The 'Shellshock' bug discovered at the heart of a core operating system technology is already being exploited by hackers.
The BBC reports that real-world applications of the security problem have been found in the wake of the bug's discovery.
The problem affects the 'Bash' console in UNIX-based operating systems, including Mac OS X.
Experts say that "millions" of computers - including those which run websites and host servers - are vulnerable to the hack, and thousands have been compromised.
Machines by Akamai were among the first to be affected, according to the BBC.
Meanwhile security firm Kaspersky said that attackers were trying to gain control of servers and install 'backdoors' to open access to other information.