This behavior essentially allows any malicious or compromised server that can trigger a 3xx redirect to force the client to disclose authenticated session tokens or proxy credentials. The vulnerability resides in the _fetch method of the Http2Adapter class, where header maps are preserved during the copyWith operation without sanitization. This flaw impacts server-side Dart applications, mobile applications, and CLI tools utilizing dio_http2_adapter for authenticated HTTP/2 communications.

Attack Chain

1. The victim application initiates an authenticated request using Dio with Http2Adapter, including sensitive headers like Authorization or Cookie.
2. The application contacts a malicious or compromised origin server that is under the attacker's control.
3. The malicious server responds with an HTTP 302 Found (or other 3xx status) and a Location header pointing to an attacker-controlled URI.
4. The Http2Adapter processes the 3xx response, parsing the Location header to determine the new redirect target.
5. The adapter calls options.copyWith to initiate the new request to the redirect URI, inadvertently cloning the full original header map into the new request options.
6. No cross-origin check is performed to see if the new URI belongs to a different authority.
7. The _fetch method emits the new HTTP/2 request, now including the sensitive Authorization and Cookie headers, to the attacker-controlled server.
8. The attacker logs the incoming request headers at the final destination, successfully exfiltrating the victim's credentials.

Impact

Successful exploitation allows for the unauthorized disclosure of sensitive credentials, including bearer tokens, session identifiers, and proxy authentication details. This enables attackers to impersonate the victim in subsequent requests to the original legitimate service. Given the prevalence of Dio in the Dart and Flutter ecosystems, any application using dio_http2_adapter with followRedirects enabled is at risk. The lack of authentication required to perform the redirect makes this a low-complexity attack for any adversary who can intercept or host an HTTP-based service.

Recommendation

• Immediately upgrade or patch the dio_http2_adapter library logic to implement origin validation during redirect processing.
• For immediate mitigation, define a list of sensitive headers (e.g., Authorization, Cookie, Proxy-Authorization) and manually remove these headers from the RequestOptions when the redirect URI differs from the original request authority.
• Developers should ensure that their Dio configurations explicitly manage redirect behavior if full library updates are delayed, though this does not replace the need for the library-level sanitization fix.
• Organizations deploying Dart-based CLI tools or backend services should audit internal codebases to confirm if dio_http2_adapter is utilized for authenticated requests to untrusted or external endpoints.