CVE-2019-18625 in Suricata
Summary
by MITRE
An issue was discovered in Suricata 5.0.0. It was possible to bypass/evade any tcp based signature by faking a closed TCP session using an evil server. After the TCP SYN packet, it is possible to inject a RST ACK and a FIN ACK packet with a bad TCP Timestamp option. The client will ignore the RST ACK and the FIN ACK packets because of the bad TCP Timestamp option. Both linux and windows client are ignoring the injected packets.
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Analysis
by VulDB Data Team • 10/23/2024
The vulnerability identified as CVE-2019-18625 represents a significant bypass mechanism within the Suricata network intrusion detection system version 5.0.0 that specifically targets TCP-based signatures. This flaw enables malicious actors to evade detection by exploiting weaknesses in how the system processes TCP session states and timestamp validation. The vulnerability stems from the improper handling of TCP packets with malformed timestamp options, creating a scenario where legitimate network traffic can be manipulated to appear as if it originates from a closed session, thereby allowing malicious payloads to bypass security monitoring.
The technical implementation of this vulnerability involves the strategic injection of TCP packets that exploit the TCP timestamp validation mechanism. An attacker can initiate a TCP connection by sending a SYN packet to establish the session, then immediately inject a RST ACK packet followed by a FIN ACK packet containing malformed TCP timestamp options. The specific flaw lies in the fact that both Linux and Windows operating systems, which are commonly targeted by network security systems, will ignore these injected packets due to the invalid timestamp values. This behavior occurs because the TCP stack implementations perform strict validation of timestamp options, and when these options are malformed, the packets are silently discarded rather than processed according to standard TCP state machine rules.
The operational impact of this vulnerability extends beyond simple signature bypass, as it fundamentally undermines the reliability of TCP-based network monitoring within Suricata. Network administrators who rely on TCP signature detection for identifying malicious traffic patterns, connection hijacking attempts, or protocol violations may find their security posture severely weakened. This vulnerability affects the core functionality of the intrusion detection system by allowing attackers to craft packets that appear to originate from closed TCP sessions, effectively creating a tunnel through which malicious traffic can flow undetected. The implications are particularly severe for organizations that depend on Suricata for protecting critical network infrastructure, as the bypass mechanism works against fundamental TCP protocol behaviors that are expected to be properly handled by network stacks.
The vulnerability aligns with CWE-119, which addresses improper restriction of operations within a memory buffer, and demonstrates how protocol-level flaws can be exploited to manipulate system behavior in unexpected ways. From an ATT&CK perspective, this vulnerability maps to T1071.004, which covers application layer protocol traffic, and T1566, which involves phishing techniques that can be enhanced by bypassing network monitoring systems. Organizations should implement immediate mitigations including updating to Suricata versions that address this specific vulnerability, implementing additional monitoring for anomalous TCP packet sequences, and considering network-level controls that can detect and block malformed TCP timestamp options. The recommended approach involves both patching the software and implementing complementary security measures that can detect the specific packet injection patterns associated with this bypass technique, ensuring that network monitoring systems maintain their integrity against sophisticated evasion attempts.