Traefik: Forwarded alias spoofing top pre-auth decision bypass_CVE-2026-39858

{
    "lastseen": "",
    "description": "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 \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.",
    "published": "2026-04-30T20:26:26.300Z",
    "modified": "2026-04-30T20:26:26.300Z",
    "type": "cve",
    "title": "Traefik: Forwarded alias spoofing top pre-auth decision bypass",
    "source": "GitHub_M",
    "references": "https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm\nhttps://github.com/traefik/traefik/releases/tag/v2.11.43\nhttps://github.com/traefik/traefik/releases/tag/v3.6.14\nhttps://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2",
    "id": "CVE-2026-39858",
    "bulletinFamily": "",
    "cwe": [
        "CWE-290",
        "CWE-306"
    ],
    "cvelist": null,
    "sourceData": "traefik traefik < 2.11.43\ntraefik traefik >= 3.0.0-beta1, < 3.6.14\ntraefik traefik >= 3.7.0-ea.1, < 3.7.0-rc.2",
    "sourceHref": "",
    "cvss": {
        "score": 7.8,
        "severity": "HIGH",
        "vector": "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"
    },
    "cvss2": [],
    "cvss3": {
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        "vectorString": "",
        "baseScore": 0,
        "baseSeverity": "",
        "attackVector": "",
        "attackComplexity": "",
        "privilegesRequired": "",
        "userInteraction": "",
        "scope": "",
        "confidentialityImpact": "",
        "integrityImpact": "",
        "availabilityImpact": "",
        "cvssV3": {
            "version": "",
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            "baseScore": 0,
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        }
    },
    "href": "",
    "category_name": "CVE",
    "post_link": "",
    "product": "traefik",
    "version": "< 2.11.43",
    "vendor": "traefik",
    "ai_description": "",
    "ai_severity": "",
    "ai_vendor": "",
    "ai_product": "",
    "ai_version": "",
    "ai_score": 0
}