FML appears to be missing any signature data. This is not a good thing (?)

I installed a fresh Forge (1.8.9-11.15.1.1722) server (with Sponge 1.8.9-1691-3.1.0-BETA-1046). The host (running Debian) is headless, Minecraft starts correctly, but one line in the startup log is worrisome:

[main/ERROR] [FML]: FML appears to be missing any signature data. This is not a good thing

There are no more errors (only INFO messages). I believe that FML means Forge Mod Loader, which then appears with its own log messages (like [FML]: Forge Mod Loader has successfully loaded 4 mods) and otherwise the server works fine (at least seems to do so, I connected from a client and walked around without issues.

Is this something I should be worried about?

Solution 1:

Is this something I should be worried about?

[main/ERROR] [FML]: FML appears to be missing any signature data. This is not a good thing

tl;dr, Nope.

Unless you have downloaded Minecraft Forge from a sketchy location, it's something you do not need to be worried about.

That basically means that it could not verify the file's integrity. It's nothing important, unless you're wary of viruses, etc.
Also, this error mostly happens when you use the Universal version of Forge.

The only thing you need to worry about is whether the game server runs or not (throws an exception and goes to the console/launcher/crash screen).