CVE-2026-74704 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: sch_cake: drop WARN_ON(1) for malformed packets in ACK filter
The sch_cake ACK filter parses packets to find the TCP header and filter duplicated ACKs if the flow is backlogged. The parsing code contains a WARN_ON(1) which can be triggered by a malformed IP header in certain cases. Depending on the system configuration, this leads either to either spamming dmesg with warnings, or a panic if panic_on_warn is set.
The code already correctly skips the offending packet in the branch that triggers the warning, so the WARN_ON itself doesn't really serve any purpose. So just drop it altogether to avoid the inconvenient side effects.
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel traffic control subsystem includes a sophisticated queueing discipline known as Cake, which implements Common Applications Kept Enhanced (CAKE) algorithms for bandwidth management and latency reduction. A critical component of this system is the ACK filter mechanism designed to identify and suppress duplicate acknowledgment packets when network flows are experiencing congestion or backlogs. This optimization helps maintain fair bandwidth distribution by preventing redundant control traffic from consuming valuable link capacity during periods of high load. The parsing logic within this module attempts to locate TCP headers within incoming IP packets to perform these checks, ensuring that only valid protocol structures trigger the filtering actions intended to optimize network performance under stress conditions.
A specific technical flaw existed in how malformed or unexpected packet structures were handled by the ACK filter parser. When processing a packet with an invalid or malformed IP header, the parsing routine encountered an edge case where it could not successfully extract the expected TCP information. Instead of silently discarding such packets as non-compliant with the filtering criteria, the code executed a WARN_ON(1) macro. This diagnostic function is typically reserved for detecting internal kernel logic errors rather than handling external input anomalies that are outside the control of the system administrator or user applications. Consequently, receiving malformed traffic from untrusted networks could trigger this warning condition repeatedly if such packets were sent in volume.
The operational impact of this vulnerability manifests primarily through log flooding and potential system instability depending on kernel configuration parameters. In standard configurations where warnings do not halt execution, a steady stream of malformed packets would cause the dmesg buffer to be rapidly filled with repetitive WARN_ON messages. This log spam can obscure other critical diagnostic information and consume unnecessary I/O resources for logging operations. More severely, if the system is configured with panic_on_warn enabled, which is common in production environments seeking strict error detection, a single malformed packet could trigger an immediate kernel panic. This results in a complete denial of service as the entire host crashes rather than gracefully dropping the invalid traffic, creating a significant availability risk for systems exposed to untrusted network segments or subjected to malicious fuzzing attacks targeting networking stacks.
From a vulnerability classification perspective, this issue aligns with CWE-754: Improper Check for Unusual or Exceptional Conditions and CWE-20: Improper Input Validation because the system failed to adequately handle unexpected input states without invoking severe diagnostic mechanisms intended for code defects rather than data anomalies. In terms of attack vectors within the MITRE ATT&CK framework, this relates to techniques involving resource exhaustion through log flooding or denial-of-service via kernel panic triggering, often categorized under impact categories that compromise availability by crashing system components. The root cause was not a failure in packet dropping logic but an inappropriate use of debugging macros for runtime error handling of external data streams.
The resolution involves removing the WARN_ON(1) statement from the ACK filter parsing code path since the surrounding conditional branches already correctly skip and discard malformed packets without further processing. By eliminating this unnecessary diagnostic trigger, the kernel maintains its robustness against malformed input while preserving the intended security posture that drops invalid traffic rather than allowing it to proceed through the queueing discipline. This change ensures that network interfaces remain stable even when subjected to high volumes of poorly formed or maliciously crafted IP packets from external sources.
To mitigate similar risks in related subsystems, developers should ensure that all input validation paths utilize appropriate error handling mechanisms such as returning error codes or silently dropping invalid data rather than invoking panic-inducing macros unless the condition represents a genuine internal logic failure unrelated to external inputs. System administrators can also review their kernel configuration settings regarding warning behaviors and consider disabling panic_on_warn in environments where network traffic from untrusted sources is anticipated, although relying on proper code handling remains the primary defense strategy for maintaining system availability during network anomalies or targeted attacks against networking components.