diff --git a/internal/api/jwks.go b/internal/api/jwks.go
index 0ca0bb4b6..0bddc9841 100644
--- a/internal/api/jwks.go
+++ b/internal/api/jwks.go
@@ -86,7 +86,7 @@ func (a *API) WellKnownOpenID(w http.ResponseWriter, r *http.Request) error {
 		SubjectTypesSupported:             []string{"public"},
 		IDTokenSigningAlgValuesSupported:  []string{"RS256", "HS256", "ES256"}, // TODO :: should create this based on signing key config?
 		TokenEndpointAuthMethodsSupported: []string{"client_secret_basic", "client_secret_post", "none"},
-		CodeChallengeMethodsSupported:     []string{"S256", "plain"},
+		CodeChallengeMethodsSupported:     []string{"S256"},
 		ScopesSupported:                   models.SupportedOAuthScopes,
 
 		// OIDC Standard Claims
diff --git a/internal/api/oauthserver/authorize.go b/internal/api/oauthserver/authorize.go
index 2addacfda..4ce97d41d 100644
--- a/internal/api/oauthserver/authorize.go
+++ b/internal/api/oauthserver/authorize.go
@@ -476,15 +476,18 @@ func (s *Server) validateRemainingAuthorizeParams(params *AuthorizeParams) error
 }
 
 func (s *Server) validatePKCEParams(codeChallengeMethod, codeChallenge string) error {
-	// PKCE is mandatory for the authorization code flow OAuth2.1
-	// Both code_challenge and code_challenge_method must be provided together
+	// PKCE is mandatory for the authorization code flow per OAuth 2.1.
+	// Both code_challenge and code_challenge_method must be provided together.
 	if codeChallenge == "" || codeChallengeMethod == "" {
 		return errors.New("PKCE flow requires both code_challenge and code_challenge_method")
 	}
 
-	// Validate code challenge method (case-insensitive)
-	if strings.ToLower(codeChallengeMethod) != "s256" && strings.ToLower(codeChallengeMethod) != "plain" {
-		return errors.New("code_challenge_method must be 'S256' or 'plain'")
+	// OAuth 2.1 (draft-ietf-oauth-v2-1-12, Section 4.1.1) permits only S256.
+	// The plain method is excluded: code_challenge is transmitted in the
+	// authorization request URL and may appear in server logs, browser history,
+	// or Referer headers — with plain, that directly exposes the code_verifier.
+	if strings.ToLower(codeChallengeMethod) != "s256" {
+		return errors.New("code_challenge_method must be 'S256'")
 	}
 
 	// Validate code challenge format and length (per OAuth2 spec)
diff --git a/internal/api/oauthserver/authorize_test.go b/internal/api/oauthserver/authorize_test.go
index c7ec7f458..9934a20ad 100644
--- a/internal/api/oauthserver/authorize_test.go
+++ b/internal/api/oauthserver/authorize_test.go
@@ -666,3 +666,86 @@ func (ts *OAuthAuthorizeTestSuite) TestConsent_InvalidActionRejected() {
 	err := ts.Server.OAuthServerConsent(w, req)
 	ts.assertHTTPError(err, http.StatusBadRequest, apierrors.ErrorCodeValidationFailed)
 }
+
+// TestValidatePKCEParams_OAuth21 verifies that the plain PKCE method is rejected
+// per OAuth 2.1 (draft-ietf-oauth-v2-1-12, Section 4.1.1) and that S256 is accepted.
+func TestValidatePKCEParams_OAuth21(t *testing.T) {
+	globalConfig, err := confload.LoadGlobal(oauthServerTestConfig)
+	require.NoError(t, err)
+
+	conn, err := test.SetupDBConnection(globalConfig)
+	require.NoError(t, err)
+	defer conn.Close()
+
+	hooksMgr := &v0hooks.Manager{}
+	tokenService := tokens.NewService(globalConfig, hooksMgr)
+	server := NewServer(globalConfig, conn, tokenService)
+
+	tests := []struct {
+		name                string
+		codeChallengeMethod string
+		codeChallenge       string
+		wantErr             bool
+		errContains         string
+	}{
+		{
+			name:                "S256 accepted",
+			codeChallengeMethod: "S256",
+			codeChallenge:       "E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM",
+			wantErr:             false,
+		},
+		{
+			name:                "s256 lowercase accepted",
+			codeChallengeMethod: "s256",
+			codeChallenge:       "E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM",
+			wantErr:             false,
+		},
+		{
+			name:                "plain rejected",
+			codeChallengeMethod: "plain",
+			codeChallenge:       "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFG",
+			wantErr:             true,
+			errContains:         "S256",
+		},
+		{
+			name:                "PLAIN rejected case-insensitively",
+			codeChallengeMethod: "PLAIN",
+			codeChallenge:       "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFG",
+			wantErr:             true,
+			errContains:         "S256",
+		},
+		{
+			name:                "unknown method rejected",
+			codeChallengeMethod: "rs256",
+			codeChallenge:       "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFG",
+			wantErr:             true,
+			errContains:         "S256",
+		},
+		{
+			name:                "missing method rejected",
+			codeChallengeMethod: "",
+			codeChallenge:       "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFG",
+			wantErr:             true,
+			errContains:         "requires both",
+		},
+		{
+			name:                "missing challenge rejected",
+			codeChallengeMethod: "S256",
+			codeChallenge:       "",
+			wantErr:             true,
+			errContains:         "requires both",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := server.validatePKCEParams(tt.codeChallengeMethod, tt.codeChallenge)
+			if tt.wantErr {
+				assert.Error(t, err)
+				assert.Contains(t, err.Error(), tt.errContains)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}
diff --git a/internal/models/oauth_authorization.go b/internal/models/oauth_authorization.go
index 2b7fab257..5313beb9d 100644
--- a/internal/models/oauth_authorization.go
+++ b/internal/models/oauth_authorization.go
@@ -111,8 +111,9 @@ func NewOAuthServerAuthorization(params NewOAuthServerAuthorizationParams) *OAut
 		auth.CodeChallenge = &params.CodeChallenge
 	}
 	if params.CodeChallengeMethod != "" {
-		// Normalize code challenge method to lowercase for database storage
-		// Database enum expects 's256' and 'plain' (lowercase)
+		// Normalize code challenge method to lowercase for database storage.
+		// The OAuth 2.1 server only accepts 's256'; the legacy flow may still
+		// pass 'plain', which is handled at the validation layer.
 		normalizedMethod := strings.ToLower(params.CodeChallengeMethod)
 		auth.CodeChallengeMethod = &normalizedMethod
 	}