ArchLinux: 201412-8: unbound: denial of service
Summary
The resolver can be tricked into following an endless series of
delegations, this consumes a lot of resources.
Resolvers fetch the content for domain names by sending queries to
authority servers on the internet. One of the responses that authority
servers can return is a referral response, which points to further
servers to continue the lookup. To continue the lookup, resolvers
may have to perform recursion, where new names, called glue, from the
referral response have to be looked up to continue the query resolution.
The issue here is a lack of limiting on the recursion fetches performed
to figure out a particular query. The authority server is a special
set-up that responds with an infinite amount of glue. This then causes
the resolver to spend a lot of resources diving into the infinite glue
looking up names, only find out it needs to look up even more names.
Resolution
Upgrade to 1.5.1-1.
# pacman -Syu "unbound>=1.5.1-1"
The problem has been fixed upstream in version 1.5.1.
References
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-8602 https://nlnetlabs.nl/downloads/unbound/CVE-2014-8602.txt
Workaround
A very simple workaround is to ignore the problem and let existing anti-DoS systems in unbound deal with the issue. It will consume a lot of resources, but other customers will (most likely) continue to get service. If affected, unbound-control flush_requestlist provides temporary relief, but the issue could resume (immediately). Putting the maliciously sent query in local-data, or using access-control to block the malicious query sending IP would workaround that exploit set-up. The config statement do-not-query-address: IPorNetblock can be used to block a specific authority server.
