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CVE-2026-63925
macsec: fix replay protection at XPN lower-PN wrap
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem drivers/net. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63925 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixed | 6.1.176-1 | Update the kernel and reboot. Fixed in 6.1.176-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
In macsec_post_decrypt(), when pn is U32_MAX, pn + 1 overflows u32 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. An attacker that captures the legitimate frame carrying pn == 0xFFFFFFFF on an XPN association can then replay it indefinitely, since lowest_pn 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 == U32_MAX advances next_pn_halves to (upper + 1, 0).
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
drivers/net/macsec.c
- What this is
- NETWORKING [MACSEC]MAINTAINERS section matched on drivers/net/macsec.c · netdev@vger.kernel.org · Maintained
- Kernel module
- macsec
Check it on the machine
grep -w CONFIG_MACSEC /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w macsecPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switch
- CONFIG_MACSEC
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 8.1 high
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:Hsource recompute it - Attack vector
- adjacent network (AV:A)source
- EPSS
- 0.34%, higher than 26.5% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- drivers/net
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.7 | 5.10.259 | dd7306779c6c |
| 5.7 | 5.15.210 | d15130461df3 |
| 5.7 | 6.1.176 | 23c0e230eab3 |
| 5.7 | 6.6.143 | 679e13a65e68 |
| 5.7 | 6.12.93 | 79495a1b0944 |
| 5.7 | 6.18.35 | 6d00f5c7e5ff |
| 5.7 | 7.0.12 | d55acbe577db |
| 5.7 | 7.1 | e68842b33564 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:AAttack vector, adjacent network
- MACsec processes attacker-supplied Ethernet frames via netdev rx_handler on the underlying link; exploitation requires L2 adjacency on the same broadcast domain or point-to-point MACsec link, not remote Internet reachability.
- AC:LAttack complexity, low
- Once XPN with replay protection is deployed, an on-link attacker can passively capture the single legitimate frame at PN 0xFFFFFFFF and then replay it reliably and indefinitely without races or conditions outside their control.
- PR:NPrivileges required, none
- Exploitation requires only the ability to send or replay Ethernet frames on the MACsec link; no local privileges, capabilities, or authentication on the victim Linux host are needed.
- UI:NUser interaction, none
- Replay injection is fully attacker-driven after capturing the target frame; no victim user or administrator action is required during exploitation.
- S:UScope, unchanged
- The flaw bypasses MACsec replay protection within the kernel network stack and does not cross a security boundary such as a VM, container, or IOMMU isolation domain.
- C:NConfidentiality impact, none
- This is a replay-protection logic flaw, not a memory corruption or decryption weakness; the attacker replays already-captured ciphertext and gains no new confidentiality beyond what passive capture already provided.
- I:HIntegrity impact, high
- Bypassing replay protection allows unlimited re-injection of previously authenticated payloads into the controlled port, defeating MACsec's integrity guarantees and enabling repeated delivery of captured control or data traffic on automotive, industrial, and datacenter MACsec links.
- A:HAvailability impact, high
- The commit confirms captured frames can be replayed indefinitely, enabling sustained injection of decrypted traffic that can overwhelm the receiver and deny service to legitimate peers on the protected link.