New submission from Antti Haapala:
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.
----------
components: Library (Lib)
messages: 191742
nosy: Ztane
priority: normal
severity: normal
status: open
title: json encoder does not support JSONP/JavaScript safe escaping
type: enhancement
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue18290>
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