[Pdns-users] PowerDNS Security Advisory 2012-01: Denial of Service vulnerability in most versions of the PowerDNS Authoritative Server

[bert hubert]

Dear PowerDNS users,

It saddens us to have to release this Security Advisory, the first one since
almost exactly two years ago.

Updated versions of the Authoritative Server are available from
http://www.powerdns.com/content/downloads.html and you will find two
workarounds and a patch below. 

A version of this announcement with clickable links is available from
http://doc.powerdns.com/powerdns-advisory-2012-01.html

If you have problems upgrading and would like us to help you, please contact
powerdns.support at netherlabs.nl for a support arrangement.

+-------------+---------------------------------------------------------------+
|CVE          |CVE-2012-0206                                                  |
+-------------+---------------------------------------------------------------+
|Date         |10th of January 2012                                           |
+-------------+---------------------------------------------------------------+
|Credit       |Ray Morris of BetterCGI.com.                                   |
+-------------+---------------------------------------------------------------+
|Affects      |Most PowerDNS Authoritative Server versions < 3.0.1 (with the  |
|             |exception of the just released 2.9.22.5)                       |
+-------------+---------------------------------------------------------------+
|Not affected |No versions of the PowerDNS Recursor ('pdns_recursor') are     |
|             |affected.                                                      |
+-------------+---------------------------------------------------------------+
|Severity     |High                                                           |
+-------------+---------------------------------------------------------------+
|             |Using well crafted UDP packets, one or more PowerDNS servers   |
|Impact       |could be made to enter a tight packet loop, causing temporary  |
|             |denial of service                                              |
+-------------+---------------------------------------------------------------+
|Exploit      |Proof of concept                                               |
+-------------+---------------------------------------------------------------+
|Risk of      |                                                               |
|system       |No                                                             |
|compromise   |                                                               |
+-------------+---------------------------------------------------------------+
|Solution     |Upgrade to PowerDNS Recursor 2.9.22.5 or 3.0.1                 |
+-------------+---------------------------------------------------------------+
|Workaround   |Several, the easiest is setting: cache-ttl=0, which does have a|
|             |performance impact. Please see below.                          |
+-------------+---------------------------------------------------------------+


Affected versions of the PowerDNS Authoritative Server can be made to
respond to DNS responses, thus enabling an attacker to setup a packet loop
between two PowerDNS servers, perpetually answering each other's answers. 
In some scenarios, a server could also be made to talk to itself, achieving
the same effect.

If enough bouncing traffic is generated, this will overwhelm the server or
network and disrupt service.

As a workaround, if upgrading to a non-affected version is not possible,
several options are available.  The issue is caused by the packet-cache,
which can be disabled by setting 'cache-ttl=0', although this does incur a
performance penalty.  This can be partially addressed by raising the
query-cache-ttl to a (far) higher value.

Alternatively, on Linux systems with a working iptables setup, 'responses'
sent to the PowerDNS Authoritative Server 'question' address can be blocked
by issuing:

# iptables -I INPUT -p udp --dst $AUTHIP --dport 53 \! -f -m u32 --u32 "0>>22&0x3C at 8>>15&0x01=1" -j DROP


If this command is used on a router or firewall, substitute FORWARD for INPUT.

To solve this issue, we recommend upgrading to the latest packages available
for your system. Tarballs and new static builds (32/64bit, RPM/DEB) of 2.9.22.5
and 3.0.1 have been uploaded to our download site. Kees Monshouwer has provided
updated CentOS/RHEL packages in his repository. Debian, Fedora and SuSE should
have packages available shortly after this announcement.

For those running custom PowerDNS versions, just applying this patch may be
easier:

--- pdns/common_startup.cc   (revision 2326)
+++ pdns/common_startup.cc   (working copy)
@@ -253,7 +253,9 @@
       numreceived4++;
     else
       numreceived6++;
-
+    if(P->d.qr)
+      continue;
+
     S.ringAccount("queries", P->qdomain+"/"+P->qtype.getName());
     S.ringAccount("remotes",P->getRemote());
     if(logDNSQueries) {

It should apply cleanly to 3.0 and with little trouble to several older
releases, including 2.9.22 and 2.9.21.

This bug resurfaced because over time, the check for 'not responding to
responses' moved to the wrong place, allowing certain responses to be processed
anyhow.

We would like to thank Ray Morris of BetterCGI.com for bringing this issue to
our attention and Aki Tuomi for helping us reproduce the problem.

Kind regards,

Bert Hubert