CVE-2025-21756
Published: 27 February 2025
Summary
CVE-2025-21756 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 27.5th 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-2025-21756 is a use-after-free vulnerability (CWE-416) in the Linux kernel's vsock subsystem. The issue arises because socket bindings, whether from explicit bind() calls or implicit autobind during connect(), are not preserved until socket destruction. This leads to premature unbinding during transport reassignment, triggering a use-after-free in functions like __vsock_bind(). Kernel debugging traces, including KASAN slab-use-after-free reports and refcount underflow/overflow warnings, confirm the defect during socket creation, binding, and release operations.
A local attacker with low privileges (PR:L) can exploit this vulnerability with low attack complexity and no user interaction required (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H). By performing socket operations such as create(), bind(), and connect(), the attacker can trigger the use-after-free, potentially leading to arbitrary code execution, data corruption, or kernel crashes due to the high impacts on confidentiality, integrity, and availability.
Mitigation involves applying upstream kernel patches, as detailed in the referenced stable commits: 3f43540166128951cc1be7ab1ce6b7f05c670d8b, 42b33381e5e1f2b967dc4fb4221ddb9aaf10d197, 645ce25aa0e67895b11d89f27bb86c9d444c40f8, b1afd40321f1c243cffbcf40ea7ca41aca87fa5e, and e48fcb403c2d0e574c19683f09399ab4cf67809c. These patches preserve socket bindings until destruction, preventing unbinding during transport reassignment.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2025-5169
Vulnerability details
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…
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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
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
The CVE describes a local use-after-free in the Linux kernel vsock subsystem that a low-privileged attacker can trigger via socket operations to achieve arbitrary code execution or kernel compromise, directly enabling privilege escalation.
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Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Mandates timely flaw remediation by applying kernel patches that preserve vsock socket bindings until destruction, directly eliminating the use-after-free vulnerability.
Provides memory protections like non-executable memory and ASLR to mitigate exploitation of the use-after-free in vsock during socket binding and transport reassignment.
Requires vulnerability scanning to identify the unpatched vsock use-after-free CVE in the Linux kernel and respond with remediation actions.