CVE-2026-80733 in Linux
Summary
by MITRE • 09/03/2026
In the Linux kernel, the following vulnerability has been resolved:
net: remove WARN_ON_ONCE() from sk_mc_loop()
sk_mc_loop() can be called for sockets that are neither AF_INET nor AF_INET6 (e.g. AF_PACKET sockets when sending packets via raw/packet socket over virtual devices such as VRF or ipvlan).
In such cases, sk_family is not AF_INET/AF_INET6 and sk_mc_loop() falls through the switch statement and triggers WARN_ON_ONCE(1).
Non-INET sockets do not support IP_MULTICAST_LOOP or IPV6_MULTICAST_LOOP options, so loopback should default to true without generating a warning.
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Analysis
by VulDB Data Team • 09/03/2026
The Linux kernel networking subsystem contains a logic flaw within the sk_mc_loop function that results in an unnecessary and disruptive kernel warning when processing non-Internet Protocol sockets. This vulnerability specifically affects scenarios where multicast loopback settings are queried or modified for socket types that do not support IP-based multicast options, such as AF_PACKET sockets used with raw packet interfaces over virtual network devices like VRF or ipvlan. The root cause lies in the function's handling of socket address families within a switch statement designed primarily for IPv4 and IPv6 contexts. When the socket family is neither AF_INET nor AF_INET6, the control flow falls through to a default case that triggers a WARN_ON_ONCE macro with an argument indicating an unexpected condition. This behavior misclassifies normal operational states as kernel warnings, leading to log noise and potential confusion during troubleshooting efforts without addressing any actual security or stability defect in the underlying network stack logic.
From a technical perspective, this issue represents a failure in input validation and state handling for non-standard socket types within the networking layer. The sk_mc_loop function is intended to determine whether multicast packets should be looped back to the sending socket based on specific IP-level options like IP_MULTICAST_LOOP or IPV6_MULTICAST_LOOP. However, these options are exclusively relevant to Internet Protocol sockets. For other socket families, particularly AF_PACKET which operates at a lower layer and handles raw frame data, there is no concept of IP multicast loopback control. Consequently, the function should default to returning true for loopback behavior rather than treating the absence of an explicit configuration as an error condition requiring developer attention via kernel warnings. The presence of WARN_ON_ONCE in this path indicates that developers anticipated only INET-family sockets would reach this code but failed to account for legitimate use cases involving virtual devices and raw packet interfaces where such calls are valid and expected.
The operational impact of this vulnerability is primarily related to system observability and stability rather than direct exploitation or data compromise. The generation of kernel warnings can clutter dmesg logs, making it difficult for administrators and automated monitoring tools to distinguish between genuine critical errors and benign informational messages. In high-throughput environments or systems heavily utilizing virtual networking features like VRF, these spurious warnings may be generated frequently, potentially impacting performance due to the overhead associated with warning generation and log writing. Furthermore, excessive kernel warnings can trigger alerting mechanisms that are configured to respond to actual security incidents, leading to false positives and unnecessary incident response activities. While this does not constitute a privilege escalation or remote code execution vector, it degrades the reliability of system diagnostics and violates best practices for clean error handling in production kernels.
This flaw aligns with CWE-754: Improper Check for Unusual or Exceptional Conditions, as the software fails to properly handle an expected but unanticipated input state within its control flow logic. It also relates to CWE-20: Improper Input Validation, specifically regarding the assumption that only certain socket families would invoke this function without verifying the context appropriately before applying INET-specific defaults. In terms of MITRE ATT&CK mapping, while not a direct attack vector, such kernel warnings can be leveraged in reconnaissance phases by an attacker to fingerprint kernel versions or identify specific configuration states through log analysis if they have access to system logs. More broadly, it reflects weaknesses often categorized under T1078: Valid Accounts or general operational security failures where noise obscures genuine threats.
To mitigate this issue and restore proper logging hygiene, the recommended action is to modify the sk_mc_loop function in the Linux kernel source code to handle non-INET socket families explicitly by defaulting the loopback flag to true without invoking warning macros. This change ensures that valid operations on AF_PACKET sockets or other non-IP protocols proceed silently as intended. System administrators should apply available kernel updates provided by their distribution vendors which incorporate this fix. Until patches are applied, monitoring tools can be configured to filter out WARN_ON_ONCE messages originating from the sk_mc_loop function path to reduce log noise and prevent false alerts. Long-term mitigation involves ensuring that all networking subsystem functions include comprehensive checks for socket family types before applying protocol-specific logic, thereby preventing similar issues in other parts of the network stack where assumptions about socket capabilities may be overly narrow.