# CVE-2026-89696

## Summary

- **CVE ID:** CVE-2026-89696
- **Severity:** HIGH
- **CVSS Score:** 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 14, 2026

## 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.

## Affected Products

- Linux — Linux (b9e8638e3d9ed8334f1f7071e081860aac37e83e)
- Linux — Linux (5.6)
- Linux — Linux (0)
- Linux — Linux (6.12.109)
- Linux — Linux (6.18.50)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)
- Linux — Linux (5.10.270)
- Linux — Linux (5.15.221)
- Linux — Linux (6.1.188)
- Linux — Linux (6.6.157)

## References

- [CNA](https://git.kernel.org/stable/c/35f248bd40b47b229d4999581df45b97daadd977)
- [CNA](https://git.kernel.org/stable/c/311f7d926630940650447cbd1c932b076b40a6c4)
- [CNA](https://git.kernel.org/stable/c/bf4d338dc8625d70c7f2cb0657d66851a7ac9154)
- [CNA](https://git.kernel.org/stable/c/c59738a00aa51b16adc1b5ceb7c80877168efb4d)
- [CNA](https://git.kernel.org/stable/c/ffdc844e673d2bcb2af8c8c3eef8cdea59d0bf32)
- [CNA](https://git.kernel.org/stable/c/00c84f4eec715e501efc080a5670114189e42507)
- [CNA](https://git.kernel.org/stable/c/2d9846fd1c767920fddd4bde30eb35cd2969df13)
- [CNA](https://git.kernel.org/stable/c/977e6f006a7a3ffc4216ae6034f768f8de6fd138)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.76%
- **EPSS Percentile:** 53.7

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