Linux: Use After Free
CVE-2025-21756 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 e7754d564579a5db9c5c9f74228df5d6dd6f1173, e48fcb403c2d0e574c19683f09399ab4cf67809c, 42b33381e5e1f2b967dc4fb4221ddb9aaf10d197 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 0.89% chance of exploitation within 30 days, higher than 60% of scored CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Linux to e7754d564579a5db9c5c9f74228df5d6dd6f1173 or e48fcb403c2d0e574c19683f09399ab4cf67809c or 42b33381e5e1f2b967dc4fb4221ddb9aaf10d197 or 3f43540166128951cc1be7ab1ce6b7f05c670d8b, whichever matches your release line, or a later version.
- Update Linux Kernel to 5.10.235 or 5.15.179 or 6.1.131 or 6.6.79, whichever matches your release line, or a later version.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Linux | Linux | c0cfa2d8a788fcf45df5bf4070ab2474c88d543a up to, but not including, e7754d564579a5db9c5c9f74228df5d6dd6f1173; c0cfa2d8a788fcf45df5bf4070ab2474c88d543a up to, but not including, e48fcb403c2d0e574c19683f09399ab4cf67809c; c0cfa2d8a788fcf45df5bf4070ab2474c88d543a up to, but not including, 42b33381e5e1f2b967dc4fb4221ddb9aaf10d197; c0cfa2d8a788fcf45df5bf4070ab2474c88d543a up to, but not including, 3f43540166128951cc1be7ab1ce6b7f05c670d8b; c0cfa2d8a788fcf45df5bf4070ab2474c88d543a up to, but not including, 645ce25aa0e67895b11d89f27bb86c9d444c40f8; c0cfa2d8a788fcf45df5bf4070ab2474c88d543a up to, but not including, b1afd40321f1c243cffbcf40ea7ca41aca87fa5e; c0cfa2d8a788fcf45df5bf4070ab2474c88d543a up to, but not including, fcdd2242c0231032fc84e1404315c245ae56322a; 5.5 | e7754d564579a5db9c5c9f74228df5d6dd6f1173, e48fcb403c2d0e574c19683f09399ab4cf67809c, 42b33381e5e1f2b967dc4fb4221ddb9aaf10d197, 3f43540166128951cc1be7ab1ce6b7f05c670d8b, 645ce25aa0e67895b11d89f27bb86c9d444c40f8, b1afd40321f1c243cffbcf40ea7ca41aca87fa5e |
| Siemens | SIMATIC S7-1500 CPU 1518-4 PN/DP MFP | V3.1.5 and later | Not stated |
| Siemens | SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP | V3.1.5 and later | Not stated |
| Siemens | SIMATIC S7-1500 TM MFP - GNU/Linux subsystem | all versions | Not stated |
| Siemens | SIPLUS S7-1500 CPU 1518-4 PN/DP MFP | V3.1.5 and later | Not stated |
| Linux | Linux Kernel | 5.5 up to, but not including, 5.10.235; 5.11 up to, but not including, 5.15.179; 5.16 up to, but not including, 6.1.131; 6.2 up to, but not including, 6.6.79; 6.7 up to, but not including, 6.12.16; 6.13 up to, but not including, 6.13.4 | 5.10.235, 5.15.179, 6.1.131, 6.6.79, 6.12.16, 6.13.4 |
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.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 7.8 | High | CNA |
| CVSS 3.1 | 7.8 | High | CISA (ADP) |
Official description
