CVE-2026-66883 Oidcc.Plug.Authorize user agent session binding inert due to case-sensitive header lookup
Case-sensitive header lookup disables user agent session binding, allowing stolen session replay from different clients if the feature is enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-66883?
CVE-2026-66883 is a medium-severity vulnerability in Erlang Ecosystem Foundation oidcc_plug, published on 4 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Improper 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.
This 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.
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.
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 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.
This issue affects oidcc_plug: from 0.1.0-alpha.3 before 0.5.0.
Weakness: CWE-178
Which versions are affected by CVE-2026-66883?
1 open source package ship an affected version, including oidcc_plug.
| Package | Affected | Fixed in |
|---|---|---|
oidcc_plug Hex | From 0.1.0-alpha.3 | 0.5.0 |
How do I fix CVE-2026-66883?
Upgrade oidcc_plug to 0.5.0 or later.
Is CVE-2026-66883 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-66883 exists.
Sources
Also known as EEF-CVE-2026-66883, GHSA-w5r8-m75h-98fc.