# CVE-2025-40072: fanotify: Validate the return value of mnt_ns_from_dentry() before dereferencing

> A local attacker can trigger a kernel NULL pointer dereference in fanotify, causing a system crash and denial of service.

- **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:** 28 Oct 2025
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2025-40072, DEBIAN-CVE-2025-40072, GHSA-w5hw-p7rf-8672, UBUNTU-CVE-2025-40072

## What is CVE-2025-40072?

CVE-2025-40072 is a vulnerability in Linux, published on 28 Oct 2025. 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:

fanotify: Validate the return value of mnt_ns_from_dentry() before dereferencing

The function do_fanotify_mark() does not validate if
mnt_ns_from_dentry() returns NULL before dereferencing mntns->user_ns.
This causes a NULL pointer dereference in do_fanotify_mark() if the
path is not a mount namespace object.

Fix this by checking mnt_ns_from_dentry()'s return value before
dereferencing it.

Before the patch

$ gcc fanotify_nullptr.c -o fanotify_nullptr
$ mkdir A
$ ./fanotify_nullptr
Fanotify fd: 3
fanotify_mark: Operation not permitted
$ unshare -Urm
Fanotify fd: 3
Killed

int main(void){
    int ffd;
    ffd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_MNT, 0);
    if(ffd < 0){
        perror("fanotify_init");
        exit(EXIT_FAILURE);
    }

    printf("Fanotify fd: %d\n",ffd);

    if(fanotify_mark(ffd, FAN_MARK_ADD | FAN_MARK_MNTNS,
FAN_MNT_ATTACH, AT_FDCWD, "A") < 0){
        perror("fanotify_mark");
        exit(EXIT_FAILURE);
    }

return 0;
}

After the patch

$ gcc fanotify_nullptr.c -o fanotify_nullptr
$ mkdir A
$ ./fanotify_nullptr
Fanotify fd: 3
fanotify_mark: Operation not permitted
$ unshare -Urm
Fanotify fd: 3
fanotify_mark: Invalid argument

[   25.694973] BUG: kernel NULL pointer dereference, address: 0000000000000038
[   25.695006] #PF: supervisor read access in kernel mode
[   25.695012] #PF: error_code(0x0000) - not-present page
[   25.695017] PGD 109a30067 P4D 109a30067 PUD 142b46067 PMD 0
[   25.695025] Oops: Oops: 0000 [#1] SMP NOPTI
[   25.695032] CPU: 4 UID: 1000 PID: 1478 Comm: fanotify_nullpt Not
tainted 6.17.0-rc4 #1 PREEMPT(lazy)
[   25.695040] Hardware name: VMware, Inc. VMware Virtual
Platform/440BX Desktop Reference Platform, BIOS 6.00 11/12/2020
[   25.695049] RIP: 0010:do_fanotify_mark+0x817/0x950
[   25.695066] Code: 04 00 00 e9 45 fd ff ff 48 8b 7c 24 48 4c 89 54
24 18 4c 89 5c 24 10 4c 89 0c 24 e8 b3 11 fc ff 4c 8b 54 24 18 4c 8b
5c 24 10 <48> 8b 78 38 4c 8b 0c 24 49 89 c4 e9 13 fd ff ff 8b 4c 24 28
85 c9
[   25.695081] RSP: 0018:ffffd31c469e3c08 EFLAGS: 00010203
[   25.695104] RAX: 0000000000000000 RBX: 0000000001000000 RCX: ffff8eb48aebd220
[   25.695110] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8eb4835e8180
[   25.695115] RBP: 0000000000000111 R08: 0000000000000000 R09: 0000000000000000
[   25.695142] R10: ffff8eb48a7d56c0 R11: ffff8eb482bede00 R12: 00000000004012a7
[   25.695148] R13: 0000000000000110 R14: 0000000000000001 R15: ffff8eb48a7d56c0
[   25.695154] FS:  00007f8733bda740(0000) GS:ffff8eb61ce5f000(0000)
knlGS:0000000000000000
[   25.695162] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   25.695170] CR2: 0000000000000038 CR3: 0000000136994006 CR4: 00000000003706f0
[   25.695201] Call Trace:
[   25.695209]  <TASK>
[   25.695215]  __x64_sys_fanotify_mark+0x1f/0x30
[   25.695222]  do_syscall_64+0x82/0x2c0
...

## Which versions are affected by CVE-2025-40072?


## How do I fix CVE-2025-40072?

Apply the vendor's patch.

## Is CVE-2025-40072 being exploited?

There is no known exploitation of CVE-2025-40072 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-40072 and fix it if it is affected.

What it is: CVE-2025-40072 is a vulnerability in Linux, published on 28 Oct 2025. 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-2025-40072 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-2025-40072/ (Markdown: https://insights.sbomify.com/CVE-2025-40072/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-40072
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-40072
- OSV: https://osv.dev/vulnerability/CVE-2025-40072

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-40072/
Summaries are generated and may contain mistakes; the official description above is the record.