In the Linux kernel, the following vulnerability has been resolved: vsock: Keep the binding until socket destruction Preserve sockets bindings; this includes both resulting from an explicit bind() and those implicitly bound through autobind during connect(). Prevents socket unbinding during a transport reassignment, which fixes a use-after-free: 1. vsock_create() (refcnt=1) calls vsock_insert_unbound() (refcnt=2) 2. transport->release() calls vsock_remove_bound() without checking if sk was bound and moved to bound list (refcnt=1) 3. vsock_bind() assumes sk is in unbound list and before __vsock_insert_bound(vsock_bound_sockets()) calls __vsock_remove_bound() which does: list_del_init(&vsk->bound_table); // nop sock_put(&vsk->sk); // refcnt=0 BUG: KASAN: slab-use-after-free in __vsock_bind+0x62e/0x730 Read of size 4 at addr ffff88816b46a74c by task a.out/2057 dump_stack_lvl+0x68/0x90 print_report+0x174/0x4f6 kasan_report+0xb9/0x190 __vsock_bind+0x62e/0x730 vsock_bind+0x97/0xe0 __sys_bind+0x154/0x1f0 __x64_sys_bind+0x6e/0xb0 do_syscall_64+0x93/0x1b0 entry_SYSCALL_64_after_hwframe+0x76/0x7e Allocated by task 2057: kasan_save_stack+0x1e/0x40 kasan_save_track+0x10/0x30 __kasan_slab_alloc+0x85/0x90 kmem_cache_alloc_noprof+0x131/0x450 sk_prot_alloc+0x5b/0x220 sk_alloc+0x2c/0x870 __vsock_create.constprop.0+0x2e/0xb60 vsock_create+0xe4/0x420 __sock_create+0x241/0x650 __sys_socket+0xf2/0x1a0 __x64_sys_socket+0x6e/0xb0 do_syscall_64+0x93/0x1b0 entry_SYSCALL_64_after_hwframe+0x76/0x7e Freed by task 2057: kasan_save_stack+0x1e/0x40 kasan_save_track+0x10/0x30 kasan_save_free_info+0x37/0x60 __kasan_slab_free+0x4b/0x70 kmem_cache_free+0x1a1/0x590 __sk_destruct+0x388/0x5a0 __vsock_bind+0x5e1/0x730 vsock_bind+0x97/0xe0 __sys_bind+0x154/0x1f0 __x64_sys_bind+0x6e/0xb0 do_syscall_64+0x93/0x1b0 entry_SYSCALL_64_after_hwframe+0x76/0x7e refcount_t: addition on 0; use-after-free. WARNING: CPU: 7 PID: 2057 at lib/refcount.c:25 refcount_warn_saturate+0xce/0x150 RIP: 0010:refcount_warn_saturate+0xce/0x150 __vsock_bind+0x66d/0x730 vsock_bind+0x97/0xe0 __sys_bind+0x154/0x1f0 __x64_sys_bind+0x6e/0xb0 do_syscall_64+0x93/0x1b0 entry_SYSCALL_64_after_hwframe+0x76/0x7e refcount_t: underflow; use-after-free. WARNING: CPU: 7 PID: 2057 at lib/refcount.c:28 refcount_warn_saturate+0xee/0x150 RIP: 0010:refcount_warn_saturate+0xee/0x150 vsock_remove_bound+0x187/0x1e0 __vsock_release+0x383/0x4a0 vsock_release+0x90/0x120 __sock_release+0xa3/0x250 sock_close+0x14/0x20 __fput+0x359/0xa80 task_work_run+0x107/0x1d0 do_exit+0x847/0x2560 do_group_exit+0xb8/0x250 __x64_sys_exit_group+0x3a/0x50 x64_sys_call+0xfec/0x14f0 do_syscall_64+0x93/0x1b0 entry_SYSCALL_64_after_hwframe+0x76/0x7e
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: poc, 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/3f43540166128951cc1be7ab1ce6b7f05c670d8b
- git.kernel.org/stable/c/42b33381e5e1f2b967dc4fb4221ddb9aaf10d197
- git.kernel.org/stable/c/645ce25aa0e67895b11d89f27bb86c9d444c40f8
- git.kernel.org/stable/c/b1afd40321f1c243cffbcf40ea7ca41aca87fa5e
- git.kernel.org/stable/c/e48fcb403c2d0e574c19683f09399ab4cf67809c
- git.kernel.org/stable/c/e7754d564579a5db9c5c9f74228df5d6dd6f1173
- git.kernel.org/stable/c/fcdd2242c0231032fc84e1404315c245ae56322a
Other references
Recent Use After Free vulnerabilities
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