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Bind DNS resolver purged of critical DoS bug

Query domain, server goes boom

Next gen security for virtualised datacentres

Makers of the internet's most widely used domain name resolution software have patched a vulnerability that allowed attackers to crash many systems that run the program.

By querying a domain with large resource record sets (or RRsets) and trying to negatively cache a response, attackers can cause the Bind server to crash. The denial-of-service vulnerability threatens systems that use various versions of Bind 9 as a caching resolver. DNS systems use negative caching to improve resolution response time by preventing servers from looking up non-existent domains over and over.

“In this vulnerability, very large RRSIG RRsets included in a negative response can trigger an assertion failure that will crash named (Bind 9 DNS) due to an off-by-one error in a buffer size check,” read an advisory published by the Internet Systems Consortium, the group that maintains Bind.

The advisory continued:

The nature of this vulnerability would allow remote exploit. An attacker can set up a DNSSEC signed authoritative DNS server with large RRSIG RRsets to act as the trigger. The attacker would then find ways to query an organization’s caching resolvers for non-existent names in the domain served by the bad server, getting a response that would “trigger” the vulnerability. The attacker would require access to an organization’s caching resolvers; access to the resolvers can be direct (open resolvers), through malware (using a BOTNET to query negative caches), or through driving DNS resolution (a SPAM run that has a domain in the E-mail that will cause the client to perform a lookup).

The advisory urges users to upgrade to Bind 9.4-ESV-R4-P1, 9.6-ESV-R4-P1, 9.7.3-P1, or 9.8.0-P2, which are available here.

As a partial workaround, users can restrict the DNS caching resolver system.

“Active exploitation can be accomplished through malware or Spam/Malvertizing actions that will force authorized clients to look up domains that would trigger this vulnerability,” the advisory warns. ®

Next gen security for virtualised datacentres

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