Bug 507717 (CVE-2009-0692)

Description from ISC's advisory:

Description:

ISC dhclient has a stack overflow vulnerability which makes it
theoretically possible for a rogue DHCP server to execute arbitrary
commands as root on the affected system through stack return
subversion.

Impact:

While generating a subnet number from the server-supplied leased address
and subnet-mask 'dhclient' copies the information into a field without
verifying if the length of the information exceeds the length of the field.

Theoretically this allows a rogue DHCP server to execute arbitrary
commands as root on the affected system through stack return subversion.

This attack has little to no risk for a client situated on a network
that is well defended, whereas clients that are roaming to potentially
hostile or ad-hoc networks can see this attack to pose a severe threat.

Factors complicating any attack would be:

1) The attacker would need to generate messages the client views as
authentic.

One option is for the attacker to present itself as a suitable DHCPv4
server for a network, in essence operating as a rogue DHCPv4 server.

Another option would be to insert messages into the conversation between
the client and the authentic DHCPv4 server.  To do this the attacker
needs to accurately guess the client's randomly chosen 16-bit
transaction ID and insert the attack precisely between the client's
request and the valid DHCPv4 server's reply.

Neither of these are likely on a well defended network but clients that
are roaming may find them, especially the first, a severe threat.

2) The attacker would then need to develop their attack within a limited
packet size.

Support for DHCPv4 total packet size may be limited from 576 octets
through the link MTU size (no support for fragmentation) up to 64KB.  Of
this, the DHCPv4 option payload space is limited by the space taken up
by the BOOTP header space, excepting the FILE and SNAME fields (which
can be used in 'option overloading' to carry option contents, such as
the subnet-mask).