# CVE-2026-90139: fuse: check for NULL root inode in fuse_fill_super_submount

> A local attacker can trigger a kernel crash via null pointer dereference in FUSE submount handling under memory pressure.

- **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:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102387, BELL-CVE-2026-90139, DEBIAN-CVE-2026-90139, GHSA-462h-76x8-ccqr, UBUNTU-CVE-2026-90139

## What is CVE-2026-90139?

CVE-2026-90139 is a vulnerability in Linux, published on 17 Sep 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:

fuse: check for NULL root inode in fuse_fill_super_submount

fuse_iget() can return NULL when its inode allocation fails, but
fuse_fill_super_submount() passed the result straight to get_fuse_inode()
and decremented fi->nlookup without checking it:

        root = fuse_iget(sb, parent_fi->nodeid, ...);
        fi = get_fuse_inode(root);
        fi->nlookup--;

Inside fuse_iget() the inode allocation can fail and return NULL.  The
submount root takes the iget5_locked() path, whose alloc_inode() can fail
under memory pressure (the auto-submount branch can fail the same way in
new_inode() or fuse_alloc_submount_lookup()):

        inode = iget5_locked(sb, nodeid, fuse_inode_eq, fuse_inode_set,
                             &nodeid);
        if (!inode)
                return NULL;

A NULL root makes get_fuse_inode() a container_of() on NULL and the
nlookup decrement a write to a bogus address, oopsing the mount.  With
CONFIG_KASAN the following null pointer dereference is reported when the
root inode allocation of an auto-submount fails (e.g. under memory
pressure):

==================================================================
BUG: KASAN: null-ptr-deref in fuse_get_tree_submount+0x656/0x8b0
Read of size 8 at addr 00000000000002b0 by task ls/942
CPU: 0 PID: 942 Comm: ls Tainted: G W 6.6 #15
Call Trace:
 <TASK>
 fuse_get_tree_submount+0x656/0x8b0
 vfs_get_tree+0x48/0x140
 fc_mount+0x13/0x50
 fuse_dentry_automount+0x7a/0xb0
 __traverse_mounts+0xca/0x330
 step_into+0x339/0xac0
 path_lookupat+0xc5/0x2f0
 filename_lookup+0x163/0x2a0
 vfs_statx+0xd5/0x200
 do_statx+0x83/0xd0
 __x64_sys_statx+0xa0/0xc0
 do_syscall_64+0x37/0x90
 entry_SYSCALL_64_after_hwframe+0x78/0xe2
 </TASK>
==================================================================

Return -ENOMEM instead; the caller tears down the partially built
superblock on error, matching the other error returns in this
function.

## Which versions are affected by CVE-2026-90139?


## How do I fix CVE-2026-90139?

Apply the vendor's patch.

## Is CVE-2026-90139 being exploited?

There is no known exploitation of CVE-2026-90139 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90139 and fix it if it is affected.

What it is: CVE-2026-90139 is a vulnerability in Linux, published on 17 Sep 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-2026-90139 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-2026-90139/ (Markdown: https://insights.sbomify.com/CVE-2026-90139/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90139
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90139
- OSV: https://osv.dev/vulnerability/CVE-2026-90139

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90139/
Summaries are generated and may contain mistakes; the official description above is the record.