Bind DNS resolver purged of critical DoS bug

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. ®