{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-88879", "assignerOrgId": "83251b91-4cc7-4094-a5c7-464a1b83ea10", "state": "PUBLISHED", "assignerShortName": "VulnCheck", "dateReserved": "2026-09-10T11:24:26.196Z", "datePublished": "2026-09-10T13:05:30.872Z", "dateUpdated": "2026-09-10T13:05:30.872Z"}, "containers": {"cna": {"providerMetadata": {"orgId": "83251b91-4cc7-4094-a5c7-464a1b83ea10", "shortName": "VulnCheck", "dateUpdated": "2026-09-10T13:05:30.872Z"}, "datePublic": "2026-08-27T00:00:00.000Z", "title": "Traefik before v2.11.56 Identity Spoofing via Header Alias", "descriptions": [{"lang": "en", "value": "Traefik is an HTTP reverse proxy and load balancer. In Traefik v1.x, v2.x through v2.11.55, and v3.0.0 through v3.7.11, header names are canonicalized only on dashes, so X-Auth-User, X_Auth_User and X.Auth.User are treated as three distinct headers by Traefik, while backends that derive variable names from header names (CGI, WSGI, PHP, NGINX and others) collapse them into a single variable. A client can therefore smuggle a dot-form alias of a header that Traefik manages past the middleware managing it \u2014 for example supplying X.Authenticated.User alongside the canonical X-Authenticated-User written by the ForwardAuth middleware \u2014 causing such a backend to read the client-supplied value instead of the identity Traefik asserted. In the tested configuration (PHP 8.2 built-in SAPI over an HTTP/1 backend path), Go's lexical header ordering makes the attacker-supplied value win deterministically, so a client that ForwardAuth admits as a low-privilege identity can be treated by the backend as a different user or role. Any header Traefik sets is affected, not only ForwardAuth's. This is an incomplete fix for GHSA-x677-9fxg-v5c5, which blocked only the underscore form. Fixed in v2.11.56 and v3.7.12, which add the aliasHeadersStrategy entry-point option; because it defaults to 'keep' for backwards compatibility, it must be explicitly set to 'delete' or 'reject' for the fix to take effect. Unmaintained release lines will not receive a patch."}], "problemTypes": [{"descriptions": [{"lang": "en", "description": "Authentication Bypass by Spoofing", "cweId": "CWE-290", "type": "CWE"}]}], "affected": [{"vendor": "traefik", "product": "traefik", "defaultStatus": "unaffected", "packageURL": "pkg:golang/Traefik", "versions": [{"version": "0", "status": "affected", "versionType": "semver", "lessThan": "2.11.56"}, {"version": "2.11.56", "status": "unaffected", "versionType": "semver"}]}, {"vendor": "traefik", "product": "traefik", "defaultStatus": "unaffected", "packageURL": "pkg:golang/Traefik", "versions": [{"version": "3.0.0", "status": "affected", "versionType": "semver", "lessThanOrEqual": "3.7.13"}]}], "cpeApplicability": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:traefik:traefik:*:*:*:*:*:*:*:*", "versionEndExcluding": "2.11.56"}]}]}, {"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:traefik:traefik:*:*:*:*:*:*:*:*", "versionStartIncluding": "3.0.0", "versionEndIncluding": "3.7.13"}]}]}], "metrics": [{"format": "CVSS", "cvssV4_0": {"version": "4.0", "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N", "attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "PRESENT", "privilegesRequired": "LOW", "userInteraction": "NONE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "NONE", "vulnAvailabilityImpact": "NONE", "subConfidentialityImpact": "HIGH", "subIntegrityImpact": "HIGH", "subAvailabilityImpact": "NONE", "baseScore": 5.3, "baseSeverity": "MEDIUM"}}], "references": [{"url": "https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c", "tags": ["vendor-advisory"], "name": "GitHub Security Advisory (GHSA-rf44-j88r-hh8c)"}, {"name": "VulnCheck Advisory: Traefik before v2.11.56 Identity Spoofing via Header Alias", "tags": ["third-party-advisory"], "url": "https://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias"}], "credits": [{"lang": "en", "value": "velgusgus599", "type": "reporter"}], "x_generator": {"engine": "vulncheck-endgame"}}}}

