ClassCastException (HTTP 500) on /oauth2/token when single-value parameters are duplicated

Describe the bug

When the OAuth2 /oauth2/token endpoint receives a request with a duplicated single-value parameter (e.g. two code values), the server responds with HTTP 500 due to an unhandled ClassCastException:

java.lang.ClassCastException: class [Ljava.lang.String; cannot be cast to class java.lang.String

Per [RFC 6749, Section 3.2](https://datatracker.ietf.org/doc/html/rfc6749#section-3.2) (Token Endpoint):

Request and response parameters MUST NOT be included more than once.

And [Section 4.1.2.1](https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.2.1) explicitly defines this case as invalid_request:

invalid_request: The request is missing a required parameter, includes an invalid parameter value, includes a parameter more than once, or is otherwise malformed.

The server should therefore reject the request with an OAuth2 invalid_request error (HTTP 400), not crash with an internal error.

Likely cause: when duplicate form parameters are sent, ServletRequest#getParameterMap() returns values as String[]. The OAuth2 token endpoint converters appear to assume each entry is a String and perform an unchecked cast, raising ClassCastException instead of a controlled OAuth2AuthenticationException(invalid_request).

To Reproduce

Send the following request against a standard Spring Authorization Server token endpoint:

POST /oauth2/token HTTP/1.1
Host: auth.example.com
Content-Type: application/x-www-form-urlencoded
Authorization: Basic <base64(client_id:client_secret)>

grant_type=authorization_code
&code=bogus-auth-code-a
&code=bogus-auth-code-b
&redirect_uri=https://example.com/callback
&code_verifier=xyz

Note the duplicated code parameter. The server returns HTTP 500 with the ClassCastException above.

Environment:

• Spring Boot: 4.0.6
• Spring Security: 7.0.5 (managed by Spring Boot 4.0.6; includes Spring Authorization Server, which has been folded into Spring Security since Boot 4.0)
• Spring Framework: 7.0.7
• Java: 25
• Client: Apache HttpClient 5.6.1

Also reproduced on Spring Boot 4.0.3 (Spring Security 7.0.2).

Expected behavior

The server should return an OAuth2-compliant error response:

HTTP/1.1 400 Bad Request
Content-Type: application/json

{
  "error": "invalid_request",
  "error_description": "OAuth 2.0 Parameter: code"
}

Single-value OAuth2 parameters (code, grant_type, redirect_uri, code_verifier, client_id, refresh_token, scope, etc.) appearing more than once should trigger OAuth2AuthenticationException with OAuth2ErrorCodes.INVALID_REQUEST, consistent with the existing "missing parameter" handling. The same defect likely affects /oauth2/authorize and should be fixed symmetrically.

Sample

(to be filled — link to a minimal reproducible sample repository)

Start by tracing the OAuth2 /oauth2/token request converters around ServletRequest#getParameterMap() and reproduce the duplicated code request described here. Verify that repeated single-value parameters produce an OAuth2 invalid_request response with HTTP 400 instead of ClassCastException and HTTP 500, then check whether the same handling applies to /oauth2/authorize.