CVE-2026-54603

Summary When an application uses OAuth2::Client (typically via an OAuth2::AccessToken) and the configured authorization server returns a redirect whose Location header is a protocol-relative URI of the form //attacker.example/leak, OAuth2::Client#request resolves the redirect with response.response.env.url.merge(location). Per RFC 3986 §5.2, an input that starts with // is a network-path reference and replaces the authority of the base URL: URI("http://idp.trusted/userinfo").merge("//attacker.example/leak") returns http://attacker.example/leak. The recursive request(verb, full_location, req_opts) call then re-sends the request to the attacker host while preserving the Authorization: Bearer <access-token> header that OAuth2::AccessToken#configure_authentication! installed on req_opts[:headers] for the original request. The result is a one-shot cross-origin credential disclosure: any 30x response from the IdP that an attacker can influence (a compromised endpoint, a tenant-controlled IdP in a multi-tenant deployment, or an open-redirect handler that does not normalise the Location it emits) can extract the bearer access token of the calling user. Affected - oauth2 v2.0.21 and all prior versions back to and including v0.4.0. - The underlying unsafe redirect-following behavior that reuses headers starts in v0.4.0 via https://github.com/ruby-oauth/oauth2/commit/b944da54cd70487c06d7252b9e4e7948eae56e73 - The vulnerability was retained when the code was refactored to a redirect helper, and in this form has been present in OAuth2::Client#request since v2.0.0 via https://github.com/ruby-oauth/oauth2/commit/b944da54cd70487c06d7252b9e4e7948eae56e73 - Ruby 4.0.5 on arm64-darwin25. The behaviour of URI#merge for protocol-relative inputs is RFC-conformant and the same on every supported Ruby (≥ 2.x). - Adapter independence: confirmed against the default faraday 2.14.2 + faraday-net_http 3.4.3 stack. The URI#merge call is in oauth2 itself, not in Faraday, so the issue is not a

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