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Millions of servers use software vulnerable to the bug, which lets attackers run commands on that system.
So far, thousands of servers have been compromised via Shellshock and some have been used to bombard web firms with data, said experts.
The number of attacks and compromises was likely to grow as the code used to exploit the bug was shared.
The Shellshock bug was discovered in a tool known as Bash that is widely used by the Unix operating system and many of its variants, including Linux open source software and Apple’s OSX.
Apple said it was working on a fix for its operating system and added that most users would not be at risk from Shellshock.
Attackers have been spotted creating networks of compromised machines, known as botnets, that were then put to other uses.
The seriousness of the bug has also led governments to act quickly. The UK government said its cybersecurity response team had issued an alertto its agencies and departments giving Shellshock the “highest possible threat ratings”.
It had this rating, said the alert, because vulnerable systems would “inevitably” include machines that formed part of the UK’s critical national infrastructure.
The US and Canada are believed to have issued similar alerts and told technology staff to patch systems as quickly as possible. Amazon, Google, Akamai and many other tech firms have also issued advisories to customers about the bug.
As well as software patches for vulnerable systems, security firms and researchers are also producing signatures and filter lists to help spot attacks based around it.
Early reports suggest up to 500 million machines could be vulnerable to Shellshock but, wrote Jen Ellis from security firm Rapid7, this figure was now being revised downwards because of the “number of factors that need to be in play for a target to be susceptible”.
“This bug is going to affect an unknowable number of products and systems, but the conditions to exploit it are fairly uncommon for remote exploitation,” said Ms Ellis.