On Friday, the Internet Systems Consortium (ISC) announced security updates for BIND DNS software that address two vulnerabilities rated with a “medium” severity rating.

Both vulnerabilities could be exploited by attackers to cause a denial-of-service (DoS) condition, the first issue tracked as CVE-2018-5737 can also cause severe operational problems such as degradation of the service.

“A problem with the implementation of the new serve-stale feature in BIND 9.12 can lead to an assertion failure in rbtdb.c, even when stale-answer-enable is off.  Additionally, problematic interaction between the serve-stale feature and NSEC aggressive negative caching can in some cases cause undesirable behavior from named, such as a recursion loop or excessive logging.” reads the security advisory published by the ISC.

“Deliberate exploitation of this condition could cause operational problems depending on the particular manifestation — either degradation or denial of service.” 

The flaw affects BIND 9.12.0 and 9.12.1 which permit recursion to clients and which have the max-stale-ttl parameter set to a non-zero value are at risk.

The Internet Systems Consortium (ISC) has addressed the flaw with the release of BIND 9.12.1-P2. Below the workaround provided by the organization:

  • Setting “max-stalettl 0;” in named.conf will prevent exploitation of this vulnerability (but will effectively disable the serve-stale feature.)
  • Setting “stale-answer enable off;” is not sufficient to prevent exploitation, max-stale-ttl needs to be set to zero.

BIND DNS sw flaw

The second flaw tracked as CVE-2018-5736 is remotely exploitable if the attacker can trigger a zone transfer.

“An error in zone database reference counting can lead to an assertion failure if a server which is running an affected version of BIND attempts several transfers of a slave zone in quick succession,” states the advisory published by the ISC.

“This defect could be deliberately exercised by an attacker who is permitted to cause a vulnerable server to initiate zone transfers (for example: by sending valid NOTIFY messages), causing the named process to exit after failing the assertion test.”

The CVE-2018-5736 flaw affects BIND 9.12.0 and 9.12.1, the ISC addressed it with the release of the version 9.12.1-P1. Experts noticed that admins need to update to version 9.12.1-P2 because version 9.12.1-P1 was affected by a problem.

This is the third time that the ISC provides security updates for BIND software this year. The first updates were released in January to address a high severity vulnerability that could cause DNS servers crash,

The second updates were released in February to address remotely exploitable vulnerabilities in DHCP.

Pierluigi Paganini

(Security Affairs – BIND DNS software, DoS)

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