Sinatra before 2.0.2 has XSS via the 400 Bad Request page that occurs upon a params parser exception.

References

• advisory https://access.redhat.com/errata/RHSA-2019:0212
• advisory https://access.redhat.com/errata/RHSA-2019:0315
• patch https://github.com/sinatra/sinatra/commit/12786867d6faaceaec62c7c2cb5b0e2dc074d71a
• report https://github.com/sinatra/sinatra/issues/1428