Executive brief
A vulnerability in the Linux kernel's MACsec implementation could allow an attacker on the local network to bypass security protections. MACsec is a standard used to secure communication between devices on a local area network. By exploiting a mathematical error in how the system tracks message numbers, an attacker could repeatedly replay captured network traffic, potentially disrupting operations or manipulating data streams.
Technical details
A vulnerability in the `macsec_post_decrypt()` function in `drivers/net/macsec.c` occurs when the 32-bit packet number (PN) reaches its maximum value (U32_MAX). Due to an integer overflow, `pn + 1` wraps to zero, causing the logic that advances the next expected packet number to fail. Under specific conditions where the lower half of the next PN is in the upper half of its range, the state remains unchanged. This allows an attacker who captures a frame with PN 0xFFFFFFFF to replay it indefinitely, as the `lowest_pn` threshold never advances and the initialization vector (IV) remains valid for that specific frame. The fix extends the XPN logic to advance the packet number halves when a wrap-around is detected.
Affected products
- Linux Linux 5.7 to 6.13
Timeline
- 2026-05-20: patched: Initial fix authored by Junrui Luo
- 2026-07-19: advisory: CVE-2026-63925 published
References
- https://git.kernel.org/stable/c/23c0e230eab397d7f68be2538790ac41d3bb91fd
- https://git.kernel.org/stable/c/679e13a65e68a67c8b3c0467c02ee89157ec6f0f
- https://git.kernel.org/stable/c/6d00f5c7e5ff7ec4795b7f5f8ed88bd346641652
- https://git.kernel.org/stable/c/79495a1b0944fe31ffd54b54b00211b493590d62
- https://git.kernel.org/stable/c/d15130461df388136b62a7b0ce9f66e7e2fa9ff1
- https://git.kernel.org/stable/c/d55acbe577db892b60547b6ef1c020b359331a6d
- https://git.kernel.org/stable/c/dd7306779c6ce1238f4cdc34f3c1f2246b854457