CVE-2006-6931 in Snort
Summary
by MITRE
Algorithmic complexity vulnerability in Snort before 2.6.1, during predicate evaluation in rule matching for certain rules, allows remote attackers to cause a denial of service (CPU consumption and detection outage) via crafted network traffic, aka a "backtracking attack."
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Analysis
by VulDB Data Team • 06/14/2025
The vulnerability identified as CVE-2006-6931 represents a critical algorithmic complexity flaw in the Snort intrusion detection system version 2.6.0 and earlier. This weakness specifically manifests during the predicate evaluation phase of rule matching, where the system's processing behavior becomes exponentially complex under certain conditions. The flaw enables remote attackers to exploit the system's pattern matching algorithms through carefully crafted network packets that trigger excessive CPU consumption and ultimately lead to complete service disruption. This vulnerability falls under the category of computational complexity attacks that target the fundamental processing mechanisms of security tools, making it particularly dangerous as it can effectively disable network monitoring capabilities without requiring privileged access or complex exploitation techniques.
The technical implementation of this vulnerability stems from the way Snort processes regular expression patterns and complex rule conditions during packet inspection. When certain rule patterns are evaluated against maliciously constructed network traffic, the system's backtracking algorithms can enter into exponential time complexity states where the processing time grows dramatically with input size. This behavior is particularly pronounced when rules contain nested quantifiers or complex alternation patterns that cause the engine to explore multiple execution paths simultaneously. The vulnerability specifically affects the rule evaluation engine's handling of regular expressions and pattern matching operations, creating a scenario where a single malicious packet can cause the system to consume excessive computational resources. This issue aligns with CWE-1333 which describes weaknesses related to inefficient algorithms and improper resource consumption in security applications.
The operational impact of CVE-2006-6931 is severe and multifaceted, as it can completely disable network monitoring capabilities through a straightforward denial of service attack. Attackers can cause sustained CPU exhaustion that renders the Snort system unable to process legitimate network traffic, effectively creating a blind spot in network security monitoring. The system may become unresponsive to new connections, fail to log events properly, or experience complete detection outages that can last for extended periods until manual intervention occurs. This vulnerability particularly affects network security infrastructures that rely heavily on Snort for real-time monitoring, as the attack can be executed remotely without requiring authentication or specialized privileges. The disruption can cascade through dependent systems that rely on Snort's alerting and logging capabilities, potentially compromising overall network security posture and creating windows of opportunity for other attacks to go undetected.
Mitigation strategies for this vulnerability require immediate patching of Snort installations to version 2.6.1 or later, which contains the necessary algorithmic complexity fixes and improved rule evaluation mechanisms. Organizations should implement network segmentation and monitoring to detect unusual CPU usage patterns that may indicate exploitation attempts, while also configuring rate limiting and connection tracking to reduce the impact of potential attacks. The fix addresses the underlying backtracking algorithm behavior by implementing more efficient pattern matching techniques and adding safeguards against exponential complexity scenarios. Security teams should also consider deploying multiple intrusion detection systems with different rule engines to provide redundancy and reduce the impact of similar vulnerabilities in any single system. This vulnerability demonstrates the importance of algorithmic complexity testing in security software development and aligns with ATT&CK technique T1499 which covers network denial of service attacks targeting security controls and monitoring systems.