RestrictedPython vulnerable to sandbox escape via string.Formatter field resolution

Impact

RestrictedPython could allow a sandbox escape when a policy exposes the standard library string module, or otherwise exposes string.Formatter, to restricted code.

string.Formatter field resolution methods such as get_field can perform attribute and item traversal internally and return live object references. This can bypass RestrictedPython's normal attribute guards and may allow access to sensitive objects such as function globals, builtins, file access, or code execution primitives.

Users are impacted if they run untrusted code with RestrictedPython and expose string.Formatter, directly or indirectly, for example through a custom import policy or globals.

Patches

The problem has been patched by blocking access to string.Formatter and unsafe string.Formatter traversal methods in safer_getattr.

Users should upgrade to the patched release once available. Affected and patched version numbers should be filled in when the release is published.

Workarounds

Do not expose the standard library string module or string.Formatter to restricted code.

If a custom import hook is used, deny imports of string or provide only a restricted wrapper that does not expose Formatter. If custom globals are supplied, ensure neither string.Formatter nor Formatter instances are available to restricted code.