What Is Directory Traversal?
Directory traversal is a type of HTTP exploit that is used by attackers to gain unauthorized access to restricted directories and files. Directory traversal, also known as path traversal, ranks #13 on the CWE/SANS Top 25 Most Dangerous Software Errors.1 Directory traversal attacks use web server software to exploit inadequate security mechanisms and access directories and files stored outside of the web root folder. An attacker that exploits a directory traversal vulnerability is capable of compromising the entire web server.
There are two security mechanisms that web servers use to restrict user access: root directory and Access Control Lists (ACLs). The root directory is the top-most directory on a server file system. User access is confined to the root directory, meaning users are unable to access directories or files outside of the root. Administrators use Access Control Lists to define user access rights and privileges for viewing, modifying and executing files.
Directory Traversal Vulnerability
A directory traversal vulnerability is the result of insufficient filtering/validation of browser input from users. Directory traversal vulnerabilities can be located in web server software/files or in application code that is executed on the server. Directory traversal vulnerabilities can exist in a variety of programming languages, including Python, PHP, Apache, ColdFusion, Perl and more. Enterprises commonly rely on vulnerability scanning and manual penetration testing techniques to detect directory traversal vulnerabilities.
Directory Traversal Examples
Directory traversal attacks can be viewed in two basic groups: attacks that target directory traversal vulnerabilities in the web server and attacks that target vulnerabilities in application code.
Attackers are able to exploit vulnerabilities in application code by sending URLs to the web server that instruct the server to return specific files to the application. For this method to work, the attacker must find a URL in which an application retrieves a file from the web server. Once attackers discover such a URL, they can simply modify the URL string with commands for the server and the name of the file they seek to access. The “../” directive is commonly used, as it instructs the web server to retrieve a file from one directory up. An attacker that is attempting to access a specific file will simply use trial-and-error to determine how many “…/” commands it takes to locate the correct directory and retrieve the file via the application.
Directory traversal vulnerabilities that exist on web servers are typically exploited to execute files. The method for this type of directory traversal attack involves sending URLs to the web server that contain the name of the targeted file and have been modified with commands and web server escape codes. Escape codes are used as workarounds when certain commands are being filtered for; for example, an attacker might use the "%2e%2e/" escape code if the “../” command is blocked. Again, this directory traversal example requires trial-and-error from the attacker, but it is still fairly easy to access and execute files when adequate preventative procedures are not in place.
Directory Traversal Prevention
There are several measures that enterprises can take to prevent directory traversal attacks and vulnerabilities. For starters, programmers should be trained to validate user input from browsers. Input validation ensures that attackers cannot use commands that leave the root directory or violate other access privileges. Beyond this, filters can be used to block certain user input. Enterprises typically employ filters to block URLs containing commands and escape codes that are commonly used by attackers. Additionally, web server software (and any software that is used) should be kept up-to-date with current patches. Regularly patching software is a critical practice for reducing security risk, as software patches typically contain security fixes.
1. "2011 CWE/SANS Top 25 Most Dangerous Software Errors." Common Weakness Enumeration. The MITRE Corporation, 13 Sept. 2011. Web. 18 Oct. 2012. http://cwe.mitre.org/top25/.