# CVE-2026-98991: namespace: queue a mount only once for mount notifications


- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 23 Oct 2026
- **Updated:** 24 Oct 2026
- **Also known as:** BELL-CVE-2026-98991, DEBIAN-CVE-2026-98991, GHSA-4qhh-mf87-xrfm, UBUNTU-CVE-2026-98991

## What is CVE-2026-98991?

CVE-2026-98991 is a high-severity vulnerability in Linux, published on 23 Oct 2026. 

### Official description

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

namespace: queue a mount only once for mount notifications

mnt_notify_add() puts a mount on notify_list. It doesn't check whether
the mount is on the list already. That's fine as long as every mount is
queued at most once per namespace_sem hold. It isn't.

propagate_umount() moves a surviving overmount off a stack of mounts
that are going away and queues it as moved. shrink_submounts() and
mark_mounts_for_expiry() call umount_tree() for several mounts under a
single namespace_sem hold. So the second umount_tree() can take down
exactly the mount the first one reparented, reparent it once more and so
end up queueing it a second time.

The second list_add_tail() cuts the mounts queued in between out of the
list while the head still points at the last of them. notify_mnt_list()
then only visits that one mount and the others are freed after the grace
period with notify_list still pointing at them. Every later mount
operation on the host walks freed memory:

  BUG: KASAN: slab-use-after-free in __list_add_valid_or_report
  Read of size 8 at addr ffff8881003d57e0 by task notify_dq/162
   __list_add_valid_or_report+0x15c/0x1a0
   mnt_add_to_ns+0x322/0x890
   attach_recursive_mnt.isra.0+0xf2a/0x1a00
   path_mount+0x139c/0x1d40

This needs a mount with MNT_SHRINKABLE to bind from. That's what
finish_automount() creates (submounts of NFS, AFS and CIFS and the
tracefs mount below debugfs). Bind mounts inherit the flag. A mount
namespace with a FAN_MARK_MNTNS mark does the rest and root in a user
namespace can have both.

Initialize to_notify in alloc_vfsmnt() and leave a mount alone that is
queued already. mnt_notify() looks at the state the mount has when the
list is drained so a single entry per mount is enough. The move event
for a mount that is unmounted under the same hold is lost. That's fine.
The detach is what matters.

## Which versions are affected by CVE-2026-98991?


## How do I fix CVE-2026-98991?

Upgrade to a fixed version.

## Is CVE-2026-98991 being exploited?

There is no known exploitation of CVE-2026-98991 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98991 and fix it if it is affected.

What it is: CVE-2026-98991 is a high-severity vulnerability in Linux, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-98991 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-98991/ (Markdown: https://insights.sbomify.com/CVE-2026-98991/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98991
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98991
- OSV: https://osv.dev/vulnerability/CVE-2026-98991

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98991/
Summaries are generated and may contain mistakes; the official description above is the record.