diff --git a/app/controllers/users/carte_controller.rb b/app/controllers/users/carte_controller.rb
index f41d55bf2..258299254 100644
--- a/app/controllers/users/carte_controller.rb
+++ b/app/controllers/users/carte_controller.rb
@@ -12,17 +12,18 @@ class Users::CarteController < UsersController
   end
 
   def save
+    safe_json_latlngs = clean_json_latlngs(params[:json_latlngs])
     dossier = current_user_dossier
 
     dossier.quartier_prioritaires.each(&:destroy)
     dossier.cadastres.each(&:destroy)
 
-    if params[:json_latlngs].present?
-      ModuleApiCartoService.save_qp! dossier, params[:json_latlngs]
-      ModuleApiCartoService.save_cadastre! dossier, params[:json_latlngs]
+    if safe_json_latlngs.present?
+      ModuleApiCartoService.save_qp! dossier, safe_json_latlngs
+      ModuleApiCartoService.save_cadastre! dossier, safe_json_latlngs
     end
 
-    dossier.update(json_latlngs: params[:json_latlngs])
+    dossier.update(json_latlngs: safe_json_latlngs)
 
     redirect_to modifier_dossier_path(dossier)
   end
@@ -62,4 +63,18 @@ class Users::CarteController < UsersController
       api_carto: true
     }
   end
+
+  private
+
+  def clean_json_latlngs(json_latlngs)
+    # a polygon must contain at least 4 points
+    # https://tools.ietf.org/html/rfc7946#section-3.1.6
+    if json_latlngs.present?
+      multipolygone = JSON.parse(json_latlngs)
+      multipolygone.reject! { |polygone| polygone.count < 4 }
+      if multipolygone.present?
+        multipolygone.to_json
+      end
+    end
+   end
 end
diff --git a/spec/controllers/users/carte_controller_shared_example.rb b/spec/controllers/users/carte_controller_shared_example.rb
index 229d2d3a3..b517ad8c4 100644
--- a/spec/controllers/users/carte_controller_shared_example.rb
+++ b/spec/controllers/users/carte_controller_shared_example.rb
@@ -59,6 +59,45 @@ shared_examples 'carte_controller_spec' do
   end
 
   describe 'POST #save' do
+    context 'it cleans json_latlngs' do
+      let(:dossier) { create(:dossier, state: 'en_construction') }
+      let(:json_latlngs) { multipolygon.to_json }
+
+      before do
+        post :save, params: { dossier_id: dossier.id, json_latlngs: json_latlngs }
+        dossier.reload
+      end
+
+      context 'when json_latlngs is invalid' do
+        let(:multipolygon) do
+          [
+            [
+              { lat: 1, lng: 1 },
+              { lat: 1, lng: 2 },
+              { lat: 1, lng: 1 }
+            ]
+          ]
+        end
+
+        it { expect(dossier.json_latlngs).to be_nil }
+      end
+
+      context 'when json_latlngs is valid' do
+        let(:multipolygon) do
+          [
+            [
+              { lat: 1, lng: 1 },
+              { lat: 1, lng: 2 },
+              { lat: 2, lng: 2 },
+              { lat: 1, lng: 1 }
+            ]
+          ]
+        end
+
+        it { expect(dossier.json_latlngs).to eq(json_latlngs) }
+      end
+    end
+
     context 'En train de modifier la localisation' do
       let(:dossier) { create(:dossier, state: 'en_construction') }
       before do