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High severity7.8NVD Advisory· Published Feb 27, 2025· Updated May 12, 2026

CVE-2025-21756

CVE-2025-21756

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

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

Use-after-free in Linux kernel vsock due to improper socket binding handling during transport reassignment, potentially enabling local privilege escalation.

Vulnerability

Description

A use-after-free vulnerability exists in the Linux kernel's vsock (virtual socket) implementation. The core issue is that socket bindings—whether from explicit bind() calls or implicit autobind during connect()—are not preserved when a transport reassignment occurs. This flaw can lead to a race condition where a socket is freed while still being referenced, resulting in a use-after-free condition [description].

Exploitation

Prerequisites

An attacker with local access to the system can trigger the vulnerability by performing a sequence of socket operations: first creating a vsock socket (via socket()), then binding it (either explicitly or implicitly via connect()), and finally causing a transport reassignment. The code flow leads to vsock_remove_bound() being called without proper checks, followed by a subsequent __vsock_remove_bound() that decrements the reference count to zero, freeing the socket. The freed memory can then be accessed, as shown in the KASAN report [description].

Impact

Successful exploitation could allow an attacker to crash the system (denial of service) or potentially escalate privileges due to the use-after-free condition. The vulnerability affects systems running the Linux kernel with vsock support, including various embedded and industrial devices that use the kernel, such as those from Siemens [1][2].

Mitigation

The fix has been applied in the Linux kernel stable releases. Users are advised to update their kernels to a version containing the commit that preserves socket bindings during transport reassignment. Siemens has also published advisories listing affected products, including SIMATIC S7-1500 CPU families and related ET 200 CPUs, and recommends applying firmware updates [1][2].

References
  1. SSA-082556
  2. SSA-265688

AI Insight generated on May 20, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.

Affected products

251

Patches

0

No patches discovered yet.

Vulnerability mechanics

AI mechanics synthesis has not run for this CVE yet.

References

11

News mentions

0

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