This attack could pose a serious security threat. You should take immediate action to stop any damage or prevent further damage from happening.
This signature detects the attempt to execute a remote code vulnerability in mIRC.
mIRC before 7.55 allows remote command execution by using argument injection through custom URI protocol handlers. The attacker can specify an irc:// URI that loads an arbitrary .ini file from a UNC share pathname. Exploitation depends on browser-specific URI handling (Chrome is not exploitable).