CVE-2019-18792 in Suricata
Summary
by MITRE
An issue was discovered in Suricata 5.0.0. It is possible to bypass/evade any tcp based signature by overlapping a TCP segment with a fake FIN packet. The fake FIN packet is injected just before the PUSH ACK packet we want to bypass. The PUSH ACK packet (containing the data) will be ignored by Suricata because it overlaps the FIN packet (the sequence and ack number are identical in the two packets). The client will ignore the fake FIN packet because the ACK flag is not set. Both linux and windows clients are ignoring the injected packet.
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Analysis
by VulDB Data Team • 10/23/2024
This vulnerability in Suricata version 5.0.0 represents a sophisticated evasion technique targeting TCP-based network signature detection mechanisms. The flaw exploits the way Suricata processes overlapping TCP segments, specifically leveraging the interaction between FIN and PUSH ACK packets to create a bypass condition. The vulnerability allows attackers to inject a fake FIN packet immediately preceding a legitimate PUSH ACK packet that contains malicious payload data. This injection occurs at the TCP layer where sequence and acknowledgment numbers align between the fake FIN and the actual data packet, creating a condition where Suricata's packet processing engine fails to properly handle the overlapping segments.
The technical implementation of this attack relies on the fundamental TCP protocol behavior where overlapping segments can cause confusion in packet reassembly and signature matching processes. When Suricata encounters the overlapping TCP segments, it treats the PUSH ACK packet containing the actual malicious content as invalid or irrelevant due to its overlap with the fake FIN packet. The fake FIN packet is crafted to have identical sequence and acknowledgment numbers to the legitimate packet, causing the detection engine to disregard the payload data. This mechanism operates because TCP stack implementations on both linux and windows systems properly ignore the injected fake FIN packet since it lacks the ACK flag, while Suricata's signature matching engine fails to properly process the overlapping packet sequence.
The operational impact of this vulnerability extends beyond simple signature bypass to represent a significant threat to network security monitoring capabilities. Organizations relying on Suricata for intrusion detection and prevention may experience complete failure of TCP-based signature detection for the affected time period. This vulnerability specifically targets the core functionality of network traffic analysis engines that depend on proper TCP segment ordering and processing. The attack vector demonstrates how protocol-level implementation details can be exploited to create blind spots in security monitoring systems, potentially allowing malicious traffic to pass undetected while maintaining normal network behavior that would not raise immediate alarms.
From a cybersecurity perspective, this vulnerability aligns with attack patterns documented in the MITRE ATT&CK framework under the technique of "T1059 - Command and Scripting Interpreter" and "T1566 - Phishing" where evasion techniques are employed to bypass network detection. The vulnerability can be classified as a CWE-129 weakness related to improper validation of input data, specifically in the context of network packet processing. The flaw represents a failure in proper TCP segment validation and overlapping packet handling within the Suricata engine, creating a condition where the system's defensive mechanisms are subverted through legitimate protocol behavior rather than through direct exploitation of implementation flaws.
Organizations should implement immediate mitigations including updating to Suricata versions that address this vulnerability, implementing additional network monitoring layers, and configuring more robust TCP session tracking mechanisms. The recommended approach involves deploying network segmentation strategies, enabling more comprehensive protocol analysis, and implementing redundant detection systems that can identify anomalous packet behavior patterns. Security teams should also consider implementing manual verification processes for suspicious traffic patterns and establish baseline network behavior to detect when signature evasion techniques are being employed. Additionally, organizations should review their existing security policies to ensure proper handling of overlapping TCP segments and implement proper network traffic analysis that can identify and flag potentially malicious packet injection patterns.