CAPEC 79

Summary

Attack Execution Flow

1. The attacker has access to a resource path and required to use slashes as resource delimiter.

2. The attacker tries variation and combination of the slashes characters in different encoding format.

3. The attacker found an unfiltered combination which maps to a valid path and accesses unauthorized resources (directories, files, etc.)

Likelihood: High

Description

Skill or Knowledge Level: Low

An attacker can try variation of the slashes characters.

Skill or Knowledge Level: Medium

An attacker can use more sophisticated tool or script to scan a website and find a path filtering problem.

An attacker can try different encoding formats for the slashes characters and see if they produce the same filtering results.

Automated tools such as fuzzer can be used to test the URL decoding and filtering. Custom scripts can also be used. For example, a good script for verifying the correct interpretation of UTF-8 encoded characters can be found at http://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt

If the first path decoding process has left some invalid or blacklisted characters, that may be a sign that the request is malicious.

Traffic filtering with IDS (or proxy) can detect request with suspicious URLs. IDS may use signature based identification to reveal such URL based attacks.

A attacker can use a fuzzer in order to probe for a UTF-8 encoding vulnerability. The fuzzer should generate suspiscious network activity.

Typically the obfuscation here is the use of different alternate encoding format (UTF-8, Unicode, etc,)

Any security checks should occur after the data has been decoded and validated as correct data format. Do not repeat decoding process, if bad character are left after decoding process, treat the data as suspicious, and fail the validation process. Refer to the RFCs to safelly decode URL.

When client input is required from web-based forms, avoid using the "GET" method to submit data, as the method causes the form data to be appended to the URL and is easily manipulated. Instead, use the "POST method whenever possible.

There are tools to scan HTTP requests to the server for valid URL such as URLScan from Microsoft (http://www.microsoft.com/technet/security/tools/urlscan.mspx)

Be aware of the threat of alternative method of data encoding and obfuscation technique such as IP address endoding. (See related guideline section)

Test your path decoding process against malicious input.

In the case of path traversals, use the principle of least privilege when determining access rights to file systems. Do not allow users to access directories/files that they should not access.

Assume all input is malicious. Create a white list that defines all valid input to the application based on the requirements specifications. Input that does not match against the white list should not be permitted to enter into the system.

UTF-8 and Unicode FAQ for Unix/Linux, by Markus Kuhn - http://www.cl.cam.ac.uk/~mgk25/unicode.html

URL encoded attacks, by Gunter Ollmann - http://www.cgisecurity.com/lib/URLEmbeddedAttacks.html