Impact

Python's "format" functionality allows someone controlling the format string to "read" all objects accessible through recursive attribute lookup and subscription from objects he can access. This can lead to critical information disclosure.
With RestrictedPython, the format functionality is available via the format and format_map methods of str (and unicode) (accessed either via the class or its instances) and via string.Formatter.
All known versions of RestrictedPython are vulnerable.

Patches

The issue will be fixed in 5.4 and 6.2.

Workarounds

There are no workarounds to fix the issue without upgrading.

References

• https://docs.python.org/3/library/stdtypes.html#str.format_map
• http://lucumr.pocoo.org/2016/12/29/careful-with-str-format/
• https://www.exploit-db.com/exploits/51580

For more information

If you have any questions or comments about this advisory:

• Open an issue in the RestrictedPython issue tracker
• Email us at security@plone.org

Credits

Thanks for analysing and reporting the go to:

• Abhishek Govindarasu
• Ankush Menat
• Ward Theunisse