CVE-2018-6794 in Suricata
Summary
by MITRE
Suricata before 4.1 is prone to an HTTP detection bypass vulnerability in detect.c and stream-tcp.c. If a malicious server breaks a normal TCP flow and sends data before the 3-way handshake is complete, then the data sent by the malicious server will be accepted by web clients such as a web browser or Linux CLI utilities, but ignored by Suricata IDS signatures. This mostly affects IDS signatures for the HTTP protocol and TCP stream content; signatures for TCP packets will inspect such network traffic as usual.
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Analysis
by VulDB Data Team • 01/01/2025
The vulnerability identified as CVE-2018-6794 represents a critical flaw in Suricata intrusion detection systems version 4.0 and earlier, specifically affecting the HTTP protocol detection mechanisms. This issue stems from improper handling of TCP flow reconstruction during the three-way handshake process, creating a window where malicious actors can exploit the system's behavior to bypass security monitoring. The flaw occurs when a compromised server intentionally disrupts normal TCP communication patterns by transmitting data before the completion of the standard three-way handshake sequence. This behavior exploits a fundamental gap in how Suricata processes TCP stream reassembly and HTTP content inspection, allowing attackers to evade detection while maintaining legitimate network communication patterns. The vulnerability primarily impacts the HTTP protocol detection capabilities of Suricata, particularly affecting signatures that rely on HTTP content inspection and TCP stream analysis.
The technical implementation of this vulnerability resides in the detect.c and stream-tcp.c modules of Suricata's codebase, where the system fails to properly validate TCP stream continuity when data arrives outside the expected handshake sequence. When a malicious server breaks a normal TCP flow and sends data prior to handshake completion, the Suricata system accepts this data as legitimate for client applications such as web browsers or Linux command-line utilities, yet simultaneously ignores the same traffic when processing IDS signatures. This selective processing creates a false sense of security for network administrators, as the system continues to monitor and log the traffic while failing to apply critical HTTP protocol signatures. The root cause can be traced to CWE-129, which addresses improper validation of input boundaries, and CWE-20, which covers input validation issues. The vulnerability demonstrates a classic case of incomplete state tracking during TCP stream processing, where the system does not maintain proper session context validation for pre-handshake traffic.
The operational impact of CVE-2018-6794 extends beyond simple detection bypass, potentially allowing attackers to execute malicious activities while remaining invisible to network monitoring systems. This vulnerability enables attackers to deliver malicious HTTP content, including payloads that could exploit client-side vulnerabilities, inject malicious code into web sessions, or perform other attacks that rely on HTTP protocol manipulation. The bypass affects both HTTP protocol signatures and TCP stream content inspection, creating a significant gap in network security monitoring capabilities. Organizations relying on Suricata for intrusion detection may experience false negatives where malicious HTTP traffic is completely ignored by signature-based detection mechanisms, potentially allowing persistent threats to establish footholds within networks without detection. The vulnerability particularly impacts enterprise environments where HTTP traffic monitoring is critical for identifying malicious activities, and where the integrity of TCP flow reconstruction is assumed by security policies.
Mitigation strategies for CVE-2018-6794 require immediate action to upgrade Suricata to version 4.1 or later, where the vulnerability has been addressed through improved TCP flow handling and enhanced validation of pre-handshake traffic. Network administrators should also implement additional monitoring measures to detect anomalous TCP behavior patterns that might indicate exploitation attempts, including tracking of unusual TCP flags combinations and unexpected data transmission before handshake completion. The fix implemented in Suricata 4.1 addresses the core issue by improving stream reassembly logic and ensuring that all TCP traffic is properly validated regardless of its timing relative to the three-way handshake. Organizations should also consider implementing complementary security measures such as deep packet inspection with multiple tools, network segmentation to limit attack surface, and enhanced logging to detect potential exploitation attempts. This vulnerability highlights the importance of maintaining up-to-date security tooling and demonstrates how seemingly minor implementation flaws in protocol handling can create significant security gaps that align with ATT&CK technique T1071.004 for application layer protocol tunneling and T1059.007 for command and scripting interpreter.