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 wagtailhooks.py module in any installed app:

python from draftjsexporter.dom import DOM from wagtail.admin.richtext.converters.htmltocontentstate import ExternalLinkElementHandler, PageLinkElementHandler from wagtail.core import hooks from wagtail.core.whitelist import checkurl

def linkentity(props): id = props.get('id') linkprops = {}

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

return DOM.createelement('a', linkprops, props['children'])

@hooks.register('registerrichtextfeatures', order=1) def registerlink(features): features.registerconverterrule('contentstate', 'link', { 'fromdatabaseformat': { 'a[href]': ExternalLinkElementHandler('LINK'), 'a[linktype="page"]': PageLinkElementHandler('LINK'), }, 'todatabaseformat': { 'entitydecorators': {'LINK': linkentity} } })

Acknowledgements Many thanks to Kevin Breen for reporting this issue.

For more information If you have any questions or comments about this advisory:

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