CVE-2026-89665
High · 8.2CVSS
8.2
EPSS
0.6%
Published
Sep 11, 2026
CISA due date
—
Cloudanix analysis
A bug in handling certain old file-sharing requests on Linux servers could let attackers quietly alter file timestamps without triggering normal error checks.
- Exploitation
- Remote Unauthenticated
- Attacker needs
- None
- Complexity
- Low
- Impact
- Data Tampering
- Component
- OS kernel
- Class
- Improper Input Validation
- Remediation
- Upgrade
- Effort
- Medium
- Patch risk
- Low
What it actually does
Allows out-of-range usecond values (>1000000) in NFSv2 SETATTR/CREATE to wrap during multiplication in svcxdr_decode_sattr on ILP32 systems, producing in-range tv_nsec values that bypass later checks and corrupt atime/mtime before proc functions run.
Why it matters to the business
May silently corrupt file timestamps on NFS servers supporting legacy NFSv2, affecting data integrity checks, backups, and compliance logs with low but non-zero exploitation likelihood.
How an attack unfolds
Attacker sends crafted NFSv2 RPCs containing usecond values such as 4294968 in atime or mtime fields; decoder multiplies into a wrapped nanosecond result that passes range validation, storing bogus timestamps on the filesystem while returning GARBAGE_ARGS only for fully invalid cases.
How to fix it
Apply the upstream kernel patch that adds an explicit guard rejecting usecond values greater than 1000000 before multiplication, symmetrically clears ATTR_ATIME_SET for the Sun convention value, and ensure the fix is present in deployed kernels.
How to confirm the fix worked
Confirm the range check (tmp2 > 1000000) exists in svcxdr_decode_sattr, recompile and test with an NFSv2 client sending usecond=1000001, and verify GARBAGE_ARGS is returned without timestamp corruption.
Typical attack path
- Craft NFSv2 SETATTR or CREATE request with usecond > 1000000 in atime/mtime
- Trigger svcxdr_decode_sattr multiplication on ILP32 kernel
- Wrapped tv_nsec evades range check and ATTR_*_SET flags remain set
- Filesystem applies corrupted timestamp
How to detect it
Review kernel NFS logs and packet captures for NFSv2 SETATTR/CREATE operations containing usecond fields exceeding 1000000; inspect filesystem metadata for unexpected atime/mtime values originating from NFSv2 clients.
What your security team should do
- Disable NFSv2 support on all servers if legacy clients are not required
- Apply the decoder guard patch to any remaining NFSv2-enabled kernels
- Audit NFSv2 usage inventory and schedule removal within 30 days
The record
In the Linux kernel, the following vulnerability has been resolved: nfsd: reject out-of-range useconds in NFSv2 SETATTR/CREATE The NFSv2 sattr decoder converts the wire useconds to nanoseconds in svcxdr_decode_sattr(): iap->ia_atime.tv_nsec = tmp2 * NSEC_PER_USEC; tmp2 is a u32 and NSEC_PER_USEC is 1000, so the product is computed in unsigned long. On ILP32 that is 32 bits, and an out-of-range useconds value such as 4294968 wraps to tv_nsec == 704. The corruption therefore happens during decode, before any proc function can inspect the value, and a later range check on tv_nsec would see an in-range result and accept it. Rejecting in the decoder yields an RPC GARBAGE_ARGS reply. NFSv2 defines no NFSERR_INVAL, so there is no NFS-level status to return for a malformed time argument, and the check cannot move to the proc function the way the v3/v4 nsec range checks do. Guard the raw useconds before the multiplication and reject values greater than 1000000. useconds == 1000000 is kept: it is the Sun convention for "set to the current server time", and the in-tree Linux NFSv2 client emits it in both the atime and the mtime field for a plain touch / utimes(file, NULL) (see encode_sattr() and xdr_encode_current_server_time() in fs/nfs/nfs2xdr.c). Rejecting 1000000 would turn that common operation into a hard decode failure for both SETATTR and CREATE. 1000000 * NSEC_PER_USEC is 10^9, which does not wrap on ILP32, so the Sun convention value passes through safely. Only genuinely out-of-range values (> 1000000) are rejected. The atime and mtime guards are therefore symmetric. The decoder only applied the Sun convention in the mtime block, which clears ATTR_ATIME_SET|ATTR_MTIME_SET when mtime useconds == 1000000. If a client puts 1000000 in the atime field but not in the mtime field, the atime block stored an out-of-range tv_nsec (10^9) and left ATTR_ATIME_SET set, so the bogus value reached the filesystem. Apply the convention in the atime block as well, clearing ATTR_ATIME_SET so the server uses its current time and ignores the value. Only ATTR_ATIME_SET is cleared there. The mtime block keeps its existing behavior, where 1000000 means "set both atime and mtime to now". [ cel: various tweaks, addenda, and clean-ups ]
Affected packages
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