CVE-2020-15701 Unhandled exception in apport

An unhandled exception in check_ignored() in apport/report.py can be exploited by a local attacker to cause a denial of service. If the mtime attribute is a string value in apport-ignore.xml, it will trigger an unhandled exception, resulting in a crash. Fixed in 2.20.1-0ubuntu2.24, 2.20.9-0ubuntu7.16, 2.20.11-0ubuntu27.6.