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PatchSiren cyber security CVE debrief

CVE-2026-63828 Linux CVE debrief

The Linux kernel has a vulnerability that has been resolved in the apparmor module. The vulnerability relates to the implicit connection of TCP fast open sendmsg. When using sendmsg()/sendto() with MSG_FASTOPEN, it combines the connect(2) and write(2) operations, opening a connection in the SYN state. The apparmor_socket_sendmsg() function only checks AA_MAY_SEND, allowing a profile that grants send but denies connect to open an outbound TCP/MPTCP connection that connect(2) would have refused. This bypasses connect mediation.

Vendor
Linux
Product
Unknown
CVSS
Unknown
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-19
Original CVE updated
2026-07-19
Advisory published
2026-07-19
Advisory updated
2026-07-19

Who should care

Users of the Linux kernel who utilize the apparmor module for security mediation should be aware of this vulnerability. This includes Linux distribution maintainers, system administrators, and users who rely on apparmor for enforcing security policies on their systems.

Technical summary

The vulnerability is in the apparmor module of the Linux kernel. Specifically, it involves the handling of MSG_FASTOPEN in sendmsg()/sendto() calls, which can implicitly establish a connection. The current implementation of apparmor_socket_sendmsg() does not adequately mediate this implicit connect when a destination is supplied and MSG_FASTOPEN is set. This allows confined tasks to bypass connect mediation, potentially leading to unauthorized outbound connections.

Defensive priority

High

Recommended defensive actions

  • Apply the official patches or updates provided by the Linux kernel maintainers to resolve the vulnerability.
  • Review and update apparmor profiles to ensure they properly handle connect and send operations.
  • Monitor system logs for unusual connection attempts that may indicate exploitation.
  • Consider implementing additional compensating controls, such as network traffic monitoring and filtering, to detect and prevent potential exploitation.
  • Perform a thorough review of the affected systems to identify potential exposure.
  • Track exceptions and retest remediated assets to ensure the vulnerability is properly mitigated.
  • Document evidence of remediation and verification for future reference.

Evidence notes

The CVE record and associated details were obtained from the Linux kernel sources and CVE.org. The vulnerability has been resolved through specific commits in the Linux kernel repository. Evidence is limited to public sources and may not reflect the full scope or impact of the vulnerability. Defenders should verify the official patches or updates provided by the Linux kernel maintainers and review apparmor profiles to ensure they properly handle connect and send operations.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-07-19T12:16:56.110Z and has not been modified since then.