"Shellshock" is the first major Internet threat to emerge since the discovery in April of "Heartbleed," which affected encryption software used in about two-thirds of all web servers, along with hundreds of technology products.
               
The latest bug has been compared to "Heartbleed" partly because the software at the heart of the "Shellshock" bug, known as Bash, is also widely used in web servers and other types of computer equipment.
               
According to security experts, "Shellshock" is unlikely to affect as many systems as "Heartbleed" because not all computers running Bash can be exploited. Still, they said the new bug has the potential to wreak more havoc because it enables hackers to gain complete control of an infected machine, which lets them destroy data, shut down networks or launch attacks on websites.
               
The "Heartbleed" bug only allowed hackers to steal data.The industry is rushing to determine which systems can be remotely compromised by hackers, but there are currently no estimates on the number of vulnerable systems.
               
Amazon.com Inc and Google Inc have released bulletins to advise web services customers how to protect themselves from the new cyber threat. A Google spokesman said the company is releasing software patches to fix the bug.
               
"We don't actually know how widespread this is. This is probably one of the most difficult-to-measure bugs that has come along in years," said Dan Kaminsky, a well-known expert on Internet threats.
               
For an attack to be successful, a targeted system must be accessible via the Internet and also running a second vulnerable set of code besides Bash, experts said.
 
"There is a lot of speculation out there as to what is vulnerable, but we just don't have the answers," said Marc Maiffret, chief technology officer of cybersecurity firm BeyondTrust. "This is going to unfold over the coming weeks and months."