CVE-2026-89711 NFSD: remove flawed WARN_ON_ONCE from nfsd_mode_check
A flawed warning check in the Linux kernel NFSD module can lead to a null pointer dereference during NFS reexports, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89711?
CVE-2026-89711 is a high-severity vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-89711?
How do I fix CVE-2026-89711?
Apply the vendor's patch.
Is CVE-2026-89711 being exploited?
There is no known exploitation of CVE-2026-89711 in the wild and no public exploit code.
Sources
Also known as AZL-100889, BELL-CVE-2026-89711, DEBIAN-CVE-2026-89711, GHSA-f2q5-7q65-xqx7, UBUNTU-CVE-2026-89711.