CVE-2026-46328: Linux AppArmor POSIX CPU Timer Rlimit Enforcement Flaw

Technical summary

CVE-2026-46328 is a resource limit enforcement issue in the Linux kernel's AppArmor module. The vulnerability stems from incomplete handling of POSIX CPU timer limits. While AppArmor's rlimit mechanism sets the resource ceiling, POSIX CPU timers require an additional enforcement step that was not being executed conditionally. The fix refactors the code to clarify the separation of concerns: when rlimits are set versus when POSIX CPU timer structures must be updated to reflect those limits. Without this conditional update, the kernel fails to properly restrict a confined process's CPU timer behavior, allowing it to exceed the administrator-defined limits.

Business impact

For organizations relying on AppArmor to enforce strict resource boundaries—common in multi-tenant hosting, container platforms, and high-security deployments—this vulnerability undermines access control guarantees. A local attacker or malicious containerized workload could consume excessive CPU timer resources, degrading performance for other processes or causing the system to become unresponsive. In regulated environments where AppArmor policies enforce compliance with resource quotas, this gap represents a control failure.

Affected systems

The vulnerability affects the Linux kernel. Systems running kernels with this AppArmor flaw are at risk if they use AppArmor policies to confine processes and rely on rlimit enforcement. This includes traditional Linux servers using AppArmor, containerized environments (Docker, Kubernetes) that delegate confinement to the host kernel's AppArmor, and embedded systems enforcing sandboxing via AppArmor policies.

Exploitability

Exploitation requires local access (AV:L) and no special privileges beyond a standard user account (PR:L). There is no user interaction needed (UI:N). The attack is straightforward: a confined process simply requests or repeatedly uses CPU timer operations that should be blocked by the rlimit. The low attack complexity (AC:L) means no special conditions or race conditions are required. An attacker within a container or with a local user account can trigger this directly.

Remediation

Apply the kernel patch that refactors AppArmor's rlimit handling for POSIX CPU timers. The fix ensures that whenever a CPU timer rlimit is modified, the kernel conditionally updates the underlying POSIX CPU timer enforcement structures. Verify the patch against your kernel version and distribution advisory, as backporting timelines vary. Distributions will release updated kernel packages; prioritize systems in multi-tenant or container environments.

Patch guidance

Monitor your Linux distribution (Ubuntu, Debian, Fedora, RHEL, etc.) for kernel security updates addressing CVE-2026-46328. Most distributions will provide patched kernel packages rather than standalone AppArmor updates. Verify that the update includes the refactored AppArmor rlimit code for POSIX CPU timers. Test the patch in a staging environment before production deployment, particularly if you rely on AppArmor policies for compliance or workload isolation. Check vendor security advisories for your specific kernel version to confirm the patch is included.

Detection guidance

Detection is challenging without seeing exploitation in progress. Monitor system logs for: (1) user-space tools reporting CPU timer limit enforcement failures or warnings; (2) processes confined by AppArmor exceeding expected resource usage metrics; (3) kernel audit logs (if enabled) showing AppArmor policy denials that seem incongruent with system behavior. Use auditctl to log AppArmor DENIED events and correlate them with actual process limits. In container environments, monitor cgroup CPU accounting to detect processes escaping expected quotas.

Why prioritize this

This merits prompt patching due to the HIGH CVSS score (7.3) reflecting the combination of local attack vector and high availability impact. The scope crossing (process affects system stability) elevates risk. While the vulnerability requires local access, it is trivial to exploit once present. Systems using AppArmor for workload isolation, particularly shared hosting and container platforms, should prioritize this over patches with lower exploitability. However, it does not appear on the CISA KEV catalog, so it lacks evidence of active exploitation in the wild—this reduces urgency relative to critical remote vulnerabilities but should not defer patching in sensitive environments.

Risk score, explained

The CVSS 3.1 score of 7.3 (HIGH) reflects: local attack vector (AV:L, +0 network exposure), low attack complexity (AC:L, trivial exploitation), low privilege requirement (PR:L, standard user sufficient), no user interaction needed, scope change (S:C, the attack impacts other processes/system stability beyond the attacker's scope), and a high availability impact (A:H, denial of service). Confidentiality and integrity impacts are not present (C:N, I:L). Organizations should weigh this against their threat model: critical for multi-tenant systems, moderate for single-user workstations.

Frequently asked questions