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All versions of io.ratpack:ratpack-core from 0.9.10 inclusive and before 1.7.6 are vulnerable to Cross-site Scripting (XSS). This affects the development mode error handler when an exception message contains untrusted data. Note the production mode error handler is not vulnerable - so for this to be utilized in production it would require users to not disable development mode.
No PoCs found on GitHub currently.