ISC Responds to Questions About SRTT Algorithm Vulnerability

Michael McNally mcnally at
Wed May 7 04:27:36 UTC 2014

This week several of our customers have contacted us to inquire
about our reaction to an article entitled "Critical Vulnerability
in BIND Software Puts DNS Protocol Security at Risk"  [1]

ISC would like to clarify that we evaluated the risk from this issue
in 2013 when it was disclosed to us, and do not judge it to be a
"critical vulnerability" or feel that it "puts DNS protocol security
at risk."  The article linked above is light on details but you can
read the original presentation from Woot '13 [2] if you would like
more background information on the SRTT algorithm flaw that allows
an attacker to influence selection of a specific nameserver from the
servers available in the NS record RRSET.

The authors of that paper responsibly reported the issue to ISC
prior to their conference presentation and we evaluated it for its
security threat potential at that time. We reached the conclusion
that the technique described did not by itself constitute an
exploitable defect in BIND security but did have potential for use
as an enhancement for other attacks.  In order to explain the matter
and make operators aware of it, we issued an Operational Notification
for BIND admins [3] and announced it on public mailing lists in
August 2013.

Renewed interest in this matter has prompted us to re-examine the
issue to see whether any new information has changed our opinion
of the issue's severity.  At this time we still believe that the
manipulation of server selection through exploitation of a flaw in
the SRTT algorithm represents at best a supplement to other attack
vectors. Nevertheless, ISC intends to correct the flaw in a future
release of BIND but has not committed to a timetable for doing so.

If you are aware of an active exploit which uses this technique,
or if you believe you are aware of an implication we may not have
considered, we encourage you to share your concerns with our ISC
Security Officers by e-mailing security-officer at Please
encrypt any communications containing sensitive security information
using the Security Officer PGP key. [4]

Thank you for the opportunity to clarify this matter,

Michael McNally,
ISC Support 


[1] "Critical Vulnerability in BIND Software Puts DNS Protocol
    Security at Risk" 

[2] "Subverting BIND's SRTT Algorithm Derandomizing NS Selection"

[3] "A Vulnerability in the SRTT Algorithm affects BIND 9 Authoritative
    Server Selection"

[4] ISC Public PGP Keys 

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