CVE-2026-31586
Published: 24 April 2026
Summary
CVE-2026-31586 is a high-severity Use After Free (CWE-416) vulnerability in Linux Linux Kernel. Its CVSS base score is 7.8 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 3.0th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-2 (Flaw Remediation) and RA-5 (Vulnerability Monitoring and Scanning).
Deeper analysis
CVE-2026-31586 is a use-after-free vulnerability in the Linux kernel's blk-cgroup subsystem, specifically within the cgwb_release_workfn() function in mm/backing-dev.c. The issue arises when cgwb_release_workfn() calls css_put() on wb->blkcg_css, which may drop the last reference and asynchronously free the blkcg via css_free_rwork_fn() and kfree(). Subsequent access to wb->blkcg_css in blkcg_unpin_online() at block/blk-cgroup.c:1367 then dereferences the freed pointer, leading to a slab-use-after-free as detected by KASAN. This affects Linux kernels, with crashes observed sporadically across multiple versions in Meta's fleet.
A local attacker with low privileges (PR:L) can exploit this vulnerability given low attack complexity (AC:L) and no user interaction (UI:N). Successful exploitation could result in high confidentiality, integrity, and availability impacts (C:H/I:H/A:H), with an overall CVSS v3.1 base score of 7.8. The unchanged security scope (S:U) indicates potential for kernel memory corruption, crashes, or arbitrary code execution via the use-after-free (CWE-416), triggered in a narrow race window during blk-cgroup workqueue processing.
Mitigation is provided through stable kernel patches, including commits such as 115a5266749dcde7fe4127e8623d19c752088f69, 50879a3c1faf06e661090015d59e2127255cff27, 67cb119d32f35e32acd0393bbeb318b2bb1fdafe, 8f5857be99f1ed1fa80991c72449541f634626ee, and dfc8292a1d6782c76b626315605e0585a5a18447. These fix the issue by reordering operations in cgwb_release_workfn() to call blkcg_unpin_online() before css_put(), ensuring the blkcg remains alive during the unpin access. Security practitioners should update to patched kernels and monitor for the KASAN-detected crash pattern.
Notably, the vulnerability has been observed in production at Meta across kernel versions based on commit 66672af7a095, with a public reproducer available at https://github.com/leitao/debug/blob/main/reproducers/repro_blkcg_uaf.sh. The race is narrow but reliably triggered under KASAN with an injected msleep(100) delay between css_put() and blkcg_unpin_online(). No widespread real-world exploitation has been reported.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-25479
Vulnerability details
In the Linux kernel, the following vulnerability has been resolved: mm: blk-cgroup: fix use-after-free in cgwb_release_workfn() cgwb_release_workfn() calls css_put(wb->blkcg_css) and then later accesses wb->blkcg_css again via blkcg_unpin_online(). If css_put() drops the last reference, the blkcg can be freed asynchronously (css_free_rwork_fn…
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-> blkcg_css_free -> kfree) before blkcg_unpin_online() dereferences the pointer to access blkcg->online_pin, resulting in a use-after-free: BUG: KASAN: slab-use-after-free in blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367) Write of size 4 at addr ff11000117aa6160 by task kworker/71:1/531 Workqueue: cgwb_release cgwb_release_workfn Call Trace: <TASK> blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367) cgwb_release_workfn (mm/backing-dev.c:629) process_scheduled_works (kernel/workqueue.c:3278 kernel/workqueue.c:3385) Freed by task 1016: kfree (./include/linux/kasan.h:235 mm/slub.c:2689 mm/slub.c:6246 mm/slub.c:6561) css_free_rwork_fn (kernel/cgroup/cgroup.c:5542) process_scheduled_works (kernel/workqueue.c:3302 kernel/workqueue.c:3385) ** Stack based on commit 66672af7a095 ("Add linux-next specific files for 20260410") I am seeing this crash sporadically in Meta fleet across multiple kernel versions. A full reproducer is available at: https://github.com/leitao/debug/blob/main/reproducers/repro_blkcg_uaf.sh (The race window is narrow. To make it easily reproducible, inject a msleep(100) between css_put() and blkcg_unpin_online() in cgwb_release_workfn(). With that delay and a KASAN-enabled kernel, the reproducer triggers the splat reliably in less than a second.) Fix this by moving blkcg_unpin_online() before css_put(), so the cgwb's CSS reference keeps the blkcg alive while blkcg_unpin_online() accesses it.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Local kernel use-after-free (CWE-416) with arbitrary code execution and high C/I/A impact directly enables exploitation for privilege escalation from low-privileged local context.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Timely remediation through application of stable kernel patches directly eliminates the use-after-free vulnerability in cgwb_release_workfn() by reordering css_put() after blkcg_unpin_online().
Memory protection mechanisms such as kernel ASLR, SMEP/SMAP, and non-executable memory mitigate exploitation of the use-after-free for arbitrary code execution or corruption.
Vulnerability scanning and monitoring identifies the CVE-2026-31586 flaw in deployed Linux kernels, enabling prioritization of patching before exploitation.