diff --git a/tools/check_conventions.py b/tools/check_conventions.py
index 01a2b012b..92d5386bb 100644
--- a/tools/check_conventions.py
+++ b/tools/check_conventions.py
@@ -143,7 +143,18 @@ class Check:
             if match and not match.group(1).startswith("3rdparty/"):
                 self.report("system-dependency", ".gitmodules", number, match.group(1))
 
+    def submodules(self):
+        result = subprocess.run(["git", "config", "--no-includes", "-z", "--blob", f"{self.head}:.gitmodules", "--list"],
+                                capture_output=True, encoding="utf-8", errors="replace")
+        values = {key: value for key, _, value in (entry.partition("\n") for entry in result.stdout.split("\0") if entry)}
+        return {path for key, path in values.items() if key.endswith(".path") and values.get(f"{key[:-5]}.url")}
+
     def repository(self):
+        gitlinks = {path for path, mode in self.modes.items() if mode == "160000" and path.startswith("3rdparty/")}
+        checked = gitlinks if ".gitmodules" in self.status else gitlinks & self.status.keys()
+        if checked:
+            for path in sorted(checked - self.submodules()):
+                self.report("system-dependency", path, 0, "submodule without a url in .gitmodules")
         for path, (added, deleted) in self.numstat.items():
             if path.startswith("3rdparty/"):
                 continue
diff --git a/tools/tests/test_check_conventions_comments.py b/tools/tests/test_check_conventions_comments.py
index 89df82af5..44616324e 100644
--- a/tools/tests/test_check_conventions_comments.py
+++ b/tools/tests/test_check_conventions_comments.py
@@ -295,6 +295,14 @@ class RepositoryChecksTests(unittest.TestCase):
         self.git("add", ".")
         self.git("commit", "-qm", "test: add convention fixture")
 
+    def add_thirdparty_submodule(self, gitmodules=None):
+        head = self.git("rev-parse", "HEAD")
+        if gitmodules is not None:
+            (self.root / ".gitmodules").write_text(gitmodules, encoding="utf-8")
+            self.git("add", ".gitmodules")
+        self.git("update-index", "--add", "--cacheinfo", "160000", head, "3rdparty/vendor")
+        self.git("commit", "-qm", "build: add submodule")
+
     def test_quoted_utf8_paths_are_checked(self):
         name = "docs/dev/測試guide.md" if os.name == "nt" else "docs/dev/測試\tguide.md"
         path = self.root / name
@@ -337,13 +345,30 @@ class RepositoryChecksTests(unittest.TestCase):
         self.assertIn(("system-dependency", "vendor", 0, "submodule outside 3rdparty/"), check.findings)
 
     def test_thirdparty_submodule_remains_allowed(self):
-        head = self.git("rev-parse", "HEAD")
-        self.git("update-index", "--add", "--cacheinfo", "160000", head, "3rdparty/vendor")
-        self.git("commit", "-qm", "build: add allowed submodule")
+        self.add_thirdparty_submodule('[submodule "vendor"]\n\tpath = 3rdparty/vendor\n'
+                                      "\turl = https://example.com/vendor.git\n")
         check = Check("HEAD~1", "HEAD")
         check.repository()
         self.assertEqual(check.findings, [])
 
+    def test_thirdparty_submodule_without_gitmodules_entry_is_reported(self):
+        self.add_thirdparty_submodule()
+        check = Check("HEAD~1", "HEAD")
+        check.repository()
+        self.assertEqual(check.findings,
+                         [("system-dependency", "3rdparty/vendor", 0, "submodule without a url in .gitmodules")])
+
+    def test_dropping_a_submodule_url_is_reported(self):
+        self.add_thirdparty_submodule('[submodule "vendor"]\n\tpath = 3rdparty/vendor\n'
+                                      "\turl = https://example.com/vendor.git\n")
+        (self.root / ".gitmodules").write_text('[submodule "vendor"]\n\tpath = 3rdparty/vendor\n', encoding="utf-8")
+        self.git("add", ".gitmodules")
+        self.git("commit", "-qm", "build: drop submodule url")
+        check = Check("HEAD~1", "HEAD")
+        check.repository()
+        self.assertEqual(check.findings,
+                         [("system-dependency", "3rdparty/vendor", 0, "submodule without a url in .gitmodules")])
+
     def test_relative_link_url_decoding(self):
         self.assertEqual(links("docs/dev/index.md", "[guide](./space%20name.md#details)"),
                          [("./space%20name.md", "docs/dev/space name.md")])