As discovered while looking at CONF-20667, Confluence stores the space key unencoded in a content tag. Considerable functionality relies on this content tag. Eg Doc Theme breaks without it. Themes choice breaks without it.
To exploit it, create a user with html in the login name, then create a personal space as that user. Finally, use a decorator=none request param when viewing a page to see the content tags.
There are actually a few places that the space key isn't encoded, so removing the ability to pass "decorator=none" is probably not a complete fix.