PYSEC-2026-1475

Description

An oversight in how the Jinja sandboxed environment detects calls to `str.format` 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 store a reference to a malicious string's `format` method, then pass that to a filter that calls it. No such filters are built-in to Jinja, but could be present through custom filters in an application. After the fix, such indirect calls are also handled by the sandbox.