In Eclipse Jetty versions 9.4.21.v20190926, 9.4.22.v20191022, and 9.4.23.v20191118, the generation of default unhandled Error response content (in text/html and text/json Content-Type) does not escape Exception messages in stacktraces included in error output.
https://www.oracle.com/security-alerts/cpuoct2020.html