Cloudanix VulnDB

CVE-2026-89674

Critical · 9.8

CVSS

9.8

EPSS

0.5%

Published

Sep 11, 2026

CISA due date

—

Cloudanix analysis

Next patch cycle SSVC: Track

A flaw in Linux's NFS server code could let remote attackers run code on the server or read its internal memory, but the risk remains low because no active attacks are known.

Exploitation
Remote Unauthenticated
Attacker needs
None
Complexity
Low
Impact
Full Compromise
Component
OS kernel
Class
Memory Corruption
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

Incorrect XDR buffer size calculations in nfsd4_ff_encode_layoutget result in out-of-bounds writes up to 5 bytes or leakage of uninitialized kernel memory when encoding file layout responses, due to mishandled padding, alignment, and header constants for file handles, uids, gids, and ds_len fields.

Why it matters to the business

Remote attackers could gain code execution or extract sensitive kernel data from Linux NFS servers, risking full system compromise; however low exploit maturity and stable low EPSS indicate limited active threat at 43 days post-publication with no KEV or ransomware use.

How an attack unfolds

An attacker sends a crafted NFSv4 LAYOUTGET request containing short strings (uid/gid=0) and an odd-sized file handle to an exposed nfsd server, triggering the miscalculated XDR reservation to either corrupt adjacent memory or return stale buffer contents containing kernel data.

How to fix it

Apply the upstream kernel patch that rewrites ds_len calculation to explicitly account for every field, uses xdr_align_size for all opaque values, and corrects header constants; rebuild and reboot affected systems.

How to confirm the fix worked

Confirm the fix by inspecting nfsd4_ff_encode_layoutget source for explicit per-field length sums and xdr_align_size calls; test with crafted short-string LAYOUTGET requests and verify no overflow or stale memory in responses.

Typical attack path

  1. Establish TCP connection to NFS server port 2049
  2. Negotiate NFSv4 and issue LAYOUTGET with flexible file layout type
  3. Supply uid/gid and fh_len values that expose the padding/alignment mismatch
  4. Receive response that either overflows the send buffer or leaks kernel memory
  5. Leverage corruption for privilege escalation or use leaked data for further attacks

How to detect it

Review kernel logs and NFS audit trails for unexpected crashes or oversized LAYOUTGET responses; inspect network captures for NFSv4 operations with unusual uid/gid lengths; correlate with any sudden memory disclosure artifacts in client responses.

What your security team should do

  • Prioritize kernel updates on all NFS servers within standard patch windows given low EPSS and no KEV status
  • Restrict NFSv4 access to trusted clients via firewalls or export options until patched
  • Enable kernel memory protection features such as KASAN or hardened usercopy in test environments

The record

In the Linux kernel, the following vulnerability has been resolved: nfsd: fix XDR length calculation in nfsd4_ff_encode_layoutget The XDR buffer size calculation in nfsd4_ff_encode_layoutget() has multiple errors that can result in either an out-of-bounds write or leaking uninitialized kernel memory to the client: - fh_len doesn't account for XDR padding on the file handle data - uid and gid lengths use "8 + len" but xdr_encode_opaque() actually writes "4 + xdr_align_size(len)" bytes - ds_len omits the flags and stats_collect_hint fields (8 bytes), while len's header constant overestimates by 8 bytes -- these partially cancel but leave a net mismatch The worst case occurs with short strings (e.g. uid=0, gid=0 with an odd-sized file handle), where the function writes up to 5 bytes past the reserved XDR buffer. Conversely, when string lengths happen to be 4-byte aligned, the reservation is too large and stale buffer content is sent to the client. Fix this by breaking out every encoded field explicitly in the ds_len calculation, using xdr_align_size() for all variable-length opaque fields, and correcting the header constants.

Affected packages

Ecosystem Package Introduced Fixed