In the var folder, a file has suddenly appeared called brute-force.ini
.
Is it harmful or not? Can I delete this file?
There is this awesome tool called google. It usually helps to look there first ;)
First result:
https://community.magento.com/t5/Security-Patches/brute-force-ini/m-p/54215#M2288
This file is saved when Magento detects failed login attemps to the downloader. (This feature was introduced on Magento 1.9.3)