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VulnerabilityAwaiting classification

CVE-2026-63925

HIGH · 8.1 CVSS v3.1 Published 2026-07-19
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
86/100
Remediation priority · High
No privileges Zero-click 5 weeks old

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
In the Linux kernel, the following vulnerability has been resolved: macsec: fix replay protection at XPN lower-PN wrap 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).

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Replay protection bypass in Linux kernel MACSEC XPN (Extended Packet Numbering) implementation. When pn reaches U32_MAX and wraps to 0, the pn+1 overflow causes the replay check logic to fail: the first branch doesn't fire due to overflow, and the XPN else-if doesn't fire because pn_same_half() returns true when next_pn_halves.lower is also in the upper half. This leaves next_pn_halves unchanged, allowing an attacker who captured a frame with pn==0xFFFFFFFF to replay it indefinitely.

MitigationApply the kernel patch that extends the XPN else-if condition to fire when pn+1 wraps to 0, advancing next_pn_halves correctly. Upgrade to a kernel version containing this fix.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Adjacent
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
High

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Verify MACsec kernel module is loaded
    Run 'lsmod | grep macsec' to check if the macsec module is loaded, or check /proc/modules for 'macsec'
    Affected if The macsec module is loaded and MACsec interfaces exist in the system
  2. Identify active MACsec interfaces
    Run 'ip -details link show type macsec' or 'macstat -i <ifname>' (if available) to list MACsec-enabled interfaces
    Affected if Any MACsec interfaces are configured and active
  3. Check if XPN mode is enabled on MACsec interfaces
    Run 'ip macsec show <interface>' and look for 'XPN' or 'extended packet number' in the output, or inspect MACsec configuration files/tools
    Affected if XPN (Extended Packet Number) mode is enabled on any MACsec interface
  4. Determine installed kernel version
    Run 'uname -r' or check /proc/version to obtain the kernel version
    Affected if The kernel version predates the fix for this vulnerability (compare to the fixed version range for your distribution)
  5. Inspect MACsec replay window configuration
    Run 'ip macsec show <interface>' and check the 'replay' window size setting; also check if 'window' parameter is set to 0 (disabled protection)
    Affected if Replay protection is disabled or set to a very small window, making the bypass more impactful

A system is affected if it runs a vulnerable Linux kernel with MACsec enabled using XPN mode, where the PN wraparound condition (pn == U32_MAX) can be triggered by an attacker.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the kernel patch that extends the XPN else-if condition to fire when pn+1 wraps to 0, advancing next_pn_halves correctly. Upgrade to a kernel version containing this fix.

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing3.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,210
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-63925 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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