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Add warning about untrusted input to the idna codec #155475

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Add warning about untrusted input to the idna codec #155475

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14 changes: 14 additions & 0 deletions Doc/library/codecs.rst
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Expand Up @@ -1395,6 +1395,15 @@ encodings.
| | | :mod:`encodings.idna`. |
| | | Only ``errors='strict'`` |
| | | is supported. |
| | | |
| | | .. warning:: |
| | | |
| | | This codec builds on |

@sethmlarson sethmlarson Aug 10, 2026

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Do we want to either link to or include the justification here that these algorithms were designed to process 63 or fewer octets? Maybe we can hint at a limit of 1,000 that we've set ourselves somehow, for users looking to adopt a limit?

@StanFromIreland StanFromIreland Aug 10, 2026

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Do we want to either link to or include the justification here that these algorithms were designed to process 63 or fewer octets?

Hmm, any suggestions as to where exactly we could link? I fear issues/PR might be too "busy" for this.

| | | ``punycode``, whose |
| | | algorithms scale |
| | | poorly, so limit the |
| | | length of untrusted |
| | | input. |
+--------------------+---------+---------------------------+
| mbcs | ansi, | Windows only: Encode the |
| | dbcs | operand according to the |
Expand Down Expand Up @@ -1646,6 +1655,11 @@ Applications) and :rfc:`3492` (Nameprep: A Stringprep Profile for
Internationalized Domain Names (IDN)). It builds upon the ``punycode`` encoding
and :mod:`stringprep`.

.. warning::

This module builds on ``punycode``, whose algorithms scale poorly, so limit
the length of untrusted input.

If you need the IDNA 2008 standard from :rfc:`5891` and :rfc:`5895`, use the
third-party :pypi:`idna` module.

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