Linux: Use After Free
CVE-2026-31586 is a high-severity use after free vulnerability in Linux, with a CVSS 3.1 base score of 7.8 out of 10. It can be exploited with local access to the system by an attacker with a low-privileged account, without any action from a user. The record lists versions 23acef4156c260e8598397a1a2e8b3a23e919893, 1bd36e93b542d9dd020190c6607c6a3663405195, 740ba1ebb223f137ff088ab74d533a13f9167bd8 as the first without the flaw, so updating Linux to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.13% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Linux to 23acef4156c260e8598397a1a2e8b3a23e919893 or 1bd36e93b542d9dd020190c6607c6a3663405195 or 740ba1ebb223f137ff088ab74d533a13f9167bd8 or 115a5266749dcde7fe4127e8623d19c752088f69, whichever matches your release line, or a later version.
- Update Linux Kernel to 6.6.136 or 6.12.83 or 6.18.24 or 6.19.14, whichever matches your release line, or a later version.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Linux | Linux | 59b57717fff8b562825d9d25e0180ad7e8048ca9 up to, but not including, 23acef4156c260e8598397a1a2e8b3a23e919893; 59b57717fff8b562825d9d25e0180ad7e8048ca9 up to, but not including, 1bd36e93b542d9dd020190c6607c6a3663405195; 59b57717fff8b562825d9d25e0180ad7e8048ca9 up to, but not including, 740ba1ebb223f137ff088ab74d533a13f9167bd8; 59b57717fff8b562825d9d25e0180ad7e8048ca9 up to, but not including, 115a5266749dcde7fe4127e8623d19c752088f69; 59b57717fff8b562825d9d25e0180ad7e8048ca9 up to, but not including, dfc8292a1d6782c76b626315605e0585a5a18447; 59b57717fff8b562825d9d25e0180ad7e8048ca9 up to, but not including, ea3af09eb87d8f8708c66747fcf1a2762902e839; 59b57717fff8b562825d9d25e0180ad7e8048ca9 up to, but not including, 50879a3c1faf06e661090015d59e2127255cff27; 59b57717fff8b562825d9d25e0180ad7e8048ca9 up to, but not including, 67cb119d32f35e32acd0393bbeb318b2bb1fdafe; and 2 more ranges | 23acef4156c260e8598397a1a2e8b3a23e919893, 1bd36e93b542d9dd020190c6607c6a3663405195, 740ba1ebb223f137ff088ab74d533a13f9167bd8, 115a5266749dcde7fe4127e8623d19c752088f69, dfc8292a1d6782c76b626315605e0585a5a18447, ea3af09eb87d8f8708c66747fcf1a2762902e839 |
| Siemens | SIMATIC S7-1500 TM MFP - GNU/Linux subsystem | all versions | Not stated |
| Linux | Linux Kernel | 4.19 up to, but not including, 6.6.136; 6.7 up to, but not including, 6.12.83; 6.13 up to, but not including, 6.18.24; 6.19 up to, but not including, 6.19.14; 7.0 up to, but not including, 7.0.1 | 6.6.136, 6.12.83, 6.18.24, 6.19.14, 7.0.1 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- LocalThe attacker needs local access or must trick a user into running something.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by CNA.
Official description
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 -> 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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.
How developers prevent this class of flaw
- Choose a language that provides automatic memory management.
- When freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
- git.kernel.org/stable/c/115a5266749dcde7fe4127e8623d19c752088f69
- git.kernel.org/stable/c/50879a3c1faf06e661090015d59e2127255cff27
- git.kernel.org/stable/c/67cb119d32f35e32acd0393bbeb318b2bb1fdafe
- git.kernel.org/stable/c/8f5857be99f1ed1fa80991c72449541f634626ee
- git.kernel.org/stable/c/dfc8292a1d6782c76b626315605e0585a5a18447
- git.kernel.org/stable/c/ea3af09eb87d8f8708c66747fcf1a2762902e839
Other references
Recent Use After Free vulnerabilities
- 9.8 Critical severity
- 7.8 High severity
- 7.8 High severity
- 7.8 High severity
- 7.8 High severity
- 7.8 High severity
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