In Tor before 0.3.3.12, 0.3.4.x before 0.3.4.11, 0.3.5.x before 0.3.5.8, and 0.4.x before 0.4.0.2-alpha, remote denial of service against Tor clients and relays can occur via memory exhaustion in the KIST cell scheduler.
https://trac.torproject.org/projects/tor/ticket/29168
https://blog.torproject.org/new-releases-tor-0402-alpha-0358-03411-and-03312
http://www.securityfocus.com/bid/107136
http://lists.opensuse.org/opensuse-security-announce/2019-04/msg00013.html