The attack aims to explore flaws in the decoding mechanism implemented on applications when decoding Unicode data format. An attacker can use this technique to encode certain characters in the URL to bypass application filters, thus accessing restricted resources on the Web server or to force browsing to protected pages.
Consider a web application which has restricted directories or files (e.g. a file containing application usernames: appusers.txt). An attacker can encode the character sequence “../” (Path Traversal Attack) using Unicode format and attempt to access the protected resource, as follows:
Original Path Traversal attack URL (without Unicode Encoding):
Path Traversal attack URL with Unicode Encoding:
The Unicode encoding for the URL above will produce the same result as the first URL (Path Traversal Attack). However, if the application has an input security filter mechanism, it could refuse any request containing “../” sequence, thus blocking the attack. However, if this mechanism doesn’t consider character encoding, the attacker can bypass and access protected resource.
Other consequences of this type of attack are privilege escalation, arbitrary code execution, data modification, and denial of service.
Related Threat Agents
- http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2000-0884 - CVE-2000-0884
- http://capec.mitre.org/data/definitions/71.html - Using Unicode Encoding to Bypass Validation Logic
- http://www.microsoft.com/technet/security/bulletin/MS00-078.mspx - Patch Available for 'Web Server Folder Traversal' Vulnerability
- http://www.kb.cert.org/vuls/id/739224 - HTTP content scanning systems full-width/half-width Unicode encoding bypass
- http://www.cgisecurity.com/lib/URLEmbeddedAttacks.html - URL encoded attacks, by Gunter Ollmann