The time and filter parameters in Fava prior to v1.22 are vulnerable to reflected XSS due to the lack of escaping of error messages which contained the parameters in verbatim.
https://huntr.dev/bounties/dbf77139-4384-4dc5-9994-45a5e0747429
https://github.com/beancount/fava/commit/ca9e3882c7b5fbf5273ba52340b9fea6a99f3711