What is a Directory Traversal attack?
Properly controlling access to web content is crucial for running a secure web server. Directory traversal is an HTTP exploit which allows attackers to access restricted directories and execute commands outside of the web server’s root directory.
Web servers provide two main levels of security mechanisms
- Access Control Lists (ACLs)
- Root directory
An Access Control List is used in the authorization process. It is a list which the web server’s administrator uses to indicate which users or groups are able to access, modify or execute particular files on the server, as well as other access rights.
The root directory is a specific directory on the server file system in which the users are confined. Users are not able to access anything above this root.
For example: the default root directory of IIS on Windows is C:\Inetpub\wwwroot and with this setup, a user does not have access to C:\Windows but has access to C:\Inetpub\wwwroot\news and any other directories and files under the root directory (provided that the user is authenticated via the ACLs).
The root directory prevents users from accessing sensitive files on the server such as cmd.exe on Windows platforms and the passwd file on Linux/UNIX platforms.
This vulnerability can exist either in the web server software itself or in the web application code.
In order to perform a directory traversal attack, all an attacker needs is a web browser and some knowledge on where to blindly find any default files and directories on the system.
What an attacker can do if your website is vulnerable
With a system vulnerable to directory traversal, an attacker can make use of this vulnerability to step out of the root directory and access other parts of the file system. This might give the attacker the ability to view restricted files, or even more dangerous, allowing the attacker to execute powerful commands on the web server which can lead to a full compromise of the system.
Depending on how the website access is set up, the attacker will execute commands by impersonating himself as the user which is associated with “the website”. Therefore it all depends on what the website user has been given access to in the system.
Example of a Directory Traversal attack via web application code
In web applications with dynamic pages, input is usually received from browsers through GET or POST request methods. Here is an example of an HTTP GET request URL
GET http://test.webarticles.com/show.asp?view=oldarchive.html HTTP/1.1 Host: test.webarticles.com
With this URL, the browser requests the dynamic page show.asp from the server and with it also sends the parameter view with the value of oldarchive.html. When this request is executed on the web server, show.asp retrieves the file oldarchive.htm from the server’s file system, renders it and then sends it back to the browser which displays it to the user. The attacker would assume that show.asp can retrieve files from the file system and sends the following custom URL.
GET http://test.webarticles.com/show.asp?view=../../../../../Windows/system.ini HTTP/1.1 Host: test.webarticles.com
This will cause the dynamic page to retrieve the file system.ini from the file system and display it to the user. The expression ../ instructs the system to go one directory up which is commonly used as an operating system directive. The attacker has to guess how many directories he has to go up to find the Windows folder on the system, but this is easily done by trial and error.
Example of a Directory Traversal attack via web server
Apart from vulnerabilities in the code, even the web server itself can be open to directory traversal attacks. The problem can either be incorporated into the web server software or inside some sample script files left available on the server.
The vulnerability has been fixed in the latest versions of web server software, but there are web servers online which are still using older versions of IIS and Apache which might be open to directory traversal attacks. Even tough you might be using a web server software version that has fixed this vulnerability, you might still have some sensitive default script directories exposed which are well known to hackers.
For example, a URL request which makes use of the scripts directory of IIS to traverse directories and execute a command can be
GET http://server.com/scripts/..%5c../Windows/System32/cmd.exe?/c+dir+c:\ HTTP/1.1 Host: server.com
The request would return to the user a list of all files in the C:\ directory by executing the cmd.exe command shell file and run the command dir c:\ in the shell. The %5c expression that is in the URL request is a web server escape code which is used to represent normal characters. In this case %5c represents the character \.
Newer versions of modern web server software check for these escape codes and do not let them through. Some older versions however, do not filter out these codes in the root directory enforcer and will let the attackers execute such commands.
How to check for Directory Traversal vulnerabilities
The best way to check whether your web site and web applications are vulnerable to directory traversal attacks is by using a Web Vulnerability Scanner. A Web Vulnerability Scanner crawls your entire website and automatically checks for directory traversal vulnerabilities. It will report the vulnerability and how to easily fix it. Besides directory traversal vulnerabilities a web application scanner will also check for SQL injection, Cross-site Scripting and other web vulnerabilities.
Preventing Directory Traversal attacks
First of all, ensure you have installed the latest version of your web server software, and sure that all patches have been applied.
Secondly, effectively filter any user input. Ideally remove everything but the known good data and filter meta characters from the user input. This will ensure that only what should be entered in the field will be submitted to the server.
Check if your website is vulnerable to attack with Acunetix
Acunetix ensures website security by automatically checking for SQL Injection, Cross-site Scripting, Directory Traversal and other vulnerabilities. It checks password strength on authentication pages and automatically audits shopping carts, forms, dynamic content and other web applications. As the scan is being completed, the software produces detailed reports that pinpoint where vulnerabilities exist. Download the trial version of Acunetix