Forced browsing

Description

Forced browsing is an attack where the aim is to enumerate and access resources that are not referenced by the application, but are still accessible.

An attacker can use Brute Force techniques to search for unlinked contents in the domain directory, such as temporary directories and files, and old backup and configuration files. These resources may store sensitive information about web applications and operational systems, such as source code, credentials, internal network addressing, and so on, thus being considered a valuable resource for intruders.

This attack is performed manually when the application index directories and pages are based on number generation or predictable values, or using automated tools for common files and directory names.

This attack is also known as Predictable Resource Location, File Enumeration, Directory Enumeration, and Resource Enumeration.

Examples

Example 1

This example presents a technique of Predictable Resource Location attack, which is based on a manual and oriented identification of resources by modifying URL parameters. The user1 wants to check his on-line agenda through the following URL:

 www.site-example.com/users/calendar.php/user1/20070715 

In the URL, it is possible to identify the username (“user1”) and the date (mm/dd/yyyy). If the user attempts to make a forced browsing attack, he could guess another user’s agenda by predicting user identification and date, as follow:

 www.site-example.com/users/calendar.php/user6/20070716 

The attack can be considered successful upon accessing other user's agenda. A bad implementation of the authorization mechanism contributed to this attack's success.

Example 2

This example presents an attack of static directory and file enumeration using an automated tool.

A scanning tool, like Nikto, has the ability to search for existing files and directories based on a database of well-know resources, such as:

/system/
/password/
/logs/
/admin/
/test/

When the tool receives an “HTTP 200” message it means that such resource was found and should be manually inspected for valuable information.

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