plone.restapi: Stored XSS by spoofing mime type

Impact

A stored XSS affecting RichText fields. RichTextValue.output returns the raw, unsanitized stored value whenever the stored mimeType equals the outputMimeType. Because the safe-HTML output type (text/x-html-safe) is the type that signifies "already sanitized", any value whose stored mimeType equals it bypasses the safe_html transform entirely on render. The transform itself is sound — it correctly strips on* event-handler attributes and javascript:/data: URIs; the defect is that it is never invoked for these values. The unsanitized value is then emitted via tal:content="structure ...", which performs no escaping, so the payload executes in the viewer's browser.

This can be a problem when a RichText field is wrongly defined in code with a mimeType and outputMimeType that are the same, or when the REST API is used to the same effect.

This is the same vulnerability as reported in plone.app.textfield: https://github.com/plone/plone.app.textfield/security/advisories/GHSA-4r4f-gg25-rmg5

Patches

The problem has been patched:

• For Plone 6.0 and 6.2, upgrade plone.restapi to 9.15.6.
• For Plone 6.2, upgrade plone.restapi to 10.0.1.

This will prevent abusing the REST API to store wrong rich text values.

This will not prevent XSS from rich text fields that already have wrong values. For that, you will need a patched plone.app.textfield version. See the plone.app.textfield advisory linked above for versions.

Workarounds

There is no known workaround.