A critical denial-of-service (DoS) vulnerability in the Domain Name System (DNS) software BIND is being exploited in the wild, researchers have warned.

The Internet Systems Consortium (ISC) rated the vulnerability “critical” because it affects almost all BIND servers and it’s difficult to defend against. The flaw (CVE-2015-5477), related to the handling of TKEY record queries, allows malicious actors to launch DoS attacks against both recursive and authoritative servers by sending them specially crafted DNS requests.

The ISC released updates (BIND 9.10.2-P3 and BIND 9.9.7-P2) to address the vulnerability on July 28, and advised users to update as soon as possible because the patch is easy to reverse engineer. In an update posted to its initial advisory on August 1, the organization revealed that a proof-of-concept (PoC) exploit was published by someone to a public source code repository. Sucuri reported seeing attacks in the wild the next day.

“Because of its severity we’ve been actively monitoring to see when the exploit would be live,” Daniel Cid, founder and CTO of Sucuri, said in a blog post. “We can confirm that the attacks have begun. DNS is one of the most critical parts of the Internet infrastructure, so having your DNS go down also means your email, HTTP and all other services will be unavailable.”

Sucuri advises DNS server owners to check their logs for the “ANY TKEY” command to detect attack attempts. Experts noted that since TKEY requests are not very common, any TKEY query could indicate that someone has attempted to crash the server.

A log entry example has been provided by Sucuri:

Aug 2 10:32:48 dns named[2717]: client a.b.c.d#42212 (foo.bar): view north_america: query: foo.bar ANY TKEY + (x.y.z.zz)

Errata Security’s Robert Graham explained that such DoS vulnerabilities are dangerous because they can be used to cause some serious damage even by an attacker with limited resources.

“I could use my ‘masscan’ tool to blanket the Internet with those packets and crash all publicly facing BIND9 DNS servers in about an hour,” Graham said.