diff --git a/backend/internal/service/oidc_service.go b/backend/internal/service/oidc_service.go
index fbce6f81..4eb4f99d 100644
--- a/backend/internal/service/oidc_service.go
+++ b/backend/internal/service/oidc_service.go
@@ -1260,7 +1260,10 @@ func (s *OidcService) createAuthorizationCode(ctx context.Context, clientID stri
 		return "", err
 	}
 
-	codeChallengeMethodSha256 := strings.ToUpper(codeChallengeMethod) == "S256"
+	codeChallengeMethodSha256, err := codeChallengeMethodIsSha256(codeChallengeMethod)
+	if err != nil {
+		return "", err
+	}
 
 	oidcAuthorizationCode := model.OidcAuthorizationCode{
 		ExpiresAt:                 datatype.DateTime(time.Now().Add(15 * time.Minute)),
@@ -1285,6 +1288,19 @@ func (s *OidcService) createAuthorizationCode(ctx context.Context, clientID stri
 	return randomString, nil
 }
 
+func codeChallengeMethodIsSha256(codeChallengeMethod string) (bool, error) {
+	switch strings.ToUpper(codeChallengeMethod) {
+	case "":
+		return false, nil
+	case "PLAIN":
+		return false, nil
+	case "S256":
+		return true, nil
+	default:
+		return false, common.NewOidcInvalidRequestError("code challenge method not supported")
+	}
+}
+
 func validateCodeVerifier(codeVerifier, codeChallenge string, codeChallengeMethodSha256 bool) bool {
 	if codeVerifier == "" || codeChallenge == "" {
 		return false
diff --git a/backend/internal/service/oidc_service_test.go b/backend/internal/service/oidc_service_test.go
index bd5e663d..d5d8ad45 100644
--- a/backend/internal/service/oidc_service_test.go
+++ b/backend/internal/service/oidc_service_test.go
@@ -763,6 +763,61 @@ func TestValidateCodeVerifier_Plain(t *testing.T) {
 	})
 }
 
+func TestCodeChallengeMethodIsSha256(t *testing.T) {
+	tests := []struct {
+		name       string
+		method     string
+		wantSha256 bool
+		wantErr    bool
+	}{
+		{
+			name:       "omitted defaults to plain",
+			method:     "",
+			wantSha256: false,
+		},
+		{
+			name:       "plain",
+			method:     "plain",
+			wantSha256: false,
+		},
+		{
+			name:       "plain case insensitive",
+			method:     "PLAIN",
+			wantSha256: false,
+		},
+		{
+			name:       "s256",
+			method:     "S256",
+			wantSha256: true,
+		},
+		{
+			name:       "s256 case insensitive",
+			method:     "s256",
+			wantSha256: true,
+		},
+		{
+			name:    "unknown method",
+			method:  "S384",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := codeChallengeMethodIsSha256(tt.method)
+			if tt.wantErr {
+				require.Error(t, err)
+				var invalidRequest *common.OidcInvalidRequestError
+				require.ErrorAs(t, err, &invalidRequest)
+				return
+			}
+
+			require.NoError(t, err)
+			assert.Equal(t, tt.wantSha256, got)
+		})
+	}
+}
+
 func TestOidcService_updateClientLogoType(t *testing.T) {
 	// Create a test database
 	db := testutils.NewDatabaseForTest(t)