CVE-2026-52973: Linux Kernel Futex Use-After-Free Local Privilege Escalation

Technical summary

CVE-2026-52973 is a use-after-free vulnerability in the Linux kernel's futex subsystem, specifically in the need_futex_hash_allocate_default() function. The vulnerability stems from improper handling of per-CPU reference counting for futex hash allocation when processes share memory via CLONE_VM. The kernel was checking strictly for CLONE_THREAD, which assumes pthread semantics and prevents proper allocation of mm->futex_ref per-CPU structures. When processes share memory through other clone mechanisms (excluding vfork), the kernel's reference counting becomes inconsistent: a +1 bias can be applied to a per-CPU counter that the memory descriptor no longer references, causing a use-after-free. This is triggered during futex_hash_put operations. The CWE-416 (use-after-free) and CWE-825 (expired pointer dereference) classifications confirm the memory safety defect. The patch changes the allocation condition to permit any CLONE_VM clone except vfork(), ensuring proper synchronization of futex reference counting across all memory-sharing scenarios.

Business impact

This vulnerability affects any organization running Linux systems where untrusted local users can execute code. Exploitation can result in kernel denial of service, making systems unstable or unavailable. In shared hosting, container, or multi-tenant environments, unprivileged users could trigger crashes affecting all tenants. While not a direct path to privilege escalation, the kernel memory corruption could be chained with other exploits. The practical impact depends on whether affected distributions expose futex operations to unprivileged users—many hardened setups do, making this a credible threat.

Affected systems

The vulnerability affects the Linux kernel across multiple versions and distributions. All Linux systems using the standard futex implementation are potentially vulnerable. The specific kernel versions and distribution releases impacted should be verified against your vendor's security advisories, as the provided data does not enumerate exact version ranges. Mainline kernel versions from the period when futex hash allocation logic was introduced through the patch date are affected. Both x86 and ARM architectures are impacted.

Exploitability

Exploitability requires local code execution as an unprivileged user (PR:L per CVSS). The attack vector is local only (AV:L), meaning remote exploitation is not possible. The complexity is low (AC:L), indicating the vulnerability can be triggered with standard futex syscalls without race conditions or timing manipulation. No user interaction is required. The attacker must craft a sequence of clone operations followed by futex operations to corrupt the reference counting. Public exploit code is not yet disclosed, but the straightforward nature of the defect (incorrect reference counting logic) suggests proof-of-concept development would be feasible for competent kernel developers.

Remediation

Apply kernel patches that implement the corrected clone checking logic. The fix modifies need_futex_hash_allocate_default() to accept any CLONE_VM clone except vfork(), rather than strictly checking CLONE_THREAD. This ensures per-CPU futex reference structures are properly allocated and tracked. Verify that your kernel version includes commits addressing this issue by checking against your distribution's advisories. Temporary mitigation is limited, but disabling local user access or restricting futex syscalls via seccomp could reduce risk in high-security environments, though this may break legitimate applications.

Patch guidance

Consult your Linux distribution's security advisory for specific patched kernel versions. Mainline kernel and stable series releases after 2026-06-24 should contain the fix. Test patched kernels in a staging environment before production deployment to ensure no regressions with workloads relying on futex-heavy operations (typically seen in multi-threaded Java, Go, and Rust applications). If your kernel is not yet patched by your vendor, escalate to your distribution maintainer for patch availability. For distributions with extended support timelines, backport patches may be available before formal releases.

Detection guidance

Monitor system logs for kernel panic messages containing futex-related stack traces, particularly those referencing futex_hash_put or use-after-free KASAN reports. Enable kernel address sanitizer (KASAN) in debug builds to catch the corruption at runtime. Monitor process behavior: repeated clone syscalls followed by futex operations from low-privileged processes may indicate exploitation attempts. Audit which users have permission to call futex syscalls; restrict if possible via SELinux, AppArmor, or seccomp policies. Check dmesg for warnings like 'BUG: KASAN: slab-use-after-free in futex_hash_put'. Network-based detection is not applicable for this local vulnerability.

Why prioritize this

This vulnerability earns HIGH severity (CVSS 7.8) due to its combination of high impact (full confidentiality, integrity, and availability compromise of the kernel) with low barrier to exploitation (local user, low complexity). Organizations should prioritize patching based on exposure: systems accepting untrusted local users (shared hosting, multi-tenant containers, university compute clusters) should be patched urgently. Single-user desktops and air-gapped systems face lower practical risk. The vulnerability is not on the KEV catalog, suggesting active exploitation in the wild is not yet confirmed, but the straightforward nature of the defect and its potential use in privilege escalation chains warrants expedited remediation.

Risk score, explained

The CVSS 3.1 score of 7.8 (HIGH) reflects a high-impact vulnerability with moderate attack friction. The AV:L (local only) and PR:L (low privilege required) constraints reduce applicability compared to network-exploitable flaws, but do not lower the severity for affected environments. AC:L (low complexity) and S:U (unchanged scope) indicate the defect is straightforward to trigger and has contained blast radius within the affected system. The C:H/I:H/A:H impact ratings are justified: use-after-free in kernel memory management can compromise kernel data structures (confidentiality), corrupt kernel state (integrity), and cause denial of service (availability). Organizations with strong process isolation and network segmentation can tolerate slightly longer remediation timelines than those with permissive local user access.

Frequently asked questions