Hackers have begun exploiting the newly identified Shellshock computer bug, also widely known as the "Bash bug," using fast-moving worm viruses to scan for vulnerable systems and then infect them, researchers warned on Thursday.

Shellshock is the first major Internet threat to emerge since the discovery in April of Heartbleed, which affected OpenSSL encryption software that is used in about two-thirds of all web servers, along with hundreds of technology products for consumers and businesses.

The latest bug has been compared to Heartbleed partly because the software at the heart of the Shellshock bug, a command prompt program known as Bash, is also widely used in web servers and other types of computer equipment.

The problem is unlikely to affect as many systems as Heartbleed because not all computers running Bash can be exploited, according to security experts. Still, they said Shellshock has the potential to wreak more havoc because it enables hackers to gain complete control of an infected machine, which could allow hackers to destroy data, shut down networks or launch attacks on websites, experts said.

The Heartbleed bug only allowed them to steal data.

Unknown severity

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.

"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."

Joe Hancock, a cybersecurity expert with insurer AEGIS in London, U.K., said in a statement that he is concerned about the potential for attacks on home broadband routers and controllers used to manage critical infrastructure facilities.

"In some areas this will be a challenge to fix, as many embedded devices are not designed with regular updates in mind and will never be able to be patched," Hancock said.

Patches could be a problem

HD Moore, chief research officer with security software maker Rapid7, said it could take weeks or even months to determine what impact the bug will have.

"At this point we don't know what we don't know, but we do expect to see additional exploit vectors surface as vendors and researchers start the assessment process for their products and services," Moore said in an email. "We are likely to see compromises as a result of this issue for years to come."

Linux makers released patches to protect against attacks on Wednesday, though security researchers uncovered flaws in those updates, prompting No. 1 Linux maker Red Hat Inc to advise customers that the patch was "incomplete."

"That's a problem. It's been a little over 24 hours and we're still in the same boat," said Mat Gangwer, lead security consultant at Rook Security. "People are kind of freaking out. Rightfully so."

Attacks underway

Russian security software maker Kaspersky Lab reported that a computer worm has begun infecting computers by exploiting Shellshock.

The malicious software can take control of an infected machine, launch denial-of-service attacks to disrupt websites, and also scan for other vulnerable devices, including routers, said Kaspersky researcher David Jacoby.

He said he did not know who was behind the attacks and could not name any victims.

Jaime Blasco, labs director at AlienVault, said he had uncovered the same piece of malware, as well as a second worm seeking to exploit Shellshock, which was designed for launching denial of service attacks.

Heartbleed is a bug in an open-source encryption software called OpenSSL. The bug put the data of millions of people at risk, as OpenSSL is used in about two-thirds of all websites. It also forced dozens of technology companies to issue security patches for hundreds of products that use OpenSSL.