diff --git a/Lib/encodings/idna.py b/Lib/encodings/idna.py
index ea4058512fe366..bf98f513366b31 100644
--- a/Lib/encodings/idna.py
+++ b/Lib/encodings/idna.py
@@ -39,23 +39,21 @@ def nameprep(label):
# Check bidi
RandAL = [stringprep.in_table_d1(x) for x in label]
- for c in RandAL:
- if c:
- # There is a RandAL char in the string. Must perform further
- # tests:
- # 1) The characters in section 5.8 MUST be prohibited.
- # This is table C.8, which was already checked
- # 2) If a string contains any RandALCat character, the string
- # MUST NOT contain any LCat character.
- if any(stringprep.in_table_d2(x) for x in label):
- raise UnicodeError("Violation of BIDI requirement 2")
-
- # 3) If a string contains any RandALCat character, a
- # RandALCat character MUST be the first character of the
- # string, and a RandALCat character MUST be the last
- # character of the string.
- if not RandAL[0] or not RandAL[-1]:
- raise UnicodeError("Violation of BIDI requirement 3")
+ if any(RandAL):
+ # There is a RandAL char in the string. Must perform further
+ # tests:
+ # 1) The characters in section 5.8 MUST be prohibited.
+ # This is table C.8, which was already checked
+ # 2) If a string contains any RandALCat character, the string
+ # MUST NOT contain any LCat character.
+ if any(stringprep.in_table_d2(x) for x in label):
+ raise UnicodeError("Violation of BIDI requirement 2")
+ # 3) If a string contains any RandALCat character, a
+ # RandALCat character MUST be the first character of the
+ # string, and a RandALCat character MUST be the last
+ # character of the string.
+ if not RandAL[0] or not RandAL[-1]:
+ raise UnicodeError("Violation of BIDI requirement 3")
return label
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index d1faf0126c1eea..37ade7d80d0222 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1532,6 +1532,12 @@ def test_builtin_encode(self):
self.assertEqual("pyth\xf6n.org".encode("idna"), b"xn--pythn-mua.org")
self.assertEqual("pyth\xf6n.org.".encode("idna"), b"xn--pythn-mua.org.")
+ def test_builtin_decode_length_limit(self):
+ with self.assertRaisesRegex(UnicodeError, "too long"):
+ (b"xn--016c"+b"a"*1100).decode("idna")
+ with self.assertRaisesRegex(UnicodeError, "too long"):
+ (b"xn--016c"+b"a"*70).decode("idna")
+
def test_stream(self):
r = codecs.getreader("idna")(io.BytesIO(b"abc"))
r.read(3)
diff --git a/Misc/NEWS.d/next/Security/2022-11-04-09-29-36.gh-issue-98433.l76c5G.rst b/Misc/NEWS.d/next/Security/2022-11-04-09-29-36.gh-issue-98433.l76c5G.rst
new file mode 100644
index 00000000000000..5185fac2e29d91
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2022-11-04-09-29-36.gh-issue-98433.l76c5G.rst
@@ -0,0 +1,6 @@
+The IDNA codec decoder used on DNS hostnames by :mod:`socket` or :mod:`asyncio`
+related name resolution functions no longer involves a quadratic algorithm.
+This prevents a potential CPU denial of service if an out-of-spec excessive
+length hostname involving bidirectional characters were decoded. Some protocols
+such as :mod:`urllib` http ``3xx`` redirects potentially allow for an attacker
+to supply such a name.
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