{"id":"CVE-2026-88879","sourceIdentifier":"disclosure@vulncheck.com","published":"2026-09-10T14:17:16.337","lastModified":"2026-10-08T16:17:58.963","vulnStatus":"Analyzed","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 — for example supplying X.Authenticated.User alongside the canonical X-Authenticated-User written by the ForwardAuth middleware — 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."}],"affected":[{"source":"disclosure@vulncheck.com","affectedData":[{"vendor":"traefik","product":"traefik","defaultStatus":"unaffected","packageURL":"pkg:golang/github.com/traefik/traefik/v2","versions":[{"version":"0","lessThan":"2.11.56","versionType":"semver","status":"affected"},{"version":"2.11.56","versionType":"semver","status":"unaffected"}]},{"vendor":"traefik","product":"traefik","defaultStatus":"unaffected","packageURL":"pkg:golang/github.com/traefik/traefik/v3","versions":[{"version":"3.0.0","lessThanOrEqual":"3.7.13","versionType":"semver","status":"affected"}]}]}],"metrics":{"cvssMetricV40":[{"source":"disclosure@vulncheck.com","type":"Secondary","cvssData":{"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/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","baseScore":5.3,"baseSeverity":"MEDIUM","attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"PRESENT","privilegesRequired":"LOW","userInteraction":"NONE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnAvailabilityImpact":"NONE","subConfidentialityImpact":"HIGH","subIntegrityImpact":"HIGH","subAvailabilityImpact":"NONE","exploitMaturity":"NOT_DEFINED","confidentialityRequirement":"NOT_DEFINED","integrityRequirement":"NOT_DEFINED","availabilityRequirement":"NOT_DEFINED","modifiedAttackVector":"NOT_DEFINED","modifiedAttackComplexity":"NOT_DEFINED","modifiedAttackRequirements":"NOT_DEFINED","modifiedPrivilegesRequired":"NOT_DEFINED","modifiedUserInteraction":"NOT_DEFINED","modifiedVulnConfidentialityImpact":"NOT_DEFINED","modifiedVulnIntegrityImpact":"NOT_DEFINED","modifiedVulnAvailabilityImpact":"NOT_DEFINED","modifiedSubConfidentialityImpact":"NOT_DEFINED","modifiedSubIntegrityImpact":"NOT_DEFINED","modifiedSubAvailabilityImpact":"NOT_DEFINED","Safety":"NOT_DEFINED","Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","valueDensity":"NOT_DEFINED","vulnerabilityResponseEffort":"NOT_DEFINED","providerUrgency":"NOT_DEFINED"}}],"cvssMetricV31":[{"source":"nvd@nist.gov","type":"Primary","cvssData":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N","baseScore":8.2,"baseSeverity":"HIGH","attackVector":"NETWORK","attackComplexity":"LOW","privilegesRequired":"NONE","userInteraction":"NONE","scope":"UNCHANGED","confidentialityImpact":"LOW","integrityImpact":"HIGH","availabilityImpact":"NONE"},"exploitabilityScore":3.9,"impactScore":4.2}],"ssvcV203":[{"source":"134c704f-9b21-4f2e-91b3-4a467353bcc0","ssvcData":{"timestamp":"2026-09-10T15:03:25.506535Z","id":"CVE-2026-88879","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}]},"weaknesses":[{"source":"disclosure@vulncheck.com","type":"Secondary","description":[{"lang":"en","value":"CWE-290"}]}],"configurations":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:a:traefik:traefik:*:*:*:*:*:*:*:*","versionStartIncluding":"1.0","versionEndExcluding":"2.11.56","matchCriteriaId":"FEC1DCAA-02FB-4FF2-B353-EEE832D5D594"},{"vulnerable":true,"criteria":"cpe:2.3:a:traefik:traefik:*:*:*:*:*:*:*:*","versionStartIncluding":"3.0.0","versionEndExcluding":"3.7.12","matchCriteriaId":"DA63A7E8-A5BB-4456-965E-F493CCC6D3C5"}]}]}],"references":[{"url":"https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c","source":"disclosure@vulncheck.com","tags":["Patch","Vendor Advisory"]},{"url":"https://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias","source":"disclosure@vulncheck.com","tags":["Third Party Advisory"]}]}