CVE-2026-74347 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
netfilter: cttimeout: detach dataplane timeout policy and repurpose refcount
Add a refcount for struct nf_ct_timeout which is used by ct extension to set the custom ct timeout policy, this tells us that the ct timeout is being used by a conntrack entry. When the last conntrack entry drops the refcount on the ct timeout, the ct timeout is released.
Remove the refcount for control plane which controls if the ruleset refers to the timeout policy. After this update, it is possible to remove the ct timeout policy from nfnetlink_cttimeout immediately. This is for simplicity not to handle two refcounts on a single object.
Remove nf_queue_nf_hook_drop(): a packet sitting in nfqueue will just hold a reference to the nf_ct_timeout object until packet is reinjected, since this is part of the ct extension, this will be released by the time the conntrack is freed.
nf_ct_untimeout() is still called to clean up in a best effort basis: the ct timeout on existing entries gets removed when the ct timeout goes away, but as long as the iptables ruleset still refers to the ct timeout through a template, new conntracks may keep attaching it and extend its lifetime until the rule is removed.
nf_ct_untimeout() is not called anymore from module removal path, this is unlikely to find timeouts give module refcount is bumped, and the new refcount already tracks the ct timeout policy use so it is released when unused.
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Analysis
by VulDB Data Team • 08/15/2026
This vulnerability addresses a critical issue in the Linux kernel's netfilter subsystem related to connection tracking timeout management. The problem stems from improper reference counting mechanisms within the nf_ct_timeout structure, which governs custom connection tracking timeout policies. When multiple conntrack entries reference the same timeout policy, the existing implementation failed to properly track these dependencies, leading to potential memory leaks and premature deallocation of active timeout policies.
The technical flaw manifests in the dual reference counting system that previously managed both dataplane usage and control plane rule references separately. This complexity created scenarios where timeout policies could be freed while still being actively referenced by conntrack entries, or conversely, remain allocated indefinitely due to stale control plane references. The vulnerability specifically affects how the kernel manages the lifecycle of connection tracking timeout objects through its interaction with netfilter subsystem components.
The operational impact of this flaw extends beyond simple memory management issues to potentially compromise network connectivity and system stability. When timeout policies are improperly released, active connections may experience unexpected timeouts or fail to maintain proper state tracking, leading to service disruption. The vulnerability also affects the reliability of connection tracking decisions since timeout policies might be applied inconsistently or not at all when policies are prematurely deallocated.
The solution implemented involves restructuring the reference counting mechanism by introducing a single refcount for the struct nf_ct_timeout structure that tracks only dataplane usage by conntrack entries. This simplification eliminates the complexity of managing two separate reference counts on the same object, reducing the potential for race conditions and memory management errors. The removal of the nf_queue_nf_hook_drop() function further streamlines the cleanup process, as packet reinjection mechanisms now properly maintain references until conntrack entries are freed.
Security implications of this vulnerability align with CWE-415 and CWE-416 categories related to double free and use after free conditions in memory management. The fix reduces attack surface by ensuring proper lifecycle management of timeout policies and prevents potential exploitation through memory corruption attacks. The implementation follows ATT&CK framework concept T1070.004 (Path Interception) by maintaining consistent connection tracking behavior and preventing unexpected timeouts that could disrupt network services.
The mitigation strategy focuses on eliminating the dual reference counting approach while preserving essential functionality for timeout policy management. The removal of nf_ct_untimeout() from the module removal path reflects the improved tracking mechanism, as the new refcount system ensures proper cleanup when timeout policies are no longer referenced by any active conntrack entries. This change also addresses potential race conditions during system shutdown or module unloading operations.
The enhanced implementation provides better resource management and reduces complexity in the connection tracking subsystem while maintaining backward compatibility with existing iptables rulesets. The solution ensures that timeout policies remain available for active connections until they are explicitly removed from firewall rules, preventing both premature deallocation and unnecessary memory retention. This improvement directly supports network security operations by ensuring predictable connection tracking behavior and reliable timeout enforcement across all network traffic flows.
The updated approach also benefits system administrators by providing more predictable resource usage patterns in environments with dynamic firewall rule changes. Connection tracking timeout policies now behave consistently regardless of whether they are referenced through active connections or firewall rules, eliminating scenarios where policy cleanup occurs at unexpected times. This stability improvement is particularly important for high-availability network services that depend on consistent connection tracking behavior.
The fix demonstrates proper kernel memory management practices by implementing a single, clear reference counting mechanism that accurately reflects the actual usage of timeout policies. This approach reduces the potential for future vulnerabilities in similar subsystems and provides a cleaner foundation for additional features or enhancements to the connection tracking functionality. The solution also improves debugging capabilities since the simplified reference counting model makes it easier to trace timeout policy usage patterns and identify potential issues during system operation.