Internet Systems Consortium (ISC) fixes High-Severity DoS flaw in BIND DNS Software

Pierluigi Paganini August 20, 2021

The Internet Systems Consortium (ISC) addressed a high-severity denial-of-service (DoS) flaw (CVE-2021-25218) affecting the BIND DNS software.

The Internet Systems Consortium (ISC) has released security updates to address a high-severity denial-of-service (DoS) vulnerability, tracked as CVE-2021-25218, that affects its BIND DNS software.

The vulnerability affects only BIND 9 releases 9.16.19, 9.17.16, and release 9.16.19-S1 of the BIND Supported Preview Edition. ISC also provided workarounds for this vulnerability.

An attacker could exploit the flaw, under specific circumstances, to trigger a DoS condition by causing BIND name server (named) process to crash.

If named attempts to respond over UDP with a response that is larger than the current effective interface maximum transmission unit (MTU), and if response-rate limiting (RRL) is active, an assertion failure is triggered (resulting in termination of the named server process).

There are two ways for named to exceed the interface MTU:

  • Direct configuration in named.conf setting max-udp-size to a value larger than the interface’s MTU, or
  • Path MTU discovery (PMTUD) informing the IP stack that it should use a smaller MTU for the interface and destination than the default max-udp-size value of 1232. Some operating systems allow packets received via other protocols to affect PMTUD values for DNS over UDP.”

ICS noted that the flaw can be triggered through misconfiguration or by deliberate exploitation, it can also be triggered during normal operating conditions,

The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has published a security advisory to warn of this vulnerability.

“The Internet Systems Consortium (ISC) has released a security advisory that addresses a vulnerability affecting multiple versions of the ISC Berkeley Internet Name Domain (BIND). A remote attacker could exploit this vulnerability to cause a denial-of-service condition.” reads the advisory published by CISA. “CISA encourages users and administrators to review ISC advisory CVE-2021-25218 and apply the necessary updates or workarounds.”

At the time of this writing, ICS is not aware of attacks in the wild exploiting the above flaw.

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Pierluigi Paganini

(SecurityAffairs – hacking, BIND DNS )

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