served, but not processed by the PHP ISAPI handler.
  3. shell.phPWND can be uploaded.
  4. SHELL~1.PHP will be expanded and returned by the OS shell, then processed by the PHP ISAPI handler.

Gray-Box Testing

White-box testing of file extension handling involves checking the server configurations in the web application architecture and verifying the rules for serving different file extensions.

If the web application relies on a load-balanced, heterogeneous infrastructure, determine whether this may introduce different behavior.


Vulnerability scanners, such as Nessus and Nikto, check for the existence of well-known web directories. They may allow the tester to download the site structure, which is helpful when trying to determine the configuration of web directories and how individual file extensions are served. Other tools that can be used for this purpose include:

  • wget
  • curl
  • Perform a Google search for “web mirroring tools”