activeadmin vulnerable to stored persistent cross-site scripting (XSS) in dynamic form legends

Impact

Users settings their active admin form legends dynamically may be
vulnerable to stored XSS, as long as its value can be injected
directly by a malicious user.

For example:

• A public web application allows users to create entities with
  arbitrary names.
• Active Admin is used to administrate these entities through a
  private backend.
• The form to edit these entities in the private backend has the
  following shape (note the dynamic name value dependent on an
  attribute of the resource):

  form do |f|
    f.inputs name: resource.name do
      f.input :name
      f.input :description
    end

    f.actions
  end

Then a malicious user could create an entity with a payload that
would get executed in the active admin administrator’s browser.

Both form blocks with an implicit or explicit name (i.e., both
form resource.name or form name: resource.name would suffer
from the problem), where the value of the name can be arbitrarily
set by non admin users.

Patches

The problem has been fixed in ActiveAdmin 3.2.2 and ActiveAdmin 4.0.0.beta7.

Workarounds

Users can workaround this problem without upgrading by explicitly
escaping the form name using an HTML escaping utility. For example:

  form do |f|
    f.inputs name: ERB::Util.html_escape(resource.name) do
      f.input :name
      f.input :description
    end

    f.actions
  end

Upgrading is of course recommended though.

References

https://owasp.org/www-community/attacks/xss/#stored-xss-attacks