# CVE-2023-53781: smc: Fix use-after-free in tcp_write_timer_handler().

> A use-after-free vulnerability in the Linux kernel's SMC module allows attackers to cause a denial of service or potentially execute code via crafted TCP operations.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 9 Dec 2025
- **Updated:** 15 Oct 2026
- **Also known as:** BELL-CVE-2023-53781, DEBIAN-CVE-2023-53781, GHSA-3h9m-xfjq-9f2f, ROOT-OS-DEBIAN-11-CVE-2023-53781, ROOT-OS-DEBIAN-12-CVE-2023-53781, ROOT-OS-UBUNTU-2204-CVE-2023-53781, UBUNTU-CVE-2023-53781

## What is CVE-2023-53781?

CVE-2023-53781 is a high-severity use after free vulnerability in Linux, published on 9 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

smc: Fix use-after-free in tcp_write_timer_handler().

With Eric's ref tracker, syzbot finally found a repro for
use-after-free in tcp_write_timer_handler() by kernel TCP
sockets. [0]

If SMC creates a kernel socket in __smc_create(), the kernel
socket is supposed to be freed in smc_clcsock_release() by
calling sock_release() when we close() the parent SMC socket.

However, at the end of smc_clcsock_release(), the kernel
socket's sk_state might not be TCP_CLOSE.  This means that
we have not called inet_csk_destroy_sock() in __tcp_close()
and have not stopped the TCP timers.

The kernel socket's TCP timers can be fired later, so we
need to hold a refcnt for net as we do for MPTCP subflows
in mptcp_subflow_create_socket().

[0]:
leaked reference.
 sk_alloc (./include/net/net_namespace.h:335 net/core/sock.c:2108)
 inet_create (net/ipv4/af_inet.c:319 net/ipv4/af_inet.c:244)
 __sock_create (net/socket.c:1546)
 smc_create (net/smc/af_smc.c:3269 net/smc/af_smc.c:3284)
 __sock_create (net/socket.c:1546)
 __sys_socket (net/socket.c:1634 net/socket.c:1618 net/socket.c:1661)
 __x64_sys_socket (net/socket.c:1672)
 do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)
==================================================================
BUG: KASAN: slab-use-after-free in tcp_write_timer_handler (net/ipv4/tcp_timer.c:378 net/ipv4/tcp_timer.c:624 net/ipv4/tcp_timer.c:594)
Read of size 1 at addr ffff888052b65e0d by task syzrepro/18091

CPU: 0 PID: 18091 Comm: syzrepro Tainted: G        W          6.3.0-rc4-01174-gb5d54eb5899a #7
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-1.amzn2022.0.1 04/01/2014
Call Trace:
 <IRQ>
 dump_stack_lvl (lib/dump_stack.c:107)
 print_report (mm/kasan/report.c:320 mm/kasan/report.c:430)
 kasan_report (mm/kasan/report.c:538)
 tcp_write_timer_handler (net/ipv4/tcp_timer.c:378 net/ipv4/tcp_timer.c:624 net/ipv4/tcp_timer.c:594)
 tcp_write_timer (./include/linux/spinlock.h:390 net/ipv4/tcp_timer.c:643)
 call_timer_fn (./arch/x86/include/asm/jump_label.h:27 ./include/linux/jump_label.h:207 ./include/trace/events/timer.h:127 kernel/time/timer.c:1701)
 __run_timers.part.0 (kernel/time/timer.c:1752 kernel/time/timer.c:2022)
 run_timer_softirq (kernel/time/timer.c:2037)
 __do_softirq (./arch/x86/include/asm/jump_label.h:27 ./include/linux/jump_label.h:207 ./include/trace/events/irq.h:142 kernel/softirq.c:572)
 __irq_exit_rcu (kernel/softirq.c:445 kernel/softirq.c:650)
 irq_exit_rcu (kernel/softirq.c:664)
 sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1107 (discriminator 14))
 </IRQ>

## Which versions are affected by CVE-2023-53781?


## How do I fix CVE-2023-53781?

Apply the vendor's patch.

## Is CVE-2023-53781 being exploited?

There is no known exploitation of CVE-2023-53781 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-53781 and fix it if it is affected.

What it is: CVE-2023-53781 is a high-severity use after free vulnerability in Linux, published on 9 Dec 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2023-53781 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2023-53781/ (Markdown: https://insights.sbomify.com/CVE-2023-53781/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-53781
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-53781
- OSV: https://osv.dev/vulnerability/CVE-2023-53781

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-53781/
Summaries are generated and may contain mistakes; the official description above is the record.