CVE-2025-39702Network attack vector

Authentication bypass via timing-leaking SRH MAC comparison

Published Sep 5, 2025 · Updated Aug 5, 2026

Observable timing discrepancy in Linux and SIMATIC CN 4100 before 5.0 allows attackers to recover SRv6 HMAC values via crafted packets. The seg6_hmac_validate_skb path compares a received Segment Routing Header MAC with memcmp, whose early exit leaks matching-prefix timing instead of using a constant-time comparison. Reachability requires SRv6 HMAC processing; upstream identifies unauthenticated remote packets, while Siemens identifies local low-privileged access for CN 4100.

CVSS severity9.8
Critical
EPSS probability0.42%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Observable timing discrepancy in Linux and SIMATIC CN 4100 before 5.0 allows attackers to recover SRv6 HMAC values via crafted packets. The seg6_hmac_validate_skb path compares a received Segment Routing Header MAC with memcmp, whose early exit leaks matching-prefix timing instead of using a constant-time comparison. Reachability requires SRv6 HMAC processing; upstream identifies unauthenticated remote packets, while Siemens identifies local low-privileged access for CN 4100.

The record

CVE
CVE-2025-39702
Published
Sep 5, 2025
Updated
Aug 5, 2026
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
CWE-208, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 5, 2025CVE publishedPublication date reported by the CVE source.
  2. Aug 5, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=4.10
  2. Affected versionversion=bf355b8d2c30a289232042cacc1cfaea4923936c <3b348c9c8d2ca2c67559ffd0e258ae7e1107d4f0
  3. Affected versionversion=bf355b8d2c30a289232042cacc1cfaea4923936c <3ddd55cf19ed6cc62def5e3af10c2a9df1b861c3
  4. Affected versionversion=bf355b8d2c30a289232042cacc1cfaea4923936c <86b6d34717fe0570afce07ee79b8eeb40341f831
  5. Affected versionversion=bf355b8d2c30a289232042cacc1cfaea4923936c <a458b2902115b26a25d67393b12ddd57d1216aaa
  6. Affected versionversion=bf355b8d2c30a289232042cacc1cfaea4923936c <b3967c493799e63f648e9c7b6cb063aa2aed04e7
  7. Affected versionversion=bf355b8d2c30a289232042cacc1cfaea4923936c <f7878d47560d61e3f370aca3cebb8f42a55b990a
  8. Affected versionversion=bf355b8d2c30a289232042cacc1cfaea4923936c <ff55a452d56490047f5233cc48c5d933f8586884

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=4.10; version=bf355b8d2c30a289232042cacc1cfaea4923936c <3b348c9c8d2ca2c67559ffd0e258ae7e1107d4f0; version=bf355b8d2c30a289232042cacc1cfaea4923936c <3ddd55cf19ed6cc62def5e3af10c2a9df1b861c3; version=bf355b8d2c30a289232042cacc1cfaea4923936c <86b6d34717fe0570afce07ee79b8eeb40341f831; version=bf355b8d2c30a289232042cacc1cfaea4923936c <a458b2902115b26a25d67393b12ddd57d1216aaa; version=bf355b8d2c30a289232042cacc1cfaea4923936c <b3967c493799e63f648e9c7b6cb063aa2aed04e7; version=bf355b8d2c30a289232042cacc1cfaea4923936c <f7878d47560d61e3f370aca3cebb8f42a55b990a; version=bf355b8d2c30a289232042cacc1cfaea4923936c <ff55a452d56490047f5233cc48c5d933f8586884
Siemens · SIMATIC CN 4100version=0 <V5.0

Published CVSS scores

7.1Siemens ProductCERTCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
9.8Linux kernel CVE teamCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-208Observable Timing Discrepancy
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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