What is CVE-2025-21756?

CVE-2025-21756 is a High severity Linux kernel vulnerability with a CVSS score of 7.8 out of 10, classified as a Use After Free flaw (CWE-416), affecting Linux kernel versions from 5.5 onward and patched in 5.10.235, 5.15.179, 6.1.131 and others. CVE-2025-21756 has not been confirmed as actively exploited and is not listed in the CISA KEV catalog.

What is the CVSS score for CVE-2025-21756?

CVE-2025-21756 has a CVSS score of 7.8 out of 10, rated High severity. The vector string is CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.

Is there a patch available for CVE-2025-21756?

Yes, CVE-2025-21756 has been patched. Fixed versions include 5.10.235, 5.15.179, 6.1.131 and others. If you are running Linux kernel 5.5 or later up to the fix versions, apply the relevant patch for your kernel branch.

Is CVE-2025-21756 actively exploited?

CVE-2025-21756 has not been confirmed as actively exploited and is not listed in the CISA KEV catalog.

What is Use After Free (CWE-416)?

The product references memory after it has been freed, which may cause it to crash, use unexpected values, or execute code. See MITRE CWE for full details: https://cwe.mitre.org/data/definitions/416.html

CVE-2025-21756

High

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

Package Linux Kernel

Published 2025-02-27

Last modified 2026-05-12

CVSS version 3.1

Patch available

Yes

CVSS 3.1 score

7.8

out of 10

High

Attack Vector

Local

Attack Complexity

Low

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Vector string

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Weakness type

CWE-416

CVE-2025-21756 is a Use After Free vulnerability

What is Use After Free?

The product references memory after it has been freed, which may cause it to crash, use unexpected values, or execute code. Learn more on MITRE CWE

Affected versions

Linux kernel versions 5.5 and later are affected. Fixed in 5.10.235, 5.15.179, 6.1.131, 6.6.79, 6.12.16, 6.13.4, 6.14 and their respective stable series.

Affected from

≥ 5.5

Fixed in

✓ 5.10.235 5.10.x ✓ 5.15.179 5.15.x ✓ 6.1.131 6.1.x ✓ 6.6.79 6.6.x ✓ 6.12.16 6.12.x ✓ 6.13.4 6.13.x ✓ 6.14

References

The following references provide additional information about CVE-2025-21756 including vendor advisories, patch commits, exploit details, and third-party analysis. Links are sourced from the NIST NVD database.

Debian Security
https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html
Debian Security
https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html
Cert-Portal
https://cert-portal.siemens.com/productcert/html/ssa-082556.html
Cert-Portal
https://cert-portal.siemens.com/productcert/html/ssa-265688.html
Kernel patch commit
https://git.kernel.org/stable/c/3f43540166128951cc1be7ab1ce6b7f05c670d8b
Patch
Kernel patch commit
https://git.kernel.org/stable/c/42b33381e5e1f2b967dc4fb4221ddb9aaf10d197
Patch
Kernel patch commit
https://git.kernel.org/stable/c/645ce25aa0e67895b11d89f27bb86c9d444c40f8
Patch

7 patch commits total View all on NVD

Details

CVE ID CVE-2025-21756

Severity High

CVSS score 7.8 / 10

CVSS version 3.1

Published 2025-02-27

Modified 2026-05-12

KEV No

Patch

Yes

Package linux

Frequently asked questions

What is CVE-2025-21756?

CVE-2025-21756 is a High severity Linux kernel vulnerability with a CVSS score of 7.8 out of 10 , classified as an Use After Free flaw (CWE-416) . It affects Linux kernel versions from 5.5 onward and has been patched in 5.10.235, 5.15.179, 6.1.131 and others. CVE-2025-21756 has not been confirmed as actively exploited and is not listed in the CISA KEV catalog.
What is the CVSS score for CVE-2025-21756?

CVE-2025-21756 has a CVSS score of 7.8 out of 10, rated High severity (CVSS 3.1). The vector string is CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.
Is there a patch available for CVE-2025-21756?

Yes — CVE-2025-21756 has been patched. Fixed versions include 5.10.235, 5.15.179, 6.1.131 and others. If you are running Linux kernel 5.5 or later up to the fix versions, apply the relevant patch for your kernel branch.
Is CVE-2025-21756 actively exploited?

No — CVE-2025-21756 has not been confirmed as actively exploited. It is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.
What is Use After Free (CWE-416)?

The product references memory after it has been freed, which may cause it to crash, use unexpected values, or execute code. View CWE-416 on MITRE CWE →