diff --git a/src/accounting/utils/next_uri.py b/src/accounting/utils/next_uri.py
index 9d276dd..48bf47f 100644
--- a/src/accounting/utils/next_uri.py
+++ b/src/accounting/utils/next_uri.py
@@ -41,11 +41,8 @@ def inherit_next(uri: str) -> str:
     :param uri: The URI.
     :return: The URI with the current next URI added at the query argument.
     """
-    next_uri: str | None = request.form.get("next") \
-        if request.method == "POST" else request.args.get("next")
-    if next_uri is None:
-        return uri
-    return __set_next(uri, next_uri)
+    next_uri: str | None = __get_next_uri()
+    return uri if next_uri is None else __set_next(uri, next_uri)
 
 
 def or_next(uri: str) -> str:
@@ -54,9 +51,22 @@ def or_next(uri: str) -> str:
     :param uri: The URI.
     :return: The next URI or the supplied URI.
     """
+    next_uri: str | None = __get_next_uri()
+    return uri if next_uri is None else next_uri
+
+
+def __get_next_uri() -> str | None:
+    """Returns the valid next URI.
+
+    :return: The valid next URI.
+    """
     next_uri: str | None = request.form.get("next") \
         if request.method == "POST" else request.args.get("next")
-    return uri if next_uri is None else next_uri
+    if next_uri is None or not next_uri.startswith("/"):
+        return None
+    if len(next_uri) > 512:
+        return next_uri[:512]
+    return next_uri
 
 
 def __set_next(uri: str, next_uri: str) -> str:
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 8d8eb07..307d8e9 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -101,6 +101,60 @@ class NextUriTestCase(unittest.TestCase):
                                                       "name": "viewer"})
         self.assertEqual(response.status_code, 200)
 
+    def test_invalid(self) -> None:
+        """Tests the next URI utilities without an invalid next URI.
+
+        :return: None.
+        """
+        def test_invalid_next_uri_view() -> str:
+            """The test view without the next URI."""
+            self.assertEqual(inherit_next(self.TARGET),
+                             request.args.get("inherit-expected"))
+            self.assertEqual(or_next(self.TARGET),
+                             request.args.get("or-expected"))
+            return ""
+
+        self.app.add_url_rule("/test-invalid-next",
+                              view_func=test_invalid_next_uri_view,
+                              methods=["GET", "POST"])
+        client: httpx.Client = httpx.Client(app=self.app, base_url=TEST_SERVER)
+        client.headers["Referer"] = TEST_SERVER
+        csrf_token: str = get_csrf_token(client)
+        next_uri: str
+        expected1: str
+        expected2: str
+        response: httpx.Response
+
+        # A foreign URI
+        next_uri = "https://example.com"
+        expected1 = self.TARGET
+        expected2 = self.TARGET
+        response = client.get(f"/test-invalid-next?next={quote_plus(next_uri)}"
+                              f"&inherit-expected={quote_plus(expected1)}"
+                              f"&or-expected={quote_plus(expected2)}")
+        self.assertEqual(response.status_code, 200)
+        response = client.post("/test-invalid-next"
+                               f"?inherit-expected={quote_plus(expected1)}"
+                               f"&or-expected={quote_plus(expected2)}",
+                               data={"csrf_token": csrf_token,
+                                     "next": next_uri})
+        self.assertEqual(response.status_code, 200)
+
+        # An extremely-long URI to trigger the error
+        next_uri = "/" + "x" * 1024
+        expected2 = next_uri[:512]
+        expected1 = f"{self.TARGET}?next={quote_plus(expected2)}"
+        response = client.get(f"/test-invalid-next?next={quote_plus(next_uri)}"
+                              f"&inherit-expected={quote_plus(expected1)}"
+                              f"&or-expected={quote_plus(expected2)}")
+        self.assertEqual(response.status_code, 200)
+        response = client.post("/test-invalid-next"
+                               f"?inherit-expected={quote_plus(expected1)}"
+                               f"&or-expected={quote_plus(expected2)}",
+                               data={"csrf_token": csrf_token,
+                                     "next": next_uri})
+        self.assertEqual(response.status_code, 200)
+
 
 class QueryKeywordParserTestCase(unittest.TestCase):
     """The test case for the query keyword parser."""