CVE-2026-89713 NFSD: check truncate permission under inode lock
A race condition in the Linux kernel NFS server allows attackers to bypass append-only file protections and truncate files via concurrent operations.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89713?
CVE-2026-89713 is a critical 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: check truncate permission under inode lock
nfsd_setattr() checks whether a size update needs NFSD_MAY_TRUNC before it takes inode_lock(). The comparison uses the file size sampled by that unlocked read, but the actual ATTR_SIZE update is applied later under inode_lock() by notify_change().
This leaves a TOCTOU window for append-only files. If a client sends a SETATTR that does not shrink the file at the time of the unlocked sample, a concurrent append can extend the file before nfsd_setattr() takes inode_lock(). notify_change() then applies a real truncation without the NFSD_MAY_TRUNC check that rejects IS_APPEND(inode). The VFS truncate syscall paths perform their own append-only checks before calling notify_change(), so NFSD must make this decision against the locked size it is about to change.
Split the write-count acquisition from the truncation permission check. Keep get_write_access() before the locked setattr work, then recheck whether the requested size is below i_size_read(inode) after inode_lock() has been acquired and before notify_change(ATTR_SIZE). This also avoids the plain unlocked inode->i_size load.
Which versions are affected by CVE-2026-89713?
How do I fix CVE-2026-89713?
Apply the vendor's patch.
Is CVE-2026-89713 being exploited?
There is no known exploitation of CVE-2026-89713 in the wild and no public exploit code.
Sources
Also known as AZL-100142, BELL-CVE-2026-89713, DEBIAN-CVE-2026-89713, GHSA-v99p-5h33-35p9, UBUNTU-CVE-2026-89713.