---
id: "CVE-2026-99110"
kind: "vulnerability"
aliases: ["CVE-2026-99110"]
title: "Linux kernel: CIFS ACL write overflows heap buffer on oversized ACLs"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["cifs", "kernel", "linux", "runtime", "self-hosted", "smb"]
not_affected_if: ["cifs-module-not-loaded", "no-cifs-mounts"]
published_at: "2026-10-28T11:17:30+00:00"
first_seen_at: "2026-10-28T11:32:34+00:00"
updated_at: "2026-10-28T11:37:45+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99110", "first_seen_at": "2026-10-28T11:32:34+00:00"}]
---

# Linux kernel: CIFS ACL write overflows heap buffer on oversized ACLs

The Linux kernel's SMB client (cifs) can write a CIFS ACL larger than the buffer it allocated when setting file ACLs, because the size check uses the POSIX xattr format rather than the larger CIFS wire format. The overflow is a kernel heap write, which can corrupt memory and may allow privilege escalation, and oversized ACLs can also truncate the packet so the server applies a partial ACL.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the running kernel version with `uname -r` and compare it to the fixed release in the kernel stable branch.
- Check whether the cifs module is loaded with `lsmod | grep cifs`; systems that never mount SMB shares are not exposed.
- unknown for a fixed version number, since the sources do not list one.

## Fix

- Update the Linux kernel to a release that includes the cifs_do_set_acl() fix (version unknown from the sources).
- Until patched, unload or avoid mounting SMB/CIFS shares with ACL support on untrusted servers.

## Timeline

- 2026-10-28T11:32:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99110

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

smb: client: fix heap overflow in cifs_do_set_acl()

cifs_set_acl() validates ACL size using posix_acl_xattr_size():

        4 + (count * 8)  // 4-byte header + 8 bytes per ACE

cifs_do_set_acl() then calls posix_acl_to_cifs() to write the CIFS
wire format into the same buffer:

        6 + (count * 10)  // 6-byte header + 10 bytes per ACE

An ACL that passes the xattr-based check in cifs_set_acl() can
overflow the heap when posix_acl_to_cifs() writes the larger CIFS
format.

Validate the CIFS format size against the remaining buffer space and
USHRT_MAX before converting--data_count is __u16, so sizes above
USHRT_MAX truncate the on-wire packet length, causing the server to
apply a partial ACL.  Replace MaxDataCount = 1000 with
min(CIFSMaxBufSize, USHRT_MAX).

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_