They say that:
We have confirmed that it was triggered by an accidental operational error that exposed a previously unknown bug in BIND, causing an internal inconsistency which is effectively prevented by the mitigation patches we have produced and distributed.
They also highlight that this could have been exploited maliciously, so if you are running a version of BIND which is vulnerable to CVE-2011-4313 then they advise you to upgrade.
Dec 5th 2011
8 years ago