# CVE-2024-49940: l2tp: prevent possible tunnel refcount underflow

> A race condition in the Linux kernel L2TP module can cause a tunnel refcount underflow, potentially leading to a denial of service.

- **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
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 21 Oct 2024
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-52917, AZL-52947, BELL-CVE-2024-49940, DEBIAN-CVE-2024-49940, GHSA-h9jc-p2cr-c4rx, ROOT-OS-DEBIAN-11-CVE-2024-49940, ROOT-OS-DEBIAN-12-CVE-2024-49940, ROOT-OS-UBUNTU-2204-CVE-2024-49940, ROOT-OS-UBUNTU-2404-CVE-2024-49940, UBUNTU-CVE-2024-49940

## What is CVE-2024-49940?

CVE-2024-49940 is a high-severity vulnerability in Linux kernel, published on 21 Oct 2024. 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:

l2tp: prevent possible tunnel refcount underflow

When a session is created, it sets a backpointer to its tunnel. When
the session refcount drops to 0, l2tp_session_free drops the tunnel
refcount if session->tunnel is non-NULL. However, session->tunnel is
set in l2tp_session_create, before the tunnel refcount is incremented
by l2tp_session_register, which leaves a small window where
session->tunnel is non-NULL when the tunnel refcount hasn't been
bumped.

Moving the assignment to l2tp_session_register is trivial but
l2tp_session_create calls l2tp_session_set_header_len which uses
session->tunnel to get the tunnel's encap. Add an encap arg to
l2tp_session_set_header_len to avoid using session->tunnel.

If l2tpv3 sessions have colliding IDs, it is possible for
l2tp_v3_session_get to race with l2tp_session_register and fetch a
session which doesn't yet have session->tunnel set. Add a check for
this case.

## Which versions are affected by CVE-2024-49940?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2024-49940?

Apply the vendor's patch.

## Is CVE-2024-49940 being exploited?

There is no known exploitation of CVE-2024-49940 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2024-49940 and fix it if it is affected.

What it is: CVE-2024-49940 is a high-severity vulnerability in Linux kernel, published on 21 Oct 2024. 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-2024-49940 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2024-49940/ (Markdown: https://insights.sbomify.com/CVE-2024-49940/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2024-49940
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2024-49940
- OSV: https://osv.dev/vulnerability/CVE-2024-49940

---
From sbomify Insights: https://insights.sbomify.com/CVE-2024-49940/
Summaries are generated and may contain mistakes; the official description above is the record.