# CVE-2025-21731

## Summary

- **CVE ID:** CVE-2025-21731
- **Severity:** HIGH
- **CVSS Score:** 7.8 (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)
- **CWE:** N/A
- **Published:** Feb 27, 2025
- **Last Modified:** Sep 8, 2026

## Description

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

nbd: don't allow reconnect after disconnect

Following process can cause nbd_config UAF:

1) grab nbd_config temporarily;

2) nbd_genl_disconnect() flush all recv_work() and release the
initial reference:

  nbd_genl_disconnect
   nbd_disconnect_and_put
    nbd_disconnect
     flush_workqueue(nbd->recv_workq)
    if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF, ...))
     nbd_config_put
     -> due to step 1), reference is still not zero

3) nbd_genl_reconfigure() queue recv_work() again;

  nbd_genl_reconfigure
   config = nbd_get_config_unlocked(nbd)
   if (!config)
   -> succeed
   if (!test_bit(NBD_RT_BOUND, ...))
   -> succeed
   nbd_reconnect_socket
    queue_work(nbd->recv_workq, &args->work)

4) step 1) release the reference;

5) Finially, recv_work() will trigger UAF:

  recv_work
   nbd_config_put(nbd)
   -> nbd_config is freed
   atomic_dec(&config->recv_threads)
   -> UAF

Fix the problem by clearing NBD_RT_BOUND in nbd_genl_disconnect(), so
that nbd_genl_reconfigure() will fail.

## Affected Products

- Linux — Linux (b7aa3d39385dc2d95899f9e379623fef446a2acd)
- Linux — Linux (4.12)
- Linux — Linux (0)
- Linux — Linux (5.4.291)
- Linux — Linux (5.10.235)
- Linux — Linux (5.15.179)
- Linux — Linux (6.1.129)
- Linux — Linux (6.6.76)
- Linux — Linux (6.12.13)
- Linux — Linux (6.13.2)
- Linux — Linux (6.14)

## References

- [CNA](https://git.kernel.org/stable/c/e70a578487a47d7cf058904141e586684d1c3381)
- [CNA](https://git.kernel.org/stable/c/6bef6222a3f6c7adb6396f77f25a3579d821b09a)
- [CNA](https://git.kernel.org/stable/c/e3be8862d73cac833e0fb7602636c19c6cb94b11)
- [CNA](https://git.kernel.org/stable/c/e7343fa33751cb07c1c56b666bf37cfca357130e)
- [CNA](https://git.kernel.org/stable/c/d208d2c52b652913b5eefc8ca434b0d6b757f68f)
- [CNA](https://git.kernel.org/stable/c/a8ee6ecde2b7bfb58c8a3afe8a9d2b848f580739)
- [CNA](https://git.kernel.org/stable/c/9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302)
- [CNA](https://git.kernel.org/stable/c/844b8cdc681612ff24df62cdefddeab5772fadf1)
- [CVE](https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html)
- [CVE](https://lists.debian.org/debian-lts-announce/2025/03/msg00028.html)
- [siemens-SADP](https://cert-portal.siemens.com/productcert/html/ssa-019113.html)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.22%
- **EPSS Percentile:** 13.3

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._