GHSA-prq8-7wvh-44qh

# Cross-origin OAuth token-request redirects can expose signed request metadata

## Summary

When an application uses `OAuth::Consumer` to request OAuth 1.0 request tokens or access tokens, the token request helper follows `300..399` redirects returned by the OAuth server. In affected versions, `OAuth::Consumer#token_request` parses the raw `Location` header, follows the redirect recursively, and can mutate the consumer's configured `site` when the redirect points to a different host with the same path.

The result is a cross-origin signed-request disclosure primitive: if an OAuth server token endpoint returns a redirect whose target an attacker controls, the client can re-sign the token request and send OAuth 1.0 request metadata, including the OAuth signature, nonce, timestamp, consumer key, and any request parameters included in the signature base string, to the attacker-controlled host. The same behavior can also be used as an SSRF or confused-deputy primitive because the application server follows the redirect and sends the next request from its own network position.

## Affected

- `oauth` v1.1.5 and prior versions back to and including v0.5.5. - The cross-host token redirect behavior was introduced by https://github.com/ruby-oauth/oauth/commit/d74b767f - The behavior is documented in the v0.5.5 changelog as "Allow redirect to different host but same path". - The vulnerable behavior is in `OAuth::Consumer#token_request`, which is used by the documented request-token and access-token flows. - The issue is not specific to a Ruby engine or platform. It is caused by the gem's redirect handling and recursive token request behavior.

Patched version: `oauth` v1.1.6.

## Impact

A consumer that calls `OAuth::Consumer#get_request_token`, `OAuth::Consumer#get_access_token`, or lower-level token request helpers against an OAuth server whose token endpoint redirect target can be influenced may lose three security properties:

1. **Cross-origin signed-request metadata disclosure.** The redirected request is signed for the attacker-controlled endpoint. Depending on the request method, scheme, and parameters, the attacker may receive OAuth 1.0 parameters such as `oauth_consumer_key`, `oauth_signature_method`, `oauth_timestamp`, `oauth_nonce`, `oauth_version`, and `oauth_signature`. 2. **SSRF from the application server.** The OAuth client follows the redirect on behalf of the application, so the redirected host is contacted from the application server's network position. 3. **Confused-deputy behavior.** A malicious or compromised token endpoint can cause an otherwise trusted application to initiate signed requests to an unintended origin.

The disclosed OAuth 1 signature is not equivalent to an OAuth 2 bearer token: it is bound to the signed request, timestamp, nonce, HTTP method, and request URL. However, it can still disclose sensitive integration metadata, may be replayable within the receiver's accepted nonce/timestamp window in some deployments, and can expose application-server reachability to attacker-selected hosts.

## Vulnerable code

[`lib/oauth/consumer.rb`](https://github.com/ruby-oauth/oauth/blob/v1.1.5/lib/oauth/consumer.rb) at tag `v1.1.5`:

```ruby def token_request(http_method, path, token = nil, request_options = {}, *arguments) request_options[:token_request] ||= true response = request(http_method, path, token, request_options, *arguments) case response.code.to_i

when (200..299) # parse token response when (300..399) # Parse redirect to follow uri = URI.parse(response["location"]) our_uri = URI.parse(site)

# Guard against infinite redirects response.error! if uri.path == path && our_uri.host == uri.host

if uri.path == path && our_uri.host != uri.host options[:site] = "#{uri.scheme}://#{uri.host}" @http = create_http end

token_request(http_method, uri.path, token, request_options, arguments) when (400..499) raise OAuth::Unauthorized, response else response.error! end end ```

The vulnerable behavior has several parts:

- `response["location"]` is trusted as the next token request target. - Redirects are followed for every `300..399` response. - There is no general redirect counter or maximum redirect limit. - Cross-host redirects with the same path can mutate `options[:site]` and rebuild the underlying HTTP client. - The recursive call continues the token request flow and signs the next request for the redirected destination.

## Reachable in production

The vulnerable path is reachable through the normal OAuth 1 token exchange:

```ruby consumer = OAuth::Consumer.new( consumer_key, consumer_secret, site: "https://provider.example" )

request_token = consumer.get_request_token ```

If `https://provider.example/oauth/request_token` returns a redirect to an attacker-controlled host, the library follows that redirect as part of the token request flow. A realistic trigger is an OAuth provider, gateway, or reverse proxy that emits `Location` based on user-controlled or tenant-controlled input, or a malicious tenant-controlled OAuth endpoint in a multi-tenant integration.

No application-level redirect handling is required. The redirect is followed inside the gem before the application receives the token response.

## Reproduction

A vulnerable application is one that uses `OAuth::Consumer` to perform an OAuth 1 token exchange against a token endpoint that can be made to return a redirect to another origin.

Example shape:

```ruby consumer = OAuth::Consumer.new( consumer_key, consumer_secret, site: "https://provider.example" )

consumer.get_request_token ```

If `https://provider.example/oauth/request_token` responds with a `30x` redirect whose `Location` points to `https://attacker.example/...`, affected versions may follow that redirect as part of the token request flow. The redirected request is then generated and signed by the application server for the new destination. Depending on the configured OAuth request scheme, OAuth 1 parameters may be sent in the `Authorization` header, request body, or query string.

Expected vulnerable behavior:

- the token request leaves the configured OAuth provider origin; - the redirected host receives a signed OAuth 1 token request; - the application does not get a chance to inspect or approve the redirect target before the redirected request is sent.

Expected patched behavior:

- same-origin redirects continue to work, subject to a redirect limit; - cross-origin token endpoint redirects are rejected by default; - applications that intentionally require cross-origin token redirects must opt in explicitly with `token_request_cross_origin_redirects`.

## Suggested fix

Reject cross-origin token endpoint redirects by default and require explicit opt-in for integrations that intentionally depend on that behavior.

The fix released in v1.1.6 does the following:

```ruby current_uri = token_request_uri(path) redirected_uri = token_request_redirect_uri(current_uri, response) response.error! unless redirected_uri

redirect_count = request_options[:token_request_redirect_count].to_i + 1 response.error! if redirect_count > token_request_max_redirects(request_options) response.error! if token_request_cross_origin?(current_uri, redirected_uri) && !token_request_cross_origin_redirects?(request_options)

redirect_options = request_options.merge(token_request_redirect_count: redirect_count) token_request(http_method, token_request_redirect_path(current_uri, redirected_uri), token, redirect_options, *arguments, &block) ```

The fix intentionally preserves same-origin redirect compatibility while making cross-origin token endpoint redirects an explicit choice. It also avoids placing internal redirect state in `request_options` passed to OAuth signing.

## Workarounds

Until a patched release is available, applications can reduce exposure by doing one or more of the following:

- Ensure configured OAuth token endpoints are fixed absolute URLs controlled by a trusted provider. - Do not use tenant-controlled OAuth token endpoint URLs unless the tenant is trusted to receive signed OAuth token requests. - Block outbound application-server traffic to internal metadata services and other sensitive internal addresses at the network layer. - Place a trusted proxy in front of OAuth providers that rejects token endpoint redirects to a different origin.

These mitigations reduce exploitability but do not remove the vulnerable redirect logic from the gem.

## Credit

Found during the follow-up audit for `GHSA-pp92-crg2-gfv9`.

Reporter/coordinator: Peter H. Boling (`pboling`).

## References

- Related OAuth 2 advisory: https://github.com/ruby-oauth/oauth2/security/advisories/GHSA-pp92-crg2-gfv9 - Fixed version: `oauth` v1.1.6 - Behavior-introducing commit: https://github.com/ruby-oauth/oauth/commit/d74b767f