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{"affected": [{"affectedData": [{"defaultStatus": "unaffected", "packageURL": "pkg:golang/Traefik", "product": "traefik", "vendor": "traefik", "versions": [{"lessThan": "2.11.56", "status": "affected", "version": "0", "versionType": "semver"}, {"status": "unaffected", "version": "2.11.56", "versionType": "semver"}]}, {"defaultStatus": "unaffected", "packageURL": "pkg:golang/Traefik", "product": "traefik", "vendor": "traefik", "versions": [{"lessThanOrEqual": "3.7.13", "status": "affected", "version": "3.0.0", "versionType": "semver"}]}], "source": "disclosure@vulncheck.com"}], "cveTags": [], "descriptions": [{"lang": "en", "value": "Traefik is an HTTP reverse proxy and load balancer. In Traefik v1.x, v2.x through v2.11.55, and v3.0.0 through v3.7.11, header names are canonicalized only on dashes, so X-Auth-User, X_Auth_User and X.Auth.User are treated as three distinct headers by Traefik, while backends that derive variable names from header names (CGI, WSGI, PHP, NGINX and others) collapse them into a single variable. A client can therefore smuggle a dot-form alias of a header that Traefik manages past the middleware managing it \u2014 for example supplying X.Authenticated.User alongside the canonical X-Authenticated-User written by the ForwardAuth middleware \u2014 causing such a backend to read the client-supplied value instead of the identity Traefik asserted. In the tested configuration (PHP 8.2 built-in SAPI over an HTTP/1 backend path), Go's lexical header ordering makes the attacker-supplied value win deterministically, so a client that ForwardAuth admits as a low-privilege identity can be treated by the backend as a different user or role. Any header Traefik sets is affected, not only ForwardAuth's. This is an incomplete fix for GHSA-x677-9fxg-v5c5, which blocked only the underscore form. Fixed in v2.11.56 and v3.7.12, which add the aliasHeadersStrategy entry-point option; because it defaults to 'keep' for backwards compatibility, it must be explicitly set to 'delete' or 'reject' for the fix to take effect. Unmaintained release lines will not receive a patch."}], "id": "CVE-2026-88879", "lastModified": "2026-09-10T14:50:07.813", "metrics": {"cvssMetricV40": [{"cvssData": {"Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "Safety": "NOT_DEFINED", "attackComplexity": "LOW", "attackRequirements": "PRESENT", "attackVector": "NETWORK", "availabilityRequirement": "NOT_DEFINED", "baseScore": 5.3, "baseSeverity": "MEDIUM", "confidentialityRequirement": "NOT_DEFINED", "exploitMaturity": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "privilegesRequired": "LOW", "providerUrgency": "NOT_DEFINED", "subAvailabilityImpact": "NONE", "subConfidentialityImpact": "HIGH", "subIntegrityImpact": "HIGH", "userInteraction": "NONE", "valueDensity": "NOT_DEFINED", "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X", "version": "4.0", "vulnAvailabilityImpact": "NONE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "NONE", "vulnerabilityResponseEffort": "NOT_DEFINED"}, "source": "disclosure@vulncheck.com", "type": "Secondary"}]}, "published": "2026-09-10T14:17:16.337", "references": [{"source": "disclosure@vulncheck.com", "url": "https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c"}, {"source": "disclosure@vulncheck.com", "url": "https://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias"}], "sourceIdentifier": "disclosure@vulncheck.com", "vulnStatus": "Awaiting Analysis", "weaknesses": [{"description": [{"lang": "en", "value": "CWE-290"}], "source": "disclosure@vulncheck.com", "type": "Primary"}]}

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