CVE-2026-39858

CRITICALNVD 10.010.0—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 10.0 under NVD CVSS v3. EPSS exploit probability: 0.5%, top 61% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
10.0EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 10.0Exploit: None knownExposed: 0

A fix is available — apply it.

Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.43, 3.6.14, and 3.7.0-rc.2, there is a high severity authentication bypass vulnerability in Traefik's ForwardAuth and snippet-based authentication middleware. Traefik's forwarded-header sanitization logic targets only canonical header names (e.g., X-Forwarded-Proto) and does not strip or normalize alias variants that use underscores instead of dashes (e.g., X_Forwarded_Proto). These unsanitized alias headers are forwarded intact to the authentication backend. When the backend normalizes underscore and dash header forms equivalently, an attacker can inject spoofed trust context — such as a trusted scheme or host — through the alias headers and bypass authentication on protected routes without valid credentials. This issue has been patched in versions 2.11.43, 3.6.14, and 3.7.0-rc.2.

CVSS v3
10.0
EG Score
10.0(medium)
EG Risk
60(Track)
EG Risk 60/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity100% × 45%
Exploitation0% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
40%
KEV
Not listed

Published

April 30, 2026

Last Modified

July 15, 2026

Advisory Details (8)

Auto-updated Sep 2, 2026
🔬 Proof of concept available. Patch available. Sources: github_release, github, redhat.
redhat Patch Available
https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-39858.json
redhat

2464234 – (CVE-2026-39858) CVE-2026-39858 traefik: Traefik: Authentication bypass via unsanitized alias headers

https://bugzilla.redhat.com/show_bug.cgi?id=2464234
redhat Patch Available

CVE-2026-39858 - Red Hat Customer Portal

Affected: Red Hat Enterprise Linux 8 or OpenShift Container Platform 4, is affected by this vulnerability and a fix may be released to a

https://access.redhat.com/security/cve/CVE-2026-39858
redhat Patch Available
https://access.redhat.com/errata/RHSA-2026:21772
github Patch Available🟡 PoC Available

Forwarded alias spoofing top pre-auth decision bypass · Advisory · traefik/traefik · GitHub

https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm
github_release Patch Available

v3.7.0-rc.2

Patch available: traefik/traefik v3.7.0-rc.2 (pre-release)

https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2
github_release Patch Available

v3.6.14

Patch available: traefik/traefik v3.6.14

https://github.com/traefik/traefik/releases/tag/v3.6.14
github_release Patch Available

v2.11.43

Patch available: traefik/traefik v2.11.43

https://github.com/traefik/traefik/releases/tag/v2.11.43

Vendor Advisories for CVE-2026-39858(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(1)

Vendor / EcosystemFixed in / PatchReleasedSource
redhatdevspaces/traefik-rhel9:17797867792026-05-28redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Affected Packages

(3 across 1 ecosystem)
Go(3)
PackageVulnerable rangeFixed inDependents
github.com/traefik/traefik———
github.com/traefik/traefik/v2—2.11.43—
github.com/traefik/traefik/v3—3.7.0-rc.2—

Weakness Classification(3)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 7× in last 7d / 80× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 275 total refreshes for this CVE.

