Daniel Roy Greenfeld

Daniel Roy Greenfeld

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Converting from bleach to nh3

Bleach is deprecated, here's how to come close to replicating bleach.clean() using the nh3 version of .clean().

import nh3

def clean_string(string: str) -> str:
    # The arguments below being passed to `nh3.clean()` are
    # the default values of the `bleach.clean()` function. 
    return nh3.clean(
            "a": {"href", "title"},
            "abbr": {"title"},
            "acronym": {"title"},
        url_schemes={"http", "https", "mailto"},

The big difference is unlike the safing of HTML done by bleach, nh3 removes the offending tags altogether. Read the comments below to see what this means.


>>> input_from_user = """<b>
<img src="">
I\'m not trying to XSS you <a href="https://example.com">Link</a>
>>> # By default, bleach version safes the HTML
>>> # rather than remove the tags altogether.
>>> bleach.clean(input_from_user)
'<b>&lt;img src=""&gt;I\'m not trying to XSS you <a href="https://example.com">Link</a></b>'
>>> # In contrast, nh3 removes the offending tags entirely
>>> # while also preserving whitespace.
>>> clean_string(input_from_user)
'<b>\n\nI\'m not trying to XSS you <a href="https://example.com">Link</a>\n</b>'

Advantages of switching to nh3 are:

  1. nh3 is actively maintained, bleach is officially deprecated.
  2. I believe the nh3 technique of stripping tags rather than allowing safing is more secure. The idea of safing is great, but I've always wondered if a creative attacker could find a way to exploit it. So I think it is better to remove the offending tags altogether.
  3. The preservation of whitespace is really useful for preserving content submitted in a textarea. This is especially true for Markdown content.
  4. nh3 is a binding to the rust-ammonia project. They claim a 15x speed increase over bleach's binding to the html5lib project. Even if that is a 3x exaggeration, that's still a 5x speed increase.

nh3 + Django

If you're coding in Django, you owe it to yourself and all your users to read Adam Johnson's fantastic article on using nh3 with Django.


  • 2023/12/14 - Added mention of Adam Johnson's article on nh3 + Django

Tags: howto python rust-lang
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