gh-149231: tomllib: Limit the number of parts in a key

Lib/test/test_tomllib/test_misc.py

@@ -119,6 +119,19 @@ def test_inline_table_recursion_limit(self):
                 recursive_table_toml = nest_count * "key = {" + nest_count * "}"
                 tomllib.loads(recursive_table_toml)
 
+    def test_key_recursion_limit(self):
+        nest_count = tomllib._parser.MAX_KEY_PARTS - 2
+        nested_key_toml = "a." * nest_count + "a = 1"
+        tomllib.loads(nested_key_toml)
+
+        nest_count = tomllib._parser.MAX_KEY_PARTS + 2
+        nested_key_toml = "a." * nest_count + "a = 1"
+        with self.assertRaisesRegex(
+            RecursionError,
+            r"TOML key has more than the allowed [0-9]+ parts",
+        ):
+            tomllib.loads(nested_key_toml)
+
     def test_types_import(self):
         """Test that `_types` module runs.
 

Lib/tomllib/_parser.py

@@ -29,6 +29,13 @@
 
     from ._types import Key, ParseFloat, Pos
 
+# Pathologically excessive number of parts in a key runs into quadratic
+# behavior (e.g. in Flags.is_).
+# Even if keys aren't currently parsed using recursion, they name a
+# recursive structure, so it makes sense to limit it using getrecursionlimit()
+# and RecursionError.
+MAX_KEY_PARTS: Final = sys.getrecursionlimit()
+
 ASCII_CTRL: Final = frozenset(chr(i) for i in range(32)) | frozenset(chr(127))
 
 # Neither of these sets include quotation mark or backslash. They are
@@ -470,6 +477,10 @@ def parse_key(src: str, pos: Pos) -> tuple[Pos, Key]:
         pos = skip_chars(src, pos, TOML_WS)
         pos, key_part = parse_key_part(src, pos)
         key += (key_part,)
+        if len(key) > MAX_KEY_PARTS:
+            raise RecursionError(
+                f"TOML key has more than the allowed {MAX_KEY_PARTS} parts"
+            )
         pos = skip_chars(src, pos, TOML_WS)
 
 

Misc/NEWS.d/next/Library/2026-05-01-16-45-31.gh-issue-149231.x2nBEE.rst

@@ -0,0 +1 @@
+In :mod:`tomllib`, the number of parts in TOML keys is now limited.