CVE-2026-89678 nfsd: fix partial-write detection in nfsd_direct_write
A logic error in the Linux kernel NFS server allows partial writes to corrupt file offsets, leading to data modification.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89678?
CVE-2026-89678 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: fix partial-write detection in nfsd_direct_write
nfsd_direct_write() walks a list of write segments and, after each vfs_iocb_iter_write(), tries to detect a short write so the loop can stop before placing the next segment at a wrong file offset:
host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter); if (host_err < 0) return host_err; *cnt += host_err; if (host_err < segments[i].iter.count) break; /* partial write */
vfs_iocb_iter_write() runs the iter through ->write_iter(), which advances the iter by the number of bytes written. By the time the check runs, segments[i].iter.count is the residual, not the original request length:
before write_iter: iter.count == original_len after write_iter: iter.count == original_len - host_err
The condition then reduces to host_err < original_len - host_err, so the break fires only when less than half of the segment was written. Any short write completing between 50% and 99% of the segment slips through; the loop advances to the next segment with kiocb->ki_pos only bumped by the short amount, writing the next segment's payload at the wrong offset and over-reporting *cnt to the NFS client.
Snapshot the segment's byte count before the write and compare host_err against that snapshot so any short write breaks the loop.
Which versions are affected by CVE-2026-89678?
How do I fix CVE-2026-89678?
Apply the vendor's patch.
Is CVE-2026-89678 being exploited?
There is no known exploitation of CVE-2026-89678 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-89678, GHSA-qpfv-2qf2-xr5w, UBUNTU-CVE-2026-89678.