code_
Registry Context
PKCE uses `code_challenge_method` in the authorization request to identify how the authorization server will transform the code verifier during token exchange. If omitted, the method defaults to `plain`; current security guidance favors `S256`.
Technical Summary
The authorization server binds `code_challenge_method` and `code_challenge` to the authorization code. At the token endpoint, it applies the bound method to the received `code_verifier` and compares the result with the associated challenge.
When Used
In PKCE authorization requests and subsequent verification of the code verifier at the token endpoint.
Normative Requirements
Clients
RFC 9700 - Section 2.1.1
Clients SHOULD use a PKCE code challenge method that does not expose the code verifier in the authorization request..
Condition: When using PKCE.
clients SHOULD use PKCE code challenge methods that do not expose the PKCE verifier
RFC 7636 - Section 4.3
The `code_challenge_method` authorization-request parameter is optional and defaults to `plain` when absent..
Condition: When sending a PKCE code challenge in an OAuth 2.0 authorization request.
code_challenge_method OPTIONAL, defaults to "plain" if not present
Authorization servers
RFC 7636 - Section 4.4
The authorization server MUST associate the `code_challenge` and `code_challenge_method` values with the authorization code..
Condition: When issuing the authorization code.
MUST associate the "code_challenge" and "code_challenge_method" values with the authorization code
RFC 9700 - Section 2.1.1
The authorization server MUST enforce correct use of the `code_verifier` at the token endpoint when the authorization request contained a valid `code_challenge`..
Condition: If the client sent a valid PKCE `code_challenge` parameter.
the authorization server MUST enforce the correct usage of code_verifier at the token endpoint
RFC 9700 - Section 2.1.1
The authorization server MUST accept a token request containing `code_verifier` only if the corresponding authorization request contained `code_challenge`..
Condition: To mitigate PKCE downgrade attacks.
Authorization servers MUST mitigate PKCE downgrade attacks by ensuring that a token request containing a code_verifier parameter is accepted only if a code_challenge parameter was present
token endpoint
RFC 7636 - Section 4.5
The token endpoint MUST use the `code_challenge_method` bound to the authorization code to verify the `code_verifier`..
Condition: When processing the token request for the authorization code.
the method that the token endpoint MUST use to verify the "code_verifier"
RFC 7636 - Section 4.6
The token endpoint MUST continue normal OAuth 2.0 processing when the transformed verifier equals the associated code challenge..
Condition: When PKCE verification succeeds.
If the values are equal, the token endpoint MUST continue processing as normal
RFC 7636 - Section 4.6
The token endpoint MUST return an `invalid_grant` error response when the transformed verifier does not equal the associated code challenge..
Condition: When PKCE verification fails.
If the values are not equal, an error response indicating "invalid_grant" ... MUST be returned
Validation Guidance
Treat an omitted `code_challenge_method` as `plain`.
Associate both the code challenge and its method with the issued authorization code.
Verify the code verifier using the method bound to the authorization code.
Continue normal token processing after successful PKCE verification.
Return `invalid_grant` when PKCE verification fails.
Prefer `S256` rather than `plain`, because `S256` does not expose the verifier in the authorization request.
Reject a token request containing `code_verifier` if the authorization request did not contain `code_challenge`.
Security Notes
RFC 9700 - Section 2.1.1
RFC 9700 identifies `S256` as the only currently defined PKCE method that does not expose the verifier in the authorization request.
Reference
Details
- Entry Id
code_challenge_ method - Name
code_challenge_ method - Parameter Usage Location
authorization request- Change Controller
IESG- Reference
RFC7636