gh-161090: Reject a leading delimiter in the punycode decoder

Lib/encodings/punycode.py

@@ -190,7 +190,10 @@ def punycode_decode(text, errors):
         text = text.encode("ascii")
     if isinstance(text, memoryview):
         text = bytes(text)
-    pos = text.rfind(b"-")
+    # RFC 3492, section 6.2: the delimiter is only consumed if at least one
+    # basic code point precedes it.  A leading "-" is left in the extended
+    # part, where it is an invalid digit.
+    pos = text.rfind(b"-", 1)
     if pos == -1:
         base = ""
         extended = text.upper()

Lib/test/test_codecs.py

@@ -1390,7 +1390,11 @@ def test_decode_invalid(self):
             (b"Proprostnemluvesky-&uyb24dma41a", "strict", UnicodeDecodeError("punycode", b"", 19, 20, "")),
             (b"Proprostnemluvesky-uyb24d&ma41a", "strict", UnicodeDecodeError("punycode", b"", 25, 26, "")),
             (b"Proprostnemluvesky-uyb24dma41a&", "strict", UnicodeDecodeError("punycode", b"", 30, 31, "")),
+            # A delimiter not preceded by any basic code point (RFC 3492, 6.2)
+            (b"-", "strict", UnicodeDecodeError("punycode", b"", 0, 1, "")),
+            (b"-80a1acny", "strict", UnicodeDecodeError("punycode", b"", 0, 1, "")),
             (b"xn--w&", "ignore", "xn-"),
+            (b"-80a1acny", "ignore", ""),
         ]
         for puny, errors, expected in testcases:
             with self.subTest(puny=puny, errors=errors):

Misc/NEWS.d/next/Library/2026-10-22-05-19-56.gh-issue-161090.pxZ1IA.rst

@@ -0,0 +1,3 @@
+Fix the ``punycode`` codec accepting a delimiter (``-``) that is not preceded
+by any basic code point. Such input, for example ``b'-80a1acny'``, now raises
+:exc:`UnicodeDecodeError`, as required by :rfc:`3492`.