  1. 2026-09-04 05:06 UTCEPSS rescore
  2. 2026-09-04 01:16 UTCOSV refresh
  3. 2026-09-02 14:12 UTCEPSS rescore
  4. 2026-09-01 13:53 UTCEPSS rescore
  5. 2026-09-01 04:39 UTCEPSS rescore
  6. 2026-08-30 19:17 UTCEPSS rescore
  7. 2026-08-30 01:22 UTCEPSS rescore
  8. 2026-08-28 21:41 UTCEPSS rescore
  9. 2026-08-27 14:25 UTCEPSS rescore
  10. 2026-08-26 14:46 UTCEPSS rescore
  11. 2026-08-25 13:49 UTCEPSS rescore
  12. 2026-08-23 00:19 UTCEPSS rescore
  13. 2026-08-21 23:49 UTCEPSS rescore
  14. 2026-08-20 22:55 UTCEPSS rescore
  15. 2026-08-19 17:03 UTCEPSS rescore
  16. 2026-08-18 13:48 UTCEPSS rescore
  17. 2026-08-17 13:48 UTCEG score recompute
  18. 2026-08-17 13:48 UTCVendor advisory
  19. 2026-08-17 13:47 UTCEPSS rescore
  20. 2026-08-16 14:56 UTCEPSS rescore
  21. 2026-08-16 09:15 UTCVendor advisory
  22. 2026-08-16 05:21 UTCEG score recompute
  23. 2026-08-16 05:21 UTCVendor advisory
  24. 2026-08-16 02:14 UTCEPSS rescore
  25. 2026-08-15 02:06 UTCEG score recompute
Show 75 more
  1. 2026-08-15 02:06 UTCVendor advisory
  2. 2026-08-15 01:30 UTCEPSS rescore
  3. 2026-08-14 22:25 UTCVendor advisory
  4. 2026-08-14 18:43 UTCVendor advisory
  5. 2026-08-14 14:53 UTCVendor advisory
  6. 2026-08-14 11:12 UTCVendor advisory
  7. 2026-08-14 07:30 UTCVendor advisory
  8. 2026-08-14 03:49 UTCEG score recompute
  9. 2026-08-14 03:49 UTCVendor advisory
  10. 2026-08-13 22:00 UTCEPSS rescore
  11. 2026-08-13 08:30 UTCVendor advisory
  12. 2026-08-13 04:48 UTCVendor advisory
  13. 2026-08-13 01:06 UTCVendor advisory
  14. 2026-08-12 21:25 UTCVendor advisory
  15. 2026-08-12 17:27 UTCEG score recompute
  16. 2026-08-12 17:27 UTCVendor advisory
  17. 2026-08-12 13:51 UTCEPSS rescore
  18. 2026-08-11 23:51 UTCVendor advisory
  19. 2026-08-11 20:09 UTCVendor advisory
  20. 2026-08-11 16:23 UTCEG score recompute
  21. 2026-08-11 16:23 UTCVendor advisory
  22. 2026-08-11 13:43 UTCEPSS rescore
  23. 2026-08-11 09:14 UTCVendor advisory
  24. 2026-08-11 05:32 UTCVendor advisory
  25. 2026-08-11 01:51 UTCEG score recompute
  26. 2026-08-11 01:51 UTCVendor advisory
  27. 2026-08-11 00:00 UTCEPSS rescore
  28. 2026-08-10 12:45 UTCVendor advisory
  29. 2026-08-10 08:42 UTCVendor advisory
  30. 2026-08-10 05:01 UTCVendor advisory
  31. 2026-08-10 01:19 UTCVendor advisory
  32. 2026-08-09 21:36 UTCVendor advisory
  33. 2026-08-09 17:55 UTCVendor advisory
  34. 2026-08-09 14:13 UTCEG score recompute
  35. 2026-08-09 14:13 UTCVendor advisory
  36. 2026-08-09 13:46 UTCEPSS rescore
  37. 2026-08-09 10:31 UTCVendor advisory
  38. 2026-08-09 06:42 UTCVendor advisory
  39. 2026-08-09 03:00 UTCEG score recompute
  40. 2026-08-09 03:00 UTCVendor advisory
  41. 2026-08-08 16:37 UTCEPSS rescore
  42. 2026-08-08 10:58 UTCVendor advisory
  43. 2026-08-08 06:56 UTCVendor advisory
  44. 2026-08-08 03:14 UTCVendor advisory
  45. 2026-08-07 23:33 UTCVendor advisory
  46. 2026-08-07 19:51 UTCEG score recompute
  47. 2026-08-07 19:51 UTCVendor advisory
  48. 2026-08-07 16:10 UTCVendor advisory
  49. 2026-08-07 12:19 UTCVendor advisory
  50. 2026-08-07 02:41 UTCVendor advisory
  51. 2026-08-06 20:50 UTCVendor advisory
  52. 2026-08-06 17:08 UTCEG score recompute
  53. 2026-08-06 17:08 UTCVendor advisory
  54. 2026-08-06 13:46 UTCEPSS rescore
  55. 2026-08-06 10:12 UTCVendor advisory
  56. 2026-08-06 06:01 UTCVendor advisory
  57. 2026-08-06 02:20 UTCVendor advisory
  58. 2026-08-05 22:38 UTCEG score recompute
  59. 2026-08-05 22:38 UTCVendor advisory
  60. 2026-08-05 19:17 UTCEPSS rescore
  61. 2026-08-04 15:10 UTCEPSS rescore
  62. 2026-08-04 10:38 UTCEPSS rescore
  63. 2026-08-04 03:52 UTCVendor advisory
  64. 2026-08-04 00:10 UTCVendor advisory
  65. 2026-08-03 20:27 UTCEG score recompute
  66. 2026-08-03 20:27 UTCVendor advisory
  67. 2026-08-03 10:36 UTCEPSS rescore
  68. 2026-08-03 00:40 UTCVendor advisory
  69. 2026-08-02 20:58 UTCVendor advisory
  70. 2026-08-02 17:17 UTCVendor advisory
  71. 2026-08-02 13:35 UTCVendor advisory
  72. 2026-08-02 09:54 UTCVendor advisory
  73. 2026-08-02 05:43 UTCEG score recompute
  74. 2026-08-02 05:43 UTCVendor advisory
  75. 2026-08-02 02:27 UTCEPSS rescore

Related CVEs(same vendor + same CWE)

Frequently asked(5)

What is CVE-2026-39858?
CVE-2026-39858 is a critical vulnerability published on April 30, 2026. Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.43, 3.6.14, and 3.7.0-rc.2, there is a high severity authentication bypass vulnerability in Traefik's ForwardAuth and snippet-based authentication middleware. Traefik's forwarded-header sanitization logic targets only…
When was CVE-2026-39858 disclosed?
CVE-2026-39858 was first published in the National Vulnerability Database on April 30, 2026, with the most recent update on July 15, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-39858 actively exploited?
CVE-2026-39858 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 60.4% of all scored CVEs.
What is the CVSS score of CVE-2026-39858?
CVE-2026-39858 has a CVSS v3 base score of 10.0 (NVD).
How do I remediate CVE-2026-39858?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-39858, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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