CVE-2026-72436 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
netfilter: ipset: Don't use test_bit() in lockless RCU readers in hash types
Sashiko pointed out that there are a few lockless RCU readers using test_bit() which is a relaxed atomic operation and provides no memory barrier guarantees. Use test_bit_acquire() instead where the operation may run parallel with add/del/gc, i.e. is not one from the next cases
- protected by region lock - in a set destroy phase - in a new/temporary set creation phase
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability in question involves a critical flaw within the Linux kernel's netfilter subsystem, specifically affecting ipset functionality when processing hash types. This issue stems from improper memory synchronization mechanisms within lockless read operations that utilize test_bit() for bit testing. The problem manifests when these relaxed atomic operations execute concurrently with add, delete, or garbage collection operations in hash-based ipset implementations, creating potential race conditions and memory consistency violations.
The technical flaw occurs because test_bit() is classified as a relaxed atomic operation that does not provide memory barrier guarantees, making it unsuitable for use within lockless RCU (Read-Copy-Update) readers that may execute simultaneously with modifying operations. This particular vulnerability falls under CWE-1164 which specifically addresses improper handling of atomic operations in concurrent environments. When test_bit() is used inappropriately within these concurrent contexts, it can lead to stale data readings and inconsistent states where the reader might observe outdated information due to missing memory ordering constraints.
The operational impact of this vulnerability extends across multiple kernel subsystems that rely on ipset functionality for network filtering and traffic management. Attackers could potentially exploit this weakness to bypass security policies, manipulate network access controls, or cause system instability through race condition exploitation. The vulnerability particularly affects scenarios where hash-based ipsets are actively modified while concurrent readers attempt to query their state, creating opportunities for inconsistent data views that could be leveraged for privilege escalation or denial of service attacks.
The fix implemented addresses this issue by replacing test_bit() with test_bit_acquire() in lockless RCU readers where concurrent modification operations may occur. This change ensures proper memory ordering semantics and maintains the integrity of atomic operations during simultaneous read and write access patterns. The solution specifically excludes three well-defined cases where test_bit() usage is appropriate: when protected by region locks, during set destroy phases, or within new/temporary set creation phases. This approach aligns with ATT&CK technique T1068 which covers privilege escalation through race conditions, and provides a targeted mitigation that preserves performance while ensuring memory safety in concurrent kernel operations.
Security implications of this fix extend beyond immediate protection against the specific race condition, as it strengthens the overall reliability of netfilter subsystems that depend on consistent atomic behavior. The resolution demonstrates proper adherence to kernel concurrency best practices by ensuring that memory ordering requirements are met when multiple threads access shared data structures with varying levels of synchronization guarantees. This vulnerability highlights the importance of careful consideration of atomic operation semantics in kernel space programming where subtle correctness issues can lead to significant security implications and system instability across enterprise network infrastructure relying on Linux-based firewalls and traffic control mechanisms.