CVE-2026-63925 Linux CVE debrief

Who should care

Linux kernel users and administrators should be aware of this vulnerability and take necessary actions to protect their systems. They should verify their systems for the presence of this vulnerability and apply the necessary patches to prevent exploitation. Additionally, they should monitor network traffic for suspicious activity and implement additional security measures to prevent replay attacks.

Technical summary

The vulnerability is caused by a flaw in the macsec_post_decrypt() function, which fails to properly handle the wrap-around of the packet number (pn). When pn is U32_MAX, pn + 1 overflows to 0, and the first branch never fires. If next_pn_halves.lower is also in the upper half, pn_same_half(pn, lower) is true, and the XPN else-if does not fire either, leaving next_pn_halves unchanged. This allows an attacker to capture a legitimate frame carrying pn == 0xFFFFFFFF on an XPN association and replay it indefinitely.

Defensive priority

High

Recommended defensive actions

• Apply the patch to the Linux kernel to fix the vulnerability
• Monitor network traffic for suspicious activity
• Implement additional security measures to prevent replay attacks
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-07-19T16:17:10.787Z and has not been modified since then. The NVD entry is currently Received. Linux kernel users should verify their systems for the presence of this vulnerability and apply the necessary patches to prevent exploitation.

Official resources