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Migrating from the html5lib sanitizer

The html5lib module deprecated its own sanitizer in version 1.1. The maintainers "recommend users migrate to Bleach." This tracks the issues encountered in the migration.

Migration path

If you upgrade to html5lib 1.1+, you may get deprecation warnings when using its sanitizer. If you follow the recommendation and switch to Bleach for sanitization, you'll need to spend time tuning the Bleach sanitizer to your needs because the Bleach sanitizer has different goals and is not a drop-in replacement for the html5lib one.

Here is an example of replacing the sanitization method:

fragment = "<a href='https://github.com'>good</a> <script>bad();</script>"

import html5lib
parser = html5lib.html5parser.HTMLParser()
parsed_fragment = parser.parseFragment(fragment)
print(html5lib.serialize(parsed_fragment, sanitize=True))

# '<a href="https://github.com">good</a> &lt;script&gt;bad();&lt;/script&gt;'

import bleach
print(bleach.clean(fragment))

# '<a href="https://github.com">good</a> &lt;script&gt;bad();&lt;/script&gt;'

Escaping differences

While html5lib will leave 'single' and "double" quotes alone, Bleach will escape them as the corresponding HTML entities (' becomes &#39; and " becomes &#34;). This should be fine in most rendering contexts.

Different allow lists

By default, html5lib and Bleach "allow" (i.e. don't sanitize) different sets of HTML elements, HTML attributes, and CSS properties. For example, html5lib will leave <u/> alone, while Bleach will sanitize it:

fragment = "<u>hi</u>"

import html5lib
parser = html5lib.html5parser.HTMLParser()
parsed_fragment = parser.parseFragment(fragment)
print(html5lib.serialize(parsed_fragment, sanitize=True))

# '<u>hi</u>'

print(bleach.clean(fragment))

# '&lt;u&gt;hi&lt;/u&gt;'

If you wish to retain the sanitization behaviour with respect to specific HTML elements, use the tags argument (see the :ref:`chapter on clean() <clean-chapter>` for more info):

fragment = "<u>hi</u>"

print(bleach.clean(fragment, tags=['u']))

# '<u>hi</u>'

If you want to stick to the html5lib sanitizer's allow lists, get them from the sanitizer code. It's probably best to copy them as static lists (as opposed to importing the module and reading them dynamically) because

  • the lists are not part of the html5lib API
  • the sanitizer module is already deprecated and might disappear
  • importing the sanitizer module gives the deprecation warning (unless you take the effort to filter it)
import bleach

from bleach.css_sanitizer import CSSSanitizer

ALLOWED_ELEMENTS = ["b", "p", "div"]
ALLOWED_ATTRIBUTES = ["style"]
ALLOWED_CSS_PROPERTIES = ["color"]

fragment = "some unsafe html"

css_sanitizer = CSSSanitizer(allowed_css_properties=ALLOWED_CSS_PROPERTIES)
safe_html = bleach.clean(
    fragment,
    tags=ALLOWED_ELEMENTS,
    attributes=ALLOWED_ATTRIBUTES,
    css_sanitizer=css_sanitizer,
)