[Uncaught exception] UnicodeError (punycode) when calling decode with arbitrary data

[pventuzelo]

Hey,

I just found another case making the program crash where idna.decode() raise a UnicodeError (due to punycode).

In the same way as #108, this could be problematic if users of the library are dealing with untrusted data since this issue will lead to a DoS. This should be detected and an idna.IDNAError should be triggered as detailed in the Readme Exception section.

Reproduction

import idna

idna.decode(b'A.A.0.a.a.A.0.a.A.A.0.a.A.0A.2.a.A.A.0.a.A.0.A.a.A0.a.a.A.0.a.fB.A.A.a.A.A.B.A.A.a.A.A.B.A.A.a.A.A.0.a.A.a.a.A.A.0.a.A.0.A.a.A0.a.a.A.0.a.fB.A.A.a.A.A.B.0A.A.a.A.A.B.A.A.a.A.A.a.A.A.B.A.A.a.A.0.a.B.A.A.a.A.B.A.a.A.A.5.a.A.0.a.Ba.A.B.A.A.a.A.0.a.Xn--B.A.A.A.a')

Traceback

python3 replay_crash2.py
Traceback (most recent call last):
  File "/usr/lib/python3.8/encodings/punycode.py", line 134, in decode_generalized_number
    char = ord(extended[extpos])
IndexError: string index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.8/encodings/punycode.py", line 207, in decode
    res = punycode_decode(input, errors)
  File "/usr/lib/python3.8/encodings/punycode.py", line 194, in punycode_decode
    return insertion_sort(base, extended, errors)
  File "/usr/lib/python3.8/encodings/punycode.py", line 164, in insertion_sort
    newpos, delta = decode_generalized_number(extended, extpos,
  File "/usr/lib/python3.8/encodings/punycode.py", line 137, in decode_generalized_number
    raise UnicodeError("incomplete punicode string")
UnicodeError: incomplete punicode string

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "replay_crash2.py", line 3, in <module>
    idna.decode(b'A.A.0.a.a.A.0.a.A.A.0.a.A.0A.2.a.A.A.0.a.A.0.A.a.A0.a.a.A.0.a.fB.A.A.a.A.A.B.A.A.a.A.A.B.A.A.a.A.A.0.a.A.a.a.A.A.0.a.A.0.A.a.A0.a.a.A.0.a.fB.A.A.a.A.A.B.0A.A.a.A.A.B.A.A.a.A.A.a.A.A.B.A.A.a.A.0.a.B.A.A.a.A.B.A.a.A.A.5.a.A.0.a.Ba.A.B.A.A.a.A.0.a.Xn--B.A.A.A.a')
  File "/home/scop/.local/lib/python3.8/site-packages/idna/core.py", line 402, in decode
    s = ulabel(label)
  File "/home/scop/.local/lib/python3.8/site-packages/idna/core.py", line 315, in ulabel
    label = label_bytes.decode('punycode')
UnicodeError: decoding with 'punycode' codec failed (UnicodeError: incomplete punicode string)

Happy to help if you have any questions ;)