CVE-2026-66883: Oidcc.Plug.Authorize user agent session binding inert due to case-sensitive header lookup

Improper Handling of Case Sensitivity vulnerability in Erlang Ecosystem Foundation oidccplug (Oidcc.Plug.Authorize module) renders the user agent session binding inert, removing a defense in depth control against replay of a stolen session.

This vulnerability is associated with program files lib/oidcc/plug/authorize.ex and lib/oidcc/plug/authorizationcallback.ex, and program routines Oidcc.Plug.Authorize.call/2 and Oidcc.Plug.AuthorizationCallback.call/2.

Oidcc.Plug.Authorize.call/2 reads the initiating client's user agent with getreqheader(conn, "User-Agent"). Plug lowercases incoming header names, but getreqheader/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 checkuseragent: 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.

The 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 checkuseragent 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.

This issue affects oidccplug: from 0.1.0-alpha.3 before 0.5.0.