PuTTY 0.68 through 0.73 has an Observable Discrepancy leading to an information leak in the algorithm negotiation. This allows man-in-the-middle attackers to target initial connection attempts (where no host key for the server has been cached by the client).
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/26TACCSQYYCPWAJYNAUIXJGZ5RGORJZV/
lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JPV4A77EDCT4BTFO5BE26ZH72BG4E5IJ/
lists.tartarus.org/pipermail/putty-announce/
security.netapp.com/advisory/ntap-20200717-0003/
www.chiark.greenend.org.uk/~sgtatham/putty/changes.html
www.fzi.de/en/news/news/detail-en/artikel/fsa-2020-2-ausnutzung-eines-informationslecks-fuer-gezielte-mitm-angriffe-auf-ssh-clients/