Attackers have started exploiting a flaw in the most widely used software for the DNS (Domain Name System), which translates domain names into IP addresses.

Last week, a patch was issued for the denial-of-service flaw, which affects all versions of BIND 9, open-source software originally developed by the University of California at Berkeley in the 1980s.

The flaw can be exploited with a single packet, crashing both authoritative and recursive DNS servers. Security analysts predicted that attackers would quickly figure out how to exploit the flaw, which has now happened.

"We can confirm that the attacks have begun," wrote Daniel Cid, CTO and founder of the security company Sucuri, in a blog post "DNS is one of the most critical parts of the Internet infrastructure, so having your DNS go down, it also means your email, HTTP and all other services will be unavailable."

There's no workaround for the flaw, so administrators need to patch to stop attacks. Major Linux distributions including Red Hat, CentOS and Ubuntu have issued patches, but it is still up to admins to apply it and restart their BIND servers.

Cid wrote via email that at least two of Sucuri's clients within different industries had seen their DNS servers crash due to attacks.

Both are popular websites that had been hit by distributed denial-of-service attacks before, "so it made sense to see them being targeted first," he wrote Sunday.

A successful attack will leave a trace in server logs, Cid wrote. The command "ANY TKEY" should appear as long as admins have querylog enabled.

The patch and advisory from the Internet Systems Consortium is available here.

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