Jinja is an extensible templating engine. Prior to 3.1.6, an
oversight in how the Jinja sandboxed environment interacts with the
|attr filter allows an attacker that controls the content of a
template to execute arbitrary Python code. To exploit the
vulnerability, an attacker needs to control the content of a template.
Whether that is the case depends on the type of application using
Jinja. This vulnerability impacts users of applications which
execute untrusted templates. Jinja's sandbox does catch calls
to str.format and ensures they don't escape the sandbox.
However, it's possible to use the |attr filter to get a reference
to a string's plain format method, bypassing the sandbox.
After the fix, the |attr filter no longer bypasses the environment's
attribute lookup. This vulnerability is fixed in 3.1.6.