# CVE-2026-72379: fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid

> Local attackers can create files with invalid ownership on idmapped mounts, potentially leading to privilege escalation or data integrity issues.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 17 Aug 2026
- **Also known as:** BELL-CVE-2026-72379, DEBIAN-CVE-2026-72379, GHSA-xpvr-4r6x-x86m, UBUNTU-CVE-2026-72379

## What is CVE-2026-72379?

CVE-2026-72379 is a vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

fs: refuse O_TMPFILE creation with an unmapped fsuid or fsgid

vfs_tmpfile() never checked that the caller's fsuid and fsgid map into
the filesystem.  On an idmapped mount whose idmapping does not cover the
caller's fs{u,g}id, the ->tmpfile() instance initializes the new inode
through inode_init_owner(), where mapped_fsuid()/mapped_fsgid() return
INVALID_UID/INVALID_GID, and the tmpfile ends up owned by (uid_t)-1.

Every other creation path already refuses this: may_o_create() (O_CREAT)
and may_create_dentry() (mkdir, mknod, symlink, link) bail out with
-EOVERFLOW via fsuidgid_has_mapping() precisely so that an object cannot
be created with an owner the filesystem cannot represent.  An O_TMPFILE
is no exception: it is created I_LINKABLE and linkat(2) can splice it
into the namespace afterwards, so the same guarantee must hold.

Add the missing fsuidgid_has_mapping() check to vfs_tmpfile().  On a
non-idmapped mount the caller's fs{u,g}id always map in the superblock's
user namespace, so this is a no-op there and only takes effect on an
idmapped mount that does not map the caller.  It applies to every
filesystem that sets FS_ALLOW_IDMAP and implements ->tmpfile() (tmpfs,
ext4, btrfs, xfs, f2fs, ...), and to overlayfs, whose upper-layer
tmpfile creation funnels through vfs_tmpfile() via backing_tmpfile_open().

## Which versions are affected by CVE-2026-72379?


## How do I fix CVE-2026-72379?

Apply the vendor's patch.

## Is CVE-2026-72379 being exploited?

There is no known exploitation of CVE-2026-72379 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-72379 and fix it if it is affected.

What it is: CVE-2026-72379 is a vulnerability in Linux, published on 15 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-72379 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-2026-72379/ (Markdown: https://insights.sbomify.com/CVE-2026-72379/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-72379
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-72379
- OSV: https://osv.dev/vulnerability/CVE-2026-72379

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-72379/
Summaries are generated and may contain mistakes; the official description above is the record.