JSON is not a strict superset of JavaScript (http://timelessrepo.com/json-isnt-a-javascript-subset). However, certain web technologies use JSON values as a part of JavaScript code (JSONP, inline <script> tags)... The Python json module, however, by default does not escape \u2028 or \u2029 when ensure_ascii is false. Furthermore, the / -> \/ escape is not supported by any switch.

Strictly speaking, json specification only requires that " be escaped to \" and \ to \\ - all other escaping is optional. The whitespace escapes only exist to aid handwriting and embedding values in HTML/code. Thus it can be argued that the choice of escapes used by json encoder is ill-adviced.

In an inline HTML <script></script> tag, no < cannot be escaped; however only the string '</script>' (or sometimes </) is interpreted as the "end of script". Thus a non-trivial XSS attack can be made by having a JSON stream {"key":"</script><script src=''></script>"} embedded in inline javascript. The only correct way to escape such content in inline html is to escape all / into \/.

The \u2028, \u2029 problem is more subtle and can break not only inline javascript but also JSONP. Thus there an incorrect value injected by a malicious or unwitting user to the database might break the entire protocol.

The current solution is to re-escape everything that comes out of JSON encoder. The best solution for python would be to make these 3 escapes default in the python json module (notice again that the current set of default escapes when ensure_ascii=False is chosen arbitrarily), or if not default, then at least they could be enabled by a switch. Furthermore, documentation should be updated appropriately, to explain why such escape is needed.