CVE-2026-39858 (GCVE-0-2026-39858)
Vulnerability from cvelistv5 – Published: 2026-04-30 20:26 – Updated: 2026-07-15 01:01
VLAI
EPSS
VEX
Title
Traefik: Forwarded alias spoofing top pre-auth decision bypass
Summary
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.
Severity
SSVC
Exploitation: none
Automatable: yes
Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-05-04 16:57 UTC
CWE
Assigner
References
8 references
| URL | Tags |
|---|---|
| https://github.com/traefik/traefik/security/advis… | x_refsource_CONFIRM |
| https://github.com/traefik/traefik/releases/tag/v… | x_refsource_MISC |
| https://github.com/traefik/traefik/releases/tag/v3.6.14 | x_refsource_MISC |
| https://github.com/traefik/traefik/releases/tag/v… | x_refsource_MISC |
| https://access.redhat.com/security/cve/CVE-2026-39858 | vdb-entryx_refsource_REDHAT |
| https://bugzilla.redhat.com/show_bug.cgi?id=2464234 | issue-trackingx_refsource_REDHAT |
| https://security.access.redhat.com/data/csaf/v2/v… | x_sadp-csaf-vex |
| https://access.redhat.com/errata/RHSA-2026:21772 | vendor-advisoryx_refsource_REDHAT |
Impacted products
2 products
| Vendor | Product | Version | CPE status | |
|---|---|---|---|---|
| traefik | traefik |
Affected:
< 2.11.43
Affected: >= 3.0.0-beta1, < 3.6.14 Affected: >= 3.7.0-ea.1, < 3.7.0-rc.2 |
guessed | |
| Red Hat | Red Hat OpenShift Dev Spaces 3.28 |
Unaffected:
1779786779 , < *
(rpm)
cpe:/a:redhat:openshift_devspaces:3.28::el9 |
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"version": "2.0.3"
},
"type": "ssvc"
}
}
],
"providerMetadata": {
"dateUpdated": "2026-05-04T16:58:09.098Z",
"orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
"shortName": "CISA-ADP"
},
"title": "CISA ADP Vulnrichment"
}
],
"cna": {
"affected": [
{
"product": "traefik",
"vendor": "traefik",
"versions": [
{
"status": "affected",
"version": "\u003c 2.11.43"
},
{
"status": "affected",
"version": "\u003e= 3.0.0-beta1, \u003c 3.6.14"
},
{
"status": "affected",
"version": "\u003e= 3.7.0-ea.1, \u003c 3.7.0-rc.2"
}
]
}
],
"descriptions": [
{
"lang": "en",
"value": "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\u0027s ForwardAuth and snippet-based authentication middleware. Traefik\u0027s 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 \u2014 such as a trusted scheme or host \u2014 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."
}
],
"metrics": [
{
"cvssV4_0": {
"attackComplexity": "LOW",
"attackRequirements": "NONE",
"attackVector": "NETWORK",
"baseScore": 7.8,
"baseSeverity": "HIGH",
"privilegesRequired": "NONE",
"subAvailabilityImpact": "NONE",
"subConfidentialityImpact": "HIGH",
"subIntegrityImpact": "HIGH",
"userInteraction": "NONE",
"vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N",
"version": "4.0",
"vulnAvailabilityImpact": "NONE",
"vulnConfidentialityImpact": "NONE",
"vulnIntegrityImpact": "NONE"
}
}
],
"problemTypes": [
{
"descriptions": [
{
"cweId": "CWE-290",
"description": "CWE-290: Authentication Bypass by Spoofing",
"lang": "en",
"type": "CWE"
}
]
},
{
"descriptions": [
{
"cweId": "CWE-306",
"description": "CWE-306: Missing Authentication for Critical Function",
"lang": "en",
"type": "CWE"
}
]
}
],
"providerMetadata": {
"dateUpdated": "2026-04-30T20:26:26.300Z",
"orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
"shortName": "GitHub_M"
},
"references": [
{
"name": "https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm",
"tags": [
"x_refsource_CONFIRM"
],
"url": "https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm"
},
{
"name": "https://github.com/traefik/traefik/releases/tag/v2.11.43",
"tags": [
"x_refsource_MISC"
],
"url": "https://github.com/traefik/traefik/releases/tag/v2.11.43"
},
{
"name": "https://github.com/traefik/traefik/releases/tag/v3.6.14",
"tags": [
"x_refsource_MISC"
],
"url": "https://github.com/traefik/traefik/releases/tag/v3.6.14"
},
{
"name": "https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2",
"tags": [
"x_refsource_MISC"
],
"url": "https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2"
}
],
"source": {
"advisory": "GHSA-5m6w-wvh7-57vm",
"discovery": "UNKNOWN"
},
"title": "Traefik: Forwarded alias spoofing top pre-auth decision bypass"
}
},
"cveMetadata": {
"assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
"assignerShortName": "GitHub_M",
"cveId": "CVE-2026-39858",
"datePublished": "2026-04-30T20:26:26.300Z",
"dateReserved": "2026-04-07T19:13:20.379Z",
"dateUpdated": "2026-05-04T16:58:14.458Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.
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