foswiki -- Script Insertion Vulnerability via unchecked user registration fields

Foswiki team reports:

When a new user registers, the new user can add arbitrary HTML and
script code into the user topic which is generated by the
RegistrationAgent via standard registration fields such as
“FirstName” or “OrganisationName”.
By design, Foswiki’s normal editing features allow arbitrary HTML
markup, including script code, to be inserted into any topic anyway,
assuming the authenticated user has CHANGE permission - which is the
case on many Foswiki sites. However, the assumption that only
authenticated users with CHANGE permission may create script content
is false if new users exploit the vulnerability detailed in this
alert to manipulate the registration agent into creating that
content for them.