bpo-42278: Replaced usage of tempfile.mktemp() with TemporaryDirectory to avoid a potential race condition. bpo-44394: Update the vendored copy of libexpat to 2.4.1 (from 2.2.8) to get the fix for the CVE-2013-0340 “Billion Laughs” vulnerability. This copy is most used on Windows and macOS. bpo-43124: Made the internal putcmd function in smtplib sanitize input for presence of \r and \n characters to avoid (unlikely) command injection. bpo-36384: ipaddress module no longer accepts any leading zeros in IPv4 address strings. Leading zeros are ambiguous and interpreted as octal notation by some libraries. For example the legacy function socket.inet_aton() treats leading zeros as octal notation. glibc implementation of modern inet_pton() does not accept any leading zeros. For a while the ipaddress module used to accept ambiguous leading zeros. It was discovered that Python incorrectly handled certain RFCs. An attacker could possibly use this issue to cause a denial of service. This issue only affected Ubuntu 16.04 ESM. (CVE-2021-3733) It was discovered that Python incorrectly handled certain server responses. An attacker could possibly use this issue to cause a denial of service. (CVE-2021-3737)