CVE-2015-8954 in Suricata
Summary
by MITRE
The MemcmpLowercase function in Suricata before 2.0.6 improperly excludes the first byte from comparisons, which might allow remote attackers to bypass intrusion-prevention functionality via a crafted HTTP request.
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Analysis
by VulDB Data Team • 09/10/2020
The vulnerability identified as CVE-2015-8954 affects the Suricata network intrusion detection system version 2.0.5 and earlier, representing a critical flaw in the memcmp_lowercase function implementation. This function is designed to perform case-insensitive string comparisons within HTTP request processing, which is essential for proper rule matching and threat detection. The flaw stems from an implementation error where the first byte of the comparison data is systematically excluded from the actual comparison operation, creating a fundamental bypass mechanism that undermines the security controls.
The technical nature of this vulnerability resides in the improper handling of string comparison logic within the Suricata engine's HTTP protocol parsing module. When processing HTTP requests, the memcmp_lowercase function is responsible for comparing user-supplied data against known threat signatures or rule patterns without regard to case sensitivity. However, due to the flawed implementation, the first byte of the compared data is omitted from the actual comparison, allowing attackers to manipulate HTTP request content in such a way that malicious payloads can evade detection. This creates a condition where crafted HTTP requests can be constructed to match rule patterns in unexpected ways, effectively bypassing the intrusion prevention system's intended security controls.
The operational impact of this vulnerability is severe and directly affects the core functionality of Suricata as an intrusion prevention system. Remote attackers can exploit this flaw to craft HTTP requests that appear to match legitimate traffic patterns or bypass security rules designed to detect malicious content. The bypass mechanism specifically targets the rule matching process, potentially allowing attackers to execute payloads that would normally be blocked by signature-based detection rules. This vulnerability undermines the fundamental trust in the system's ability to detect and prevent network-based attacks, making it particularly dangerous in environments where Suricata serves as a primary security control for network monitoring and intrusion prevention.
The vulnerability aligns with CWE-120, which describes buffer overflow conditions that can occur when the first byte is improperly handled in comparison operations, and relates to ATT&CK technique T1071.004 for application layer protocol manipulation. Organizations using affected versions of Suricata face significant risk of undetected malicious activity, as the bypass allows attackers to craft requests that evade signature-based detection mechanisms. The impact extends beyond simple evasion to potentially enable more sophisticated attack vectors, as the bypass can be combined with other techniques to create stealthy attack patterns that are difficult to trace. This vulnerability demonstrates the critical importance of proper string handling and comparison logic in security systems, where even seemingly minor implementation flaws can create substantial security gaps that adversaries can exploit.
The recommended mitigation strategy involves immediate upgrade to Suricata version 2.0.6 or later, which contains the fixed memcmp_lowercase function implementation. Organizations should also implement additional monitoring and anomaly detection measures to identify potential exploitation attempts, while reviewing existing rules to ensure they are not inadvertently vulnerable to similar bypass techniques. Network administrators should consider implementing complementary security controls and maintaining strict version control to prevent deployment of vulnerable software versions in production environments.