CVE-2024-55629: Oisf Suricata TCP Urgent Data DoS Flaw

CVE-2024-55629 Overview

CVE-2024-55629 is a network security monitoring evasion vulnerability affecting Suricata, the open-source Intrusion Detection System (IDS), Intrusion Prevention System (IPS), and Network Security Monitoring (NSM) engine maintained by the Open Information Security Foundation (OISF). Versions prior to 7.0.8 mishandle Transmission Control Protocol (TCP) streams containing urgent data, also known as out-of-band (OOB) data. Suricata interprets these packets differently than the applications at the TCP endpoints. Attackers can exploit this parsing divergence to evade identification and slip malicious payloads past the sensor. The issue is classified under [CWE-436] Interpretation Conflict and [CWE-437] Incomplete Model of Endpoint Features.

Critical Impact
Remote attackers can bypass Suricata IDS/IPS identification by crafting TCP streams with urgent flags, allowing malicious traffic to reach protected endpoints without generating alerts.

Affected Products

OISF Suricata versions prior to 7.0.8
Deployments running Suricata as an Intrusion Detection System (IDS)
Deployments running Suricata as an Intrusion Prevention System (IPS)

Discovery Timeline

2025-01-06 - CVE-2024-55629 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-55629

Vulnerability Analysis

The vulnerability stems from inconsistent handling of TCP urgent data between Suricata and the actual TCP endpoints receiving the traffic. TCP urgent data is signaled by the URG flag combined with an urgent pointer indicating the byte offset of the out-of-band data within the segment. Different operating systems and applications interpret this urgent data in incompatible ways. Some strip the urgent byte from the stream, some deliver it inline, and others deliver it through a separate channel.

Suricata prior to version 7.0.8 applied a single interpretation to reconstructed TCP streams before matching rules against the payload. When the endpoint applied a different interpretation, the reconstructed byte sequence analyzed by Suricata diverged from the byte sequence the application actually processed. An attacker who controls this divergence can position malicious content so that Suricata sees benign data while the target application sees the attack payload.

Root Cause

The root cause is an interpretation conflict [CWE-436] combined with an incomplete model of endpoint TCP behavior [CWE-437]. Suricata's stream reassembly did not offer configurable policies to align its urgent data handling with the specific TCP stack of the protected host. This mirrors the classic IDS insertion and evasion class of attacks first documented by Ptacek and Newsham, applied here to the URG mechanism.

Attack Vector

Exploitation requires network access to send TCP traffic through a Suricata sensor to a monitored endpoint. No authentication or user interaction is needed. The attacker crafts TCP segments with the URG flag set and manipulates the urgent pointer so that portions of the payload are treated as urgent data. When Suricata reassembles the stream, the resulting byte sequence differs from what the endpoint application consumes. Malicious signatures such as exploit strings, malware downloads, or command-and-control traffic can be split across urgent boundaries to defeat pattern matching.

No public proof-of-concept exploit code has been released. The vulnerability mechanism is described in the GitHub Security Advisory GHSA-69wr-vhwg-84h2 and the Open Information Security Foundation Issue #7411.

Detection Methods for CVE-2024-55629

Indicators of Compromise

TCP segments with the URG flag set traversing sensor boundaries, particularly toward services that do not use urgent data in their protocol
Unusual density of TCP packets carrying non-zero urgent pointers from a single source
Discrepancies between Suricata flow logs and application-layer logs on protected hosts
Alerts on the protected host or downstream tooling for activity that Suricata did not flag

Detection Strategies

Deploy the Suricata rule drop tcp any any -> any any (sid:1; tcp.flags:U*;) in IPS mode to block traffic with the URG flag set, as recommended in the vendor advisory
Correlate Suricata eve.json events with endpoint telemetry to surface traffic reaching applications without a corresponding sensor identification
Baseline normal URG flag usage across monitored segments and alert on deviations

Monitoring Recommendations

Enable stream and packet logging on Suricata sensors to retain evidence of URG-flagged flows for retrospective analysis
Forward Suricata identifications and endpoint process telemetry into a centralized data lake for cross-source correlation
Track the ratio of URG-flagged sessions per destination service and investigate outliers

How to Mitigate CVE-2024-55629

Immediate Actions Required

Upgrade all Suricata deployments to version 7.0.8 or later
In IPS mode, deploy a rule to drop traffic with the URG flag set for protocols that do not require urgent data
Review Suricata configuration files to enable the new TCP urgent data handling options introduced in 7.0.8
Audit historical Suricata logs for prior URG-flagged traffic that may indicate past evasion attempts

Patch Information

OISF fixed the vulnerability in Suricata 7.0.8. The patch introduces configuration options that let operators define how urgent data is handled during stream reassembly, aligning sensor behavior with the endpoint TCP stack. The upstream fixes are recorded in commits 6882bcb3 and 779f9d8b.

Workarounds

In IPS mode, apply the rule drop tcp any any -> any any (sid:1; tcp.flags:U*;) to block all TCP packets carrying the URG flag
Restrict the rule scope to specific destination ports where urgent data is never expected to avoid impacting legitimate protocols
Where upgrade is not immediately possible, supplement Suricata with host-based identification on protected endpoints to reduce reliance on network-layer visibility

bash

# Suricata IPS rule to drop TCP packets with the URG flag set
drop tcp any any -> any any (msg:"Block TCP URG flag - CVE-2024-55629 mitigation"; sid:1; tcp.flags:U*;)