This exploit is for a combination of fail2ban and `mail`. Reading how it works, it seems much more of a vulnerability for anything using the `mail` command than in fail2ban per se.

That ~! escape is really dangerous. What percentage of sysadmins are even aware of its existence? I can see how it can be useful, but there is a lot of potential for exploit if you aren't extremely careful.

The `mail` command shouldn't so easily accommodate executing arbitrary commands from input. The ~! escape should probably be either removed from `mail` entirely, or enabled only if you pass it a flag. It seems like a vestige from an earlier, more innocent time.

This isn't to absolve sysadmins who fail to sanitize their inputs, but let's not make their job so difficult.