Linux: Use After Free
CVE-2025-39881 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 34d9cafd469c69ad85e6a36b4303c78382cf5c79, 854baafc00c433cccbe0ab4231b77aeb9b637b77, 7e64474aba78d240f7804f48f2d454dcca78b15f 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 0.15% 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 34d9cafd469c69ad85e6a36b4303c78382cf5c79 or 854baafc00c433cccbe0ab4231b77aeb9b637b77 or 7e64474aba78d240f7804f48f2d454dcca78b15f or ac5cda4fae8818cf1963317bb699f7f2f85b60af, whichever matches your release line, or a later version.
- Update Linux Kernel to 6.1.153 or 6.6.107 or 6.12.48 or 6.16.8, whichever matches your release line, or a later version.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Linux | Linux | 34f26a15611afb03c33df6819359d36f5b382589 up to, but not including, 34d9cafd469c69ad85e6a36b4303c78382cf5c79; 34f26a15611afb03c33df6819359d36f5b382589 up to, but not including, 854baafc00c433cccbe0ab4231b77aeb9b637b77; 34f26a15611afb03c33df6819359d36f5b382589 up to, but not including, 7e64474aba78d240f7804f48f2d454dcca78b15f; 34f26a15611afb03c33df6819359d36f5b382589 up to, but not including, ac5cda4fae8818cf1963317bb699f7f2f85b60af; 34f26a15611afb03c33df6819359d36f5b382589 up to, but not including, 3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f; 6.1 | 34d9cafd469c69ad85e6a36b4303c78382cf5c79, 854baafc00c433cccbe0ab4231b77aeb9b637b77, 7e64474aba78d240f7804f48f2d454dcca78b15f, ac5cda4fae8818cf1963317bb699f7f2f85b60af, 3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f |
| Linux | Linux Kernel | 6.1 up to, but not including, 6.1.153; 6.2 up to, but not including, 6.6.107; 6.7 up to, but not including, 6.12.48; 6.13 up to, but not including, 6.16.8; 6.17 | 6.1.153, 6.6.107, 6.12.48, 6.16.8 |
| Debian | Debian Linux | 11.0 | Not stated |
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 NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 7.8 | High | CNA |
| CVSS 3.1 | 7.8 | High | NVD |
Official description
In the Linux kernel, the following vulnerability has been resolved: kernfs: Fix UAF in polling when open file is released A use-after-free (UAF) vulnerability was identified in the PSI (Pressure Stall Information) monitoring mechanism: BUG: KASAN: slab-use-after-free in psi_trigger_poll+0x3c/0x140 Read of size 8 at addr ffff3de3d50bd308 by task systemd/1 psi_trigger_poll+0x3c/0x140 cgroup_pressure_poll+0x70/0xa0 cgroup_file_poll+0x8c/0x100 kernfs_fop_poll+0x11c/0x1c0 ep_item_poll.isra.0+0x188/0x2c0 Allocated by task 1: cgroup_file_open+0x88/0x388 kernfs_fop_open+0x73c/0xaf0 do_dentry_open+0x5fc/0x1200 vfs_open+0xa0/0x3f0 do_open+0x7e8/0xd08 path_openat+0x2fc/0x6b0 do_filp_open+0x174/0x368 Freed by task 8462: cgroup_file_release+0x130/0x1f8 kernfs_drain_open_files+0x17c/0x440 kernfs_drain+0x2dc/0x360 kernfs_show+0x1b8/0x288 cgroup_file_show+0x150/0x268 cgroup_pressure_write+0x1dc/0x340 cgroup_file_write+0x274/0x548 Reproduction Steps: 1. Open test/cpu.pressure and establish epoll monitoring 2. Disable monitoring: echo 0 > test/cgroup.pressure 3. Re-enable monitoring: echo 1 > test/cgroup.pressure The race condition occurs because: 1. When cgroup.pressure is disabled (echo 0 > cgroup.pressure), it: - Releases PSI triggers via cgroup_file_release() - Frees of->priv through kernfs_drain_open_files() 2. While epoll still holds reference to the file and continues polling 3. Re-enabling (echo 1 > cgroup.pressure) accesses freed of->priv epolling disable/enable cgroup.pressure fd=open(cpu.pressure) while(1) ... epoll_wait kernfs_fop_poll kernfs_get_active = true echo 0 > cgroup.pressure ... cgroup_file_show kernfs_show // inactive kn kernfs_drain_open_files cft->release(of); kfree(ctx); ... kernfs_get_active = false echo 1 > cgroup.pressure kernfs_show kernfs_activate_one(kn); kernfs_fop_poll kernfs_get_active = true cgroup_file_poll psi_trigger_poll // UAF ... end: close(fd) To address this issue, introduce kernfs_get_active_of() for kernfs open files to obtain active references. This function will fail if the open file has been released. Replace kernfs_get_active() with kernfs_get_active_of() to prevent further operations on released file descriptors.
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.
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
- git.kernel.org/stable/c/34d9cafd469c69ad85e6a36b4303c78382cf5c79
- git.kernel.org/stable/c/3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f
- git.kernel.org/stable/c/7e64474aba78d240f7804f48f2d454dcca78b15f
- git.kernel.org/stable/c/854baafc00c433cccbe0ab4231b77aeb9b637b77
- git.kernel.org/stable/c/ac5cda4fae8818cf1963317bb699f7f2f85b60af
Third-party and government advisories
- lists.debian.org/debian-lts-announce/2025/10/msg00008.htmlThird Party Advisory
Recent Use After Free vulnerabilities
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