Apple doesn't regularly release a critical update for previous iOS versions but DarkSword appears to be a serious threat.
You Need to Download This iOS 18 Update ASAP if You Aren't on iOS 26
Apple released an emergency iOS 18 update to patch a critical vulnerability (codenamed 'DarkSword') exploited in the wild. Users of iOS 18.x devices are at risk of remote code execution or privilege escalation attacks if they do not update immediately.