Race condition in the _get_masked_mode function in Lib/os.py in Python 3.2 through 3.5, when exist_ok is set to true and multiple threads are used, might allow local users to bypass intended file permissions by leveraging a separate application vulnerability before the umask has been set to the expected value.
http://bugs.python.org/issue21082
http://lists.opensuse.org/opensuse-updates/2014-05/msg00007.html
http://lists.opensuse.org/opensuse-updates/2014-05/msg00008.html
http://www.openwall.com/lists/oss-security/2014/03/28/15
http://www.openwall.com/lists/oss-security/2014/03/29/5