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.
- 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
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: 1 proofs of concept on GitHub.
Sources
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.