CVE-2026-63828 in Linux
Summary
by MITRE • 07/19/2026
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.
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability resides in the Linux kernel's AppArmor security module where a mediation bypass occurs during TCP Fast Open operations. The issue manifests when applications use sendmsg() or sendto() functions with the MSG_FASTOPEN flag, which internally combines connect() and write() operations into a single SYN packet transmission. The fundamental flaw lies in how AppArmor handles permission checks for these combined operations, specifically in the apparmor_socket_sendmsg() function that only validates AA_MAY_SEND permissions while neglecting proper connect mediation.
The technical implementation reveals that when MSG_FASTOPEN is utilized with a destination address, the kernel creates an implicit connection that bypasses normal connect() permission checks. This occurs because the existing code path does not enforce the same connect permissions that would normally be required for establishing outbound TCP or MPTCP connections. A confined application profile that grants send permissions but explicitly denies connect operations can therefore circumvent this restriction by using MSG_FASTOPEN, effectively opening network connections that should have been blocked.
The operational impact of this vulnerability extends beyond simple permission bypasses as it undermines the core security model of AppArmor confinement. An attacker with access to a confined process could exploit this weakness to establish outbound connections to arbitrary destinations, potentially exfiltrating data or establishing command and control channels. The vulnerability affects both traditional TCP and MPTCP protocols, though the implementation requires explicit handling for MPTCP due to the sk_is_tcp() function limitation that does not cover MPTCP fast open scenarios.
Security mitigations for this issue involve modifying the apparmor_socket_sendmsg() function to explicitly check for implicit connect permissions when MSG_FASTOPEN is detected. The solution requires direct calls to aa_sk_perm() instead of relying on shared helpers used by both sendmsg and recvmsg operations, ensuring that the same permission checks apply regardless of the communication direction. This approach mirrors similar fixes implemented in SELinux and Tomoyo security modules, establishing consistency across different Linux security frameworks. The fix specifically addresses both SOCK_STREAM and IPPROTO_MPTCP protocols to ensure comprehensive coverage of modern network transport mechanisms.
This vulnerability classification aligns with CWE-693: Protection Mechanism Failure, where an access control mechanism fails to properly enforce required permissions for network operations. The issue also relates to ATT&CK technique T1071.004: Application Layer Protocol: DNS, as it enables unauthorized network communication that could facilitate data exfiltration through bypassed security controls. The remediation approach follows established security patterns for kernel-level access control enforcement and demonstrates the importance of comprehensive permission checking in combined system calls where multiple operations are performed atomically.