escape csv formulae

This is only needed for unsecure spreadsheet applications (hi Google Docs and MS Excel) that load formulae by default.

See https://owasp.org/www-community/attacks/CSV_Injection for some mitigation explanation. This is not complete, but it should be OK for now.

ihatemoney/tests/import_test.py

@@ -596,6 +596,38 @@ class ExportTestCase(IhatemoneyTestCase):
                 set(line.split(",")), set(received_lines[i].strip("\r").split(","))
             )
 
+    def test_export_escape_formulae(self):
+        self.post_project("raclette", default_currency="EUR")
+
+        # add participants
+        self.client.post("/raclette/members/add", data={"name": "zorglub"})
+
+        # create bills
+        self.client.post(
+            "/raclette/add",
+            data={
+                "date": "2016-12-31",
+                "what": "=COS(36)",
+                "payer": 1,
+                "payed_for": [1],
+                "amount": "10.0",
+                "original_currency": "EUR",
+            },
+        )
+
+        # generate csv export of bills
+        resp = self.client.get("/raclette/export/bills.csv")
+        expected = [
+            "date,what,amount,currency,payer_name,payer_weight,owers",
+            "2016-12-31,'=COS(36),10.0,EUR,zorglub,1.0,zorglub",
+        ]
+        received_lines = resp.data.decode("utf-8").split("\n")
+
+        for i, line in enumerate(expected):
+            self.assertEqual(
+                set(line.split(",")), set(received_lines[i].strip("\r").split(","))
+            )
+
 
 class ImportTestCaseJSON(CommonTestCase.Import):
     def generate_form_data(self, data):

ihatemoney/utils.py

@@ -152,6 +152,17 @@ def list_of_dicts2json(dict_to_convert):
     return BytesIO(dumps(dict_to_convert).encode("utf-8"))
 
 
+def escape_csv_formulae(value):
+    # See https://owasp.org/www-community/attacks/CSV_Injection
+    if (
+        value
+        and isinstance(value, str)
+        and value[0] in ["=", "+", "-", "@", "\t", "\n"]
+    ):
+        return f"'{value}"
+    return value
+
+
 def list_of_dicts2csv(dict_to_convert):
     """Take a list of dictionnaries and turns it into
     a csv in-memory file, assume all dict have the same keys
@@ -164,7 +175,10 @@ def list_of_dicts2csv(dict_to_convert):
         # (expecting a sequence getting a view)
         csv_data = [list(dict_to_convert[0].keys())]
         for dic in dict_to_convert:
-            csv_data.append([dic[h] for h in dict_to_convert[0].keys()])
+            csv_data.append(
+                [escape_csv_formulae(dic[h]) for h in dict_to_convert[0].keys()]
+            )
+            # csv_data.append([dic[h] for h in dict_to_convert[0].keys()])
     except (KeyError, IndexError):
         csv_data = []
     writer = csv.writer(csv_file)