This is an emergency patch for 7.4.0 to work around an exploit in vanilla Minecraft. Anyone who attempts to send a potentially vulnerable chat message will be told that they cannot send it.
USERS SHOULD UPDATE TO THIS SERVER VERSION AS SOON AS POSSIBLE
IN ADDITION, we strongly recommend that everyone adds the flag -Dlog4j2.formatMsgNoLookups=true to their server/client startup parameters (on the client, you can do this in the launcher by going to installations at the top, hovering over the version of Minecraft you're running, selecting "Edit", the "More Options", then adding the flag in there.)
Full Changelog: v7.4.0...v7.4.1
Server owners should download spongevanilla-1.12.2-7.4.2.jar