By Jim Finkle

BOSTON, Sept 25 (Reuters) - Hackers have launched attacks exploiting the newly identified "Shellshock" computer bug, researchers warned on Thursday, as news surfaced that an initial patch for the issue was incomplete, suggesting even updated systems were vulnerable.

The attacks came as security experts scrambled to determine how many systems and what types of computers are vulnerable to "Shellshock," which some say may be as serious as the "Heartbleed" vulnerability that surfaced in April.

"Shellshock" is a bug in a piece of software known as "Bash" that runs the command prompt on many Unix computers, including some Linux servers that run websites, and tiny computers inside consumer devices such as routers and web cams.

"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, computer 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, 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.

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."

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 on websites to disrupt their operations and 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.

"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. (Additional reporting by Carolyn Cohn; Editing by Dan Grebler)