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

> AppArmor fails to mediate implicit connections in TCP Fast Open, allowing confined processes to bypass network restrictions.

- **Severity:** High 8.4 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N`)
- **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:** 19 Jul 2026
- **Updated:** 17 Aug 2026
- **Also known as:** BELL-CVE-2026-63828, DEBIAN-CVE-2026-63828, GHSA-fvhv-v8p3-vrwq, ROOT-OS-DEBIAN-11-CVE-2026-63828, ROOT-OS-UBUNTU-2204-CVE-2026-63828, UBUNTU-CVE-2026-63828

## What is CVE-2026-63828?

CVE-2026-63828 is a high-severity vulnerability in Linux, published on 19 Jul 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:

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.

## Which versions are affected by CVE-2026-63828?


## How do I fix CVE-2026-63828?

Apply the vendor's patch.

## Is CVE-2026-63828 being exploited?

There is no known exploitation of CVE-2026-63828 in the wild and no public exploit code.

Public exploit code:

- Using TCP Fast Open to bypass syscall-based networking rules (CVE-2026-63828/CVE-2026-72243 PoC) (github_poc): https://github.com/4n4s4zi/tfo-connect-bypass

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63828 and fix it if it is affected.

What it is: CVE-2026-63828 is a high-severity vulnerability in Linux, published on 19 Jul 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-2026-63828 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-2026-63828/ (Markdown: https://insights.sbomify.com/CVE-2026-63828/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63828
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63828
- OSV: https://osv.dev/vulnerability/CVE-2026-63828

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63828/
Summaries are generated and may contain mistakes; the official description above is the record.