Unstable Minecraft Versions

Unstable Minecraft Versions
Unstable versions are on the left, and the reasons are on the right.

Using these versions may not be a serious threat to private servers between friends, but you always want to ensure you don't have these versions at all anyway. There are newer fixes for each major update, so use those. Hacked clients can access your servers even if you think they're private, and an example of one that does this is "Minecraft Wurst"

1.6.1, 1.6.2 - "Handshake Exploit"
1.7.2, 1.7.4, 1.7.5, 1.7.6, 1.7.7 - MC-52755
1.8, 1.8.1, 1.8.2, 1.8.3 - MC-79079 – Malicious clients can force a server to freeze
1.8, 1.8.1, 1.8.2, 1.8.3, 1.8.4, 1.8.5 - MC-75630 – Exploit with signs and command blocks (can access server files, force ops, stop server, etc) https://mojang.com/2015/05/minecraft-1-8-5-security-release/ https://bugs.mojang.com/browse/MC-80671?focusedCommentId=228546&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-228546
1.8, 1.8.1, 1.8.2, 1.8.3, 1.8.4, 1.8.5, 1.8.6 - https://mojang.com/2015/06/minecraft-1-8-7-security-release/
1.8, 1.8.1, 1.8.2, 1.8.3, 1.8.4, 1.8.5, 1.8.6, 1.8.7 - https://mojang.com/2015/07/minecraft-1-8-8-release/