Multiple denial-of-service attacks that can be triggered by writing to the terminal exist in PuTTY versions before 0.71.
https://www.debian.org/security/2019/dsa-4423
https://www.chiark.greenend.org.uk/~sgtatham/putty/changes.html
https://security.netapp.com/advisory/ntap-20190404-0001/
https://seclists.org/bugtraq/2019/Apr/6
https://lists.debian.org/debian-lts-announce/2019/04/msg00023.html
http://lists.opensuse.org/opensuse-security-announce/2019-04/msg00020.html
http://lists.opensuse.org/opensuse-security-announce/2019-04/msg00004.html