CVE-2026-89695 nfsd: cap decoded POSIX ACL count to bound sort cost
A remote attacker can cause a denial of service in the Linux kernel NFS server by sending a crafted POSIX ACL count that triggers excessive CPU usage during sorting.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89695?
CVE-2026-89695 is a high-severity denial of service 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: cap decoded POSIX ACL count to bound sort cost
nfsd4_decode_posixacl() reads a u32 entry count off the wire and passes it straight to posix_acl_alloc() and sort_pacl_range(). The latter is an O(n^2) bubble sort, so a client-chosen count drives unbounded CPU in the server's compound processing path.
nfsd4_decode_posixacl() xdr_stream_decode_u32(&count) /* uncapped u32 */ posix_acl_alloc(count, GFP_KERNEL) sort_pacl_range(*acl, 0, count - 1) /* O(n^2) bubble sort */
The encoder side in the same file already rejects ACLs whose a_count exceeds NFS_ACL_MAX_ENTRIES, but the decoder introduced in commit 5fc51dfc2eb1 ("NFSD: Add support for XDR decoding POSIX draft ACLs") omitted the symmetric check.
Fix by rejecting a wire count greater than NFS_ACL_MAX_ENTRIES with nfserr_inval, before any allocation, so the sort is bounded by NFS_ACL_MAX_ENTRIES^2 comparisons.
While we're in here, also fix the nfserr_resource return if posix_acl_alloc() fails. That's not a legal error code for v4.1+. Change it to return nfserr_jukebox as that's more appropriate for memory allocation failures.
Which versions are affected by CVE-2026-89695?
How do I fix CVE-2026-89695?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-89695 being exploited?
There is no known exploitation of CVE-2026-89695 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-89695, GHSA-rh54-vrwc-232g, UBUNTU-CVE-2026-89695.