# CVE-2026-98541: fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT

> A local attacker with high privileges can cause a memory leak in the Linux kernel's FOU module by omitting a protocol attribute, leading to denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** An administrator
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-109124, BELL-CVE-2026-98541, DEBIAN-CVE-2026-98541, GHSA-rj48-hq74-h6fv, UBUNTU-CVE-2026-98541

## What is CVE-2026-98541?

CVE-2026-98541 is a memory corruption vulnerability in Linux, published on 18 Oct 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:

fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT

Commit 7a9bc9e3f423 ("fou: Don't allow 0 for FOU_ATTR_IPPROTO.") added
NLA_POLICY_MIN(NLA_U8, 1) to fou_nl_policy[FOU_ATTR_IPPROTO], which
rejects an explicitly supplied FOU_ATTR_IPPROTO == 0 attribute with
-ERANGE.

However, FOU_ATTR_IPPROTO is an optional netlink attribute. When a user
sends FOU_CMD_ADD with FOU_ATTR_TYPE set to FOU_ENCAP_DIRECT and omits
FOU_ATTR_IPPROTO entirely, nla_policy validation succeeds and
parse_nl_config() leaves cfg->protocol as 0 (from memset(cfg, 0,
sizeof(*cfg))). fou_create() then creates a FOU_ENCAP_DIRECT socket with
fou->protocol == 0.

In fou_udp_recv(), returning -fou->protocol to udp_queue_rcv_one_skb()
triggers IP protocol resubmission when fou->protocol > 0, whereas
returning 0 tells the UDP tunnel layer that the skb was consumed without
freeing it. When fou->protocol == 0, every packet received on the socket
returns 0 from fou_udp_recv() and leaks the sk_buff.

Reject FOU_ENCAP_DIRECT when !cfg->protocol in fou_create() so that
creating a direct encapsulation port without FOU_ATTR_IPPROTO fails with
-EINVAL while leaving FOU_CMD_DEL and FOU_CMD_GET (which share
parse_nl_config()) unaffected.

Tested in QEMU against Linux 7.3.0-rc3 by sending a FOU_CMD_ADD Generic
Netlink request with FOU_ATTR_PORT = 5555 and FOU_ATTR_TYPE =
FOU_ENCAP_DIRECT while omitting FOU_ATTR_IPPROTO. On the unfixed kernel,
FOU_CMD_ADD succeeds (err = 0), FOU_CMD_GET reports fou->type = 1 and
fou->protocol = 0, and sending 4000 UDP packets to 127.0.0.1:5555 leaks
all 4000 sk_buffs (SUnreclaim in /proc/meminfo grows from 41456 kB to
59008 kB, +17552 kB); with this patch applied, FOU_CMD_ADD is rejected
with -EINVAL (-22).

## Which versions are affected by CVE-2026-98541?


## How do I fix CVE-2026-98541?

Apply the vendor's patch.

## Is CVE-2026-98541 being exploited?

There is no known exploitation of CVE-2026-98541 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98541 and fix it if it is affected.

What it is: CVE-2026-98541 is a memory corruption vulnerability in Linux, published on 18 Oct 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-2026-98541 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-2026-98541/ (Markdown: https://insights.sbomify.com/CVE-2026-98541/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98541
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98541
- OSV: https://osv.dev/vulnerability/CVE-2026-98541

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98541/
Summaries are generated and may contain mistakes; the official description above is the record.