diff --git a/kinto/core/resource/__init__.py b/kinto/core/resource/__init__.py
index 994f43cf4..f8aaa7b15 100644
--- a/kinto/core/resource/__init__.py
+++ b/kinto/core/resource/__init__.py
@@ -1058,6 +1058,10 @@ def _extract_limit(self):
 
     def _extract_filters(self):
         """Extracts filters from QueryString parameters."""
+
+        def is_valid_timestamp(value):
+            return isinstance(value, int) or re.match(r'^"?\d+"?$', str(value))
+
         queryparams = self.request.validated["querystring"]
 
         filters = []
@@ -1090,7 +1094,7 @@ def _extract_filters(self):
                         send_alert(self.request, message, url)
                     operator = COMPARISON.LT
 
-                if value == "" or not isinstance(value, (int, str, type(None))):
+                if value is not None and not is_valid_timestamp(value):
                     raise_invalid(self.request, **error_details)
 
                 filters.append(Filter(self.model.modified_field, value, operator))
@@ -1127,7 +1131,7 @@ def _extract_filters(self):
                 error_details["description"] = "Invalid character 0x00"
                 raise_invalid(self.request, **error_details)
 
-            if field == self.model.modified_field and value == "":
+            if field == self.model.modified_field and not is_valid_timestamp(value):
                 raise_invalid(self.request, **error_details)
 
             filters.append(Filter(field, value, operator))
diff --git a/tests/core/resource/test_filter.py b/tests/core/resource/test_filter.py
index ba8603c90..d34eb8f0a 100644
--- a/tests/core/resource/test_filter.py
+++ b/tests/core/resource/test_filter.py
@@ -83,6 +83,15 @@ def test_filter_raises_error_if_last_modified_value_is_empty(self):
         self.validated["querystring"] = {"lt_last_modified": ""}
         self.assertRaises(httpexceptions.HTTPBadRequest, self.resource.plural_get)
 
+    def test_filter_raises_error_if_last_modified_value_is_not_int(self):
+        bad_value = "171103608603432920249' or '7127'='7127"
+        self.validated["querystring"] = {"last_modified": bad_value}
+        self.assertRaises(httpexceptions.HTTPBadRequest, self.resource.plural_get)
+        self.validated["querystring"] = {"_since": bad_value}
+        self.assertRaises(httpexceptions.HTTPBadRequest, self.resource.plural_get)
+        self.validated["querystring"] = {"lt_last_modified": bad_value}
+        self.assertRaises(httpexceptions.HTTPBadRequest, self.resource.plural_get)
+
     def test_filter_works_with_since_none(self):
         self.validated["querystring"] = {"_since": None}
         result = self.resource.plural_get()