{"id":"EEF-CVE-2026-66883","summary":"Oidcc.Plug.Authorize user agent session binding inert due to case-sensitive header lookup","details":"## Summary\n\nImproper Handling of Case Sensitivity vulnerability in Erlang Ecosystem Foundation oidcc_plug (`Oidcc.Plug.Authorize` module) renders the user agent session binding inert, removing a defense in depth control against replay of a stolen session.\n\nThis vulnerability is associated with program files `lib/oidcc/plug/authorize.ex` and `lib/oidcc/plug/authorization_callback.ex`, and program routines `Oidcc.Plug.Authorize.call/2` and `Oidcc.Plug.AuthorizationCallback.call/2`.\n\n`Oidcc.Plug.Authorize.call/2` reads the initiating client's user agent with `get_req_header(conn, \"User-Agent\")`. Plug lowercases incoming header names, but `get_req_header/2` matches the supplied key exactly and performs no normalization of its own, so the mixed-case lookup always returns an empty list and `nil` is written into the session. On the callback side, `Oidcc.Plug.AuthorizationCallback` treats a stored `nil` user agent as nothing to compare and returns `:ok` without inspecting the request. The two behaviours combine so that the check passes unconditionally on every request, including for deployments that explicitly opted in with `check_useragent: true`, and an authorization callback can be completed from a different user agent than the one that initiated the flow without detection. The check fails open silently, with no error and no log entry, so a deployment cannot tell the binding is absent.\n\nThe impact is limited to defense in depth. The inert check does not by itself allow an attacker to complete an authorization flow; it removes one layer that would otherwise hinder use of a stolen or leaked session, such as an exfiltrated session cookie replayed from a different client. The CSRF/state, nonce, and PKCE checks are unaffected and continue to function. Deployments that never enabled `check_useragent` are not affected in practice, since they never expected the binding. The corresponding lookup in `Oidcc.Plug.AuthorizationCallback` correctly uses the lowercase key and is not affected.\n\nThis issue affects oidcc_plug: from 0.1.0-alpha.3 before 0.5.0.","aliases":["CVE-2026-66883","GHSA-w5r8-m75h-98fc"],"modified":"2026-09-08T08:00:47.928246283Z","published":"2026-08-04T11:46:05.191Z","database_specific":{"cwe_ids":["CWE-178"],"capec_ids":["CAPEC-60"],"cpe_ids":["cpe:2.3:a:erlef:oidcc_plug:*:*:*:*:*:erlang:*:*"]},"references":[{"type":"ADVISORY","url":"https://github.com/erlef/oidcc_plug/security/advisories/GHSA-w5r8-m75h-98fc"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-66883.html"},{"type":"FIX","url":"https://github.com/erlef/oidcc_plug/commit/f15e52c5750aaba701104b10ed96cfac063bd557"},{"type":"PACKAGE","url":"https://hex.pm/packages/oidcc_plug"}],"affected":[{"package":{"name":"oidcc_plug","ecosystem":"Hex","purl":"pkg:hex/oidcc_plug"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.1.0-alpha.3"},{"fixed":"0.5.0"}]}],"versions":["0.1.0","0.1.0-alpha.3","0.1.0-alpha.4","0.1.0-rc.1","0.1.0-rc.2","0.1.1","0.1.2","0.2.0","0.2.0-beta.1","0.2.1","0.2.2","0.3.0","0.3.1","0.3.2","0.4.0"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-66883.json"}},{"ranges":[{"type":"GIT","repo":"https://github.com/erlef/oidcc_plug","events":[{"introduced":"0a75fb139d8e2686d1b75e0d48962323d2181673"},{"fixed":"f15e52c5750aaba701104b10ed96cfac063bd557"}]}],"versions":["v0.4.0","v0.3.2","v0.3.1","v0.3.0","v0.2.2","v0.2.1","v0.2.0","v0.2.0-beta.1","v0.1.2","v0.1.1","v0.1.0","v0.1.0-rc.2","v0.1.0-rc.1","v0.1.0-alpha.4","v0.1.0-alpha.3"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-66883.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N"}],"credits":[{"name":"Mathias Polligkeit","type":"FINDER"},{"name":"Jonatan Männchen / EEF","type":"REMEDIATION_DEVELOPER"},{"name":"Mathias Polligkeit","type":"REMEDIATION_REVIEWER"},{"name":"Jonatan Männchen / EEF","type":"COORDINATOR"}]}