CVE-2026-63925: macsec: fix replay protection at XPN lower-PN wrap
In the Linux kernel, the following vulnerability has been resolved:
macsec: fix replay protection at XPN lower-PN wrap
In macsecpostdecrypt(), when pn is U32MAX, pn + 1 overflows u32 to 0 and the first branch never fires. If nextpnhalves.lower is also in the upper half, pnsamehalf(pn, lower) is true and the XPN else-if does not fire either, leaving nextpnhalves unchanged. An attacker that captures the legitimate frame carrying pn == 0xFFFFFFFF on an XPN association can then replay it indefinitely, since lowestpn never rises above the captured pn and macsecdecrypt() reconstructs the same IV.
Extend the XPN else-if to also fire when pn + 1 wraps to 0, so receipt of pn == U32MAX advances nextpnhalves to (upper + 1, 0).
Affected Software
Remediation
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In macsec_post_decrypt(), extend the XPN else-if to also fire when pn + 1 wraps to 0, so receipt of pn == U32_MAX advances next_pn_halves to (upper + 1, 0).
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Systems using Linux kernel MACsec with an XPN association are affected under the described packet-number state. The issue concerns replay protection when the lower packet number reaches 0xFFFFFFFF.
What does an attacker need to exploit it?
An attacker needs to capture a legitimate frame sent with packet number 0xFFFFFFFF on an XPN association. They can then replay that captured frame indefinitely because the replay state does not advance correctly.
What is the impact of successful exploitation?
Successful exploitation bypasses replay protection for the captured frame. The kernel reconstructs the same IV for each replay, allowing repeated acceptance of that frame.
What changes in the fix?
The fix handles the lower packet-number wrap from 0xFFFFFFFF to 0 by advancing the XPN state to the next upper half with a lower value of 0. This prevents the replay state from remaining unchanged at the wrap boundary.