Versions of Ratpack from 0.9.10 through 1.7.5 are vulnerable to CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (aka. XSS) in the development error handler. An attacker can utilize this to perform XSS when an exception message contains untrusted data.
As a simplistic example:
This vulnerability has been patched in Ratpack version 1.7.6.