diff --git a/src/specify_cli/catalogs.py b/src/specify_cli/catalogs.py
index ec8014127..e4df8eae2 100644
--- a/src/specify_cli/catalogs.py
+++ b/src/specify_cli/catalogs.py
@@ -71,8 +71,12 @@ class CatalogStackBase:
         """Validate that a catalog URL uses HTTPS, except localhost HTTP."""
         from urllib.parse import urlparse
 
-        parsed = urlparse(url)
-        is_localhost = parsed.hostname in ("localhost", "127.0.0.1", "::1")
+        try:
+            parsed = urlparse(url)
+            hostname = parsed.hostname
+        except ValueError:
+            raise cls._error(f"Catalog URL is malformed: {url}") from None
+        is_localhost = hostname in ("localhost", "127.0.0.1", "::1")
         if parsed.scheme != "https" and not (parsed.scheme == "http" and is_localhost):
             raise cls._error(
                 f"Catalog URL must use HTTPS (got {parsed.scheme}://). "
@@ -81,7 +85,7 @@ class CatalogStackBase:
         # Check hostname, not netloc: netloc is truthy for host-less URLs like
         # "https://:8080" or "https://user@", so the host guarantee this error
         # promises would not actually hold. hostname is None in those cases (#3209).
-        if not parsed.hostname:
+        if not hostname:
             raise cls._error("Catalog URL must be a valid URL with a host.")
 
     def _load_catalog_config(self, config_path: Path) -> list[CatalogEntry] | None:
diff --git a/tests/integrations/test_integration_catalog.py b/tests/integrations/test_integration_catalog.py
index 44f4a1d78..a0323b3ea 100644
--- a/tests/integrations/test_integration_catalog.py
+++ b/tests/integrations/test_integration_catalog.py
@@ -84,6 +84,20 @@ class TestCatalogURLValidation:
         with pytest.raises(IntegrationCatalogError, match="valid URL"):
             IntegrationCatalog._validate_catalog_url(url)
 
+    @pytest.mark.parametrize(
+        "url",
+        [
+            "https://[::1",                 # unclosed ipv6 bracket
+            "https://[not-an-ip]/c.json",   # bracketed non-ip host
+        ],
+    )
+    def test_malformed_url_rejected_cleanly(self, url):
+        # A malformed authority makes urlparse/hostname raise ValueError. The
+        # validator must turn that into its normal catalog error, not leak a
+        # raw ValueError to the caller.
+        with pytest.raises(IntegrationCatalogError, match="malformed"):
+            IntegrationCatalog._validate_catalog_url(url)
+
 
 # ---------------------------------------------------------------------------
 # IntegrationCatalog — active catalogs