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VulnerabilityAwaiting classification

CVE-2026-63828

HIGH · 8.4 CVSS v3.1 Published 2026-07-19
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
86/100
Remediation priority · High
Zero-click 5 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

AppArmor does not properly mediate TCP Fast Open (TFO) when sendmsg()/sendto() is called with MSG_FASTOPEN flag. This flag combines connect(2) and write(2) into a single operation that opens a TCP connection in the SYN packet. The apparmor_socket_sendmsg() function only checks AA_MAY_SEND permission, allowing a confined application with send but not connect permissions to bypass connect mediation and establish outbound TCP/MPTCP connections that would be refused by a proper connect(2) call.

MitigationUpdate the Linux kernel to the version containing this fix. The vulnerability is in the kernel's AppArmor LSM and cannot be mitigated through AppArmor profile configuration alone.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm AppArmor is active
    Run `aa-status` or check `/sys/kernel/security/apparmor/profiles` to verify AppArmor is loaded and enforcing profiles.
    Affected if AppArmor is enabled and enforcing, but the kernel is unpatched for the implicit connect bypass.
  2. Identify profiles with send-only permissions
    Audit AppArmor profiles for rules granting `aa_raw_socket::send` or `network tcp send` without corresponding `connect` or `network tcp connect` permissions. Use `aa-audit` or manually inspect profiles in `/etc/apparmor.d/`.
    Affected if A profile grants send permission but denies connect; an attacker could bypass the connect denial using MSG_FASTOPEN.
  3. Check for MSG_FASTOPEN usage
    Inspect application configurations and code for TCP_FASTOPEN usage (setsockopt with TCP_FASTOPEN, or search for MSG_FASTOPEN in source). On the system, check `/proc/sys/net/ipv4/tcp_fastopen` for the client enable bit (value 1 or 3).
    Affected if TCP_FASTOPEN is enabled for clients (bit 1 set) and the kernel lacks the fix, allowing confined apps to implicitly connect via sendmsg with MSG_FASTOPEN.
  4. Verify kernel version lacks the fix
    Run `uname -r` and compare the kernel version to the upstream release containing the AppArmor MSG_FASTOPEN connect mediation patch. Since no specific version was provided, treat any pre-patch kernel as potentially affected.
    Affected if The running kernel predates the patch that adds `apparmor_socket_sendmsg` mediation for MSG_FASTOPEN implicit connects.
  5. Monitor for anomalous outbound connections
    Use `ss -tan` or `netstat -tan` to list established outbound TCP connections from confined processes. Cross-reference with AppArmor deny logs in `/var/log/kern.log` or `/var/log/syslog` for blocked connect attempts.
    Affected if Outbound TCP connections exist from a confined domain that has no explicit connect permission, indicating a possible bypass.

A user is affected if their kernel lacks the AppArmor MSG_FASTFASTOPEN fix AND they run confined profiles that grant send but deny connect while using applications that employ TCP_FASTOPEN.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Update the Linux kernel to the version containing this fix. The vulnerability is in the kernel's AppArmor LSM and cannot be mitigated through AppArmor profile configuration alone.

Have this fixed Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing12.0 h
  • Review / QA4.0 h
28.0 hours of engineering $4,760
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-63828 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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