diff --git a/internal/oauth/server.go b/internal/oauth/server.go
index 05e8cad44..05872318a 100644
--- a/internal/oauth/server.go
+++ b/internal/oauth/server.go
@@ -126,7 +126,6 @@ func New(
 				oauth2.ClientCredentials,
 			},
 			AllowedCodeChallengeMethods: []oauth2.CodeChallengeMethod{
-				oauth2.CodeChallengePlain,
 				oauth2.CodeChallengeS256,
 			},
 		},