CVE-2026-63925

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 macsec_decrypt() reconstructs the same IV.

Extend the XPN else-if to also fire when pn + 1 wraps to 0, so receipt of pn == U32MAX advances nextpn_halves to (upper + 1, 0).