{"id":"EEF-CVE-2026-82756","summary":"ash_authentication_oauth2_server interpolates a tenant-derived value into the WWW-Authenticate challenge without escaping, allowing header parameter injection","details":"## Summary\n\nImproper Encoding or Escaping of Output vulnerability in ash-project ash_authentication_oauth2_server allows an unauthenticated attacker to inject arbitrary authentication parameters into the `WWW-Authenticate` challenge header.\n\n`BearerPlug` and `RequireScopePlug` built the `Bearer resource_metadata=\"...\"` challenge by interpolating a `resource_metadata` URL derived from the request tenant directly into the quoted value. In a multi-tenant application that sets the Ash tenant from request-controlled data (a subdomain, the `Host`, a path segment, or a header), a tenant containing a `\"` closes the quoted value and appends attacker-chosen auth-params, including a second `resource_metadata` URL pointing at an attacker-controlled authorization server that spec-following clients follow. Carriage returns and line feeds are rejected by Plug, so this is parameter injection within one header, not response splitting.\n\nThis issue affects ash_authentication_oauth2_server: from 0.1.3 before 0.3.1.\n\n## Configurations\n\nReachable only in a multi-tenant application that derives the Ash tenant from request-controlled input (subdomain, `Host`, path, or header) and uses `BearerPlug` or `RequireScopePlug`; the tenant reaches the challenge through the server's tenant-aware `resource_url`.","aliases":["CVE-2026-82756","GHSA-2h3v-83jg-2qmm"],"modified":"2026-09-08T22:45:02.816934255Z","published":"2026-09-07T22:30:55.280Z","database_specific":{"cwe_ids":["CWE-116"],"capec_ids":["CAPEC-153"],"cpe_ids":["cpe:2.3:a:ash-project:ash_authentication_oauth2_server:*:*:*:*:*:*:*:*"]},"references":[{"type":"ADVISORY","url":"https://github.com/ash-project/ash_authentication_oauth2_server/security/advisories/GHSA-2h3v-83jg-2qmm"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-82756.html"},{"type":"FIX","url":"https://github.com/ash-project/ash_authentication_oauth2_server/commit/09f97476715da031b136eaec7b2cda2363ad8149"},{"type":"PACKAGE","url":"https://hex.pm/packages/ash_authentication_oauth2_server"}],"affected":[{"package":{"name":"ash_authentication_oauth2_server","ecosystem":"Hex","purl":"pkg:hex/ash_authentication_oauth2_server"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.1.3"},{"fixed":"0.3.1"}]}],"versions":["0.1.3","0.2.0","0.2.1","0.2.2","0.3.0"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-82756.json"}},{"ranges":[{"type":"GIT","repo":"https://github.com/ash-project/ash_authentication_oauth2_server","events":[{"introduced":"99de0a1cacb5ef667c4533278b7c81ca98c00231"},{"fixed":"09f97476715da031b136eaec7b2cda2363ad8149"}]}],"versions":["v0.3.0","v0.2.2","v0.2.1","v0.2.0","v0.1.3"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-82756.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N"}],"credits":[{"name":"Peter Ullrich","type":"FINDER"},{"name":"Peter Ullrich","type":"REPORTER"},{"name":"Zach Daniel / Ash Project","type":"REMEDIATION_DEVELOPER"},{"name":"Jonatan Männchen / EEF","type":"COORDINATOR"}]}