CVE-2001-0669 in IDS
Summary
by MITRE
Various Intrusion Detection Systems (IDS) including (1) Cisco Secure Intrusion Detection System, (2) Cisco Catalyst 6000 Intrusion Detection System Module, (3) Dragon Sensor 4.x, (4) Snort before 1.8.1, (5) ISS RealSecure Network Sensor 5.x and 6.x before XPU 3.2, and (6) ISS RealSecure Server Sensor 5.5 and 6.0 for Windows, allow remote attackers to evade detection of HTTP attacks via non-standard "%u" Unicode encoding of ASCII characters in the requested URL.
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Analysis
by VulDB Data Team • 12/18/2024
The vulnerability described in CVE-2001-0669 represents a significant weakness in intrusion detection systems that affects multiple commercial and open-source security products from the early 2000s. This flaw specifically targets the ability of IDS solutions to properly identify and block malicious HTTP traffic by exploiting a particular encoding technique that bypasses signature-based detection mechanisms. The vulnerability impacts a diverse range of security appliances and software including Cisco's secure intrusion detection systems, Dragon Sensor 4.x, Snort versions prior to 1.8.1, and various ISS RealSecure products across multiple versions. The core issue stems from how these systems process and analyze HTTP request URLs that contain non-standard Unicode encoding using the "%u" format for ASCII characters, which allows attackers to obfuscate malicious payloads that would normally be detected by standard signatures.
The technical flaw manifests in the way these IDS systems parse and interpret URL-encoded data, particularly when encountering the "%u" Unicode escape sequence format that represents ASCII characters in a non-standard manner. This encoding method, while valid in web applications, creates a parsing inconsistency between the IDS detection engine and the actual HTTP request processing that occurs at the web server or application level. When attackers utilize this encoding technique, the IDS systems fail to recognize that the encoded payload corresponds to known malicious patterns, effectively allowing the attack traffic to slip through undetected. This represents a classic case of signature evasion where the attack payload is transformed in a way that maintains its functional maliciousness while circumventing the pattern matching algorithms that form the core of traditional signature-based intrusion detection.
The operational impact of this vulnerability extends beyond simple detection failure to represent a fundamental weakness in network security monitoring capabilities. Organizations relying on these IDS solutions for protection against web-based attacks would be left vulnerable to malicious traffic that appears benign to their security infrastructure. The attack vector specifically targets HTTP protocol interactions, making it particularly dangerous for web applications and services that are common targets for exploitation. Security administrators would experience false confidence in their protection mechanisms, potentially leading to delayed incident response or complete oversight of malicious activities. This vulnerability essentially undermines the trust placed in signature-based detection systems and demonstrates the critical importance of proper encoding handling in security appliances. The impact is particularly severe given that this vulnerability affects major commercial products from established vendors, suggesting that the flaw existed in widely deployed security infrastructure.
The mitigation strategies for this vulnerability involve multiple approaches that address both the immediate threat and the underlying architectural issues. Organizations should immediately upgrade their IDS systems to versions that properly handle Unicode encoding in HTTP requests, with particular attention to Snort versions 1.8.1 and later, and ISS RealSecure versions with XPU 3.2 or later. System administrators should implement additional monitoring and logging mechanisms to detect unusual URL encoding patterns that might indicate attempted exploitation. Network administrators should also consider implementing application-level protections and web application firewalls that can handle encoding variations more effectively than traditional signature-based systems. This vulnerability aligns with CWE-1037, which describes weaknesses in encoding handling within software systems, and represents a clear example of how improper input validation can lead to security bypasses. The issue also maps to ATT&CK technique T1071.004, which covers application layer protocol traffic, and highlights the need for comprehensive network security solutions that can handle various encoding methods used in modern web applications. Organizations should also consider implementing network segmentation and additional layers of security monitoring to reduce the impact of such signature evasion techniques.