A potential denial of service (DoS) vulnerability was discovered in bind9, the popular DNS server.
Every named instance configured to run as a recursive resolver maintains a
cache database holding the responses to the queries it has recently sent to
authoritative servers. The size limit for that cache database can be configured
using the max-cache-size statement in the configuration file which defaults
to 90% of the total amount of memory available on the host. When the size of
the cache reaches 7/8 of the configured limit, a cache-cleaning algorithm
starts to remove expired and/or least-recently used RRsets from the cache in
order to keep memory use below the configured limit.
However, it was discovered that the effectiveness of the cache-cleaning
algorithm used in named can be severely diminished by querying the resolver
for specific RRsets in a certain order, effectively allowing the configured
max-cache-size limit to be significantly exceeded.
By exploiting this flaw, an attacker could have caused the amount of memory
used by a named resolver to go well beyond the configured max-cache-size
limit. The effectiveness of the attack depends on a number of factors (e.g.
query load, query patterns), but since the default value of the
max-cache-size statement is 90%, in the worst case, the attacker can exhaust
all available memory on the host running named, leading to a
denial-of-service condition.