CVE-2021-29434: Improper validation of URLs ('Cross-site Scripting') in Wagtail rich text fields

Impact

When saving the contents of a rich text field in the admin interface, Wagtail does not apply server-side checks to ensure that link URLs use a valid protocol. A malicious user with access to the admin interface could thus craft a POST request to publish content with javascript: URLs containing arbitrary code. The vulnerability is not exploitable by an ordinary site visitor without access to the Wagtail admin.

Patches

Patched versions have been released as Wagtail 2.11.7 (for the LTS 2.11 branch) and Wagtail 2.12.4 (for the current 2.12 branch).

Workarounds

For sites that cannot easily upgrade to a current supported version, the vulnerability can be patched by adding the following code to a wagtail_hooks.py module in any installed app:

from draftjs_exporter.dom import DOM
from wagtail.admin.rich_text.converters.html_to_contentstate import ExternalLinkElementHandler, PageLinkElementHandler
from wagtail.core import hooks
from wagtail.core.whitelist import check_url


def link_entity(props):
    id_ = props.get('id')
    link_props = {}

    if id_ is not None:
        link_props['linktype'] = 'page'
        link_props['id'] = id_
    else:
        link_props['href'] = check_url(props.get('url'))

    return DOM.create_element('a', link_props, props['children'])


@hooks.register('register_rich_text_features', order=1)
def register_link(features):
    features.register_converter_rule('contentstate', 'link', {
        'from_database_format': {
            'a[href]': ExternalLinkElementHandler('LINK'),
            'a[linktype="page"]': PageLinkElementHandler('LINK'),
        },
        'to_database_format': {
            'entity_decorators': {'LINK': link_entity}
        }
    })

Acknowledgements

Many thanks to Kevin Breen for reporting this issue.

For more information

If you have any questions or comments about this advisory:

• Visit Wagtail's support channels
• Email us at security@wagtail.io (if you wish to send encrypted email, the public key ID is 0x6ba1e1a86e0f8ce8)