[SECURITY] [DLA 192-1] ntp security update

Package : ntp
Version : 1:4.2.6.p2+dfsg-1+deb6u3
CVE ID : CVE-2015-1798 CVE-2015-1799
Debian Bug : #782095

Brief introduction

CVE-2015-1798

When ntpd is configured to use a symmetric key to authenticate a remote NTP
server/peer, it checks if the NTP message authentication code (MAC) in received
packets is valid, but not if there actually is any MAC included. Packets without
a MAC are accepted as if they had a valid MAC. This allows a MITM attacker to
send false packets that are accepted by the client/peer without having to know
the symmetric key. The attacker needs to know the transmit timestamp of the
client to match it in the forged reply and the false reply needs to reach the
client before the genuine reply from the server. The attacker doesn't
necessarily need to be relaying the packets between the client and the server.

Authentication using autokey doesn't have this problem as there is a check that
requires the key ID to be larger than NTP_MAXKEY, which fails for packets
without a MAC.

CVE-2015-1799

An attacker knowing that NTP hosts A and B are peering with each other
(symmetric association) can send a packet to host A with source address of B
which will set the NTP state variables on A to the values sent by the attacker.
Host A will then send on its next poll to B a packet with originate timestamp
that doesn't match the transmit timestamp of B and the packet will be dropped.
If the attacker does this periodically for both hosts, they won't be able to
synchronize to each other. This is a known denial-of-service attack, described
at https://www.eecis.udel.edu/~mills/onwire.html .

According to the document the NTP authentication is supposed to protect
symmetric associations against this attack, but that doesn't seem to be the
case. The state variables are updated even when authentication fails and the
peers are sending packets with originate timestamps that don't match the
transmit timestamps on the receiving side.

ntp-keygen on big endian hosts

Using ntp-keygen to generate an MD5 key on big endian hosts resulted in
either an infite loop or an key of only 93 possible keys.

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