Security consultancy iSec Partners yesterday detailed half a dozen ways to compromise VoIP-based phone systems based on the H.323 and Inter Asterisk eXchange protocols.

After detailing the ways they uncovered to hack into these two types of VoIP phone systems, Himanshu Dwivedi, principal partner at iSec, and Zane Lackey, security analyst there, also released exploit tools on the firm’s Web site to prove their assertions about the weakness of H.323 and IAX. Their presentation was made at the Black Hat conference in Las Vegas.

The researchers said they concentrated on uncovering security problems with authentication and authorization in VoIP phones based on H.323 and IAX because there has not been as much investigation of those protocols as there has been of another VoIP protocol, the Session Initiation Protocol (SIP).

“There are a lot of known problems with SIP, but we’re here to say H.323 and IAX are just as bad,” Dwivedi said, adding that the equipment from Cisco, Avaya and Polycom that supports H.323 could be affected by the types of attacks he described at Black Hat.

Specifically, iSec Partners discussed how an attacker could sniff a corporate network to compromise H.323-based authentication, which uses an MD5 hash technique to authenticate the phone user. “We do an offline dictionary attack on all the stuff we sniffed off the network,” said Dwivedi. “A password is usually only four or five digits on a phone extension. You don’t have anything but numeric passwords.”

Although a time stamp in H.323 authentication is meant to set a time period for the authentication process, and can be exact to the millisecond, one basic weakness in practice is that the time stamp is usually valid for as long as thirty minutes or even an hour, says Dwivedi. “You sniff the MD5 hash and replay it because it’s valid for 30 minutes.”

In addition to this man-in-the-middle attack that spoofs the victim’s VoIP phone, iSec Partners also detailed how to break into H.323 Gatekeeper servers used for registering and authenticating VoIP phones and launch a registration-rejection attack with an automated tool.

Lackey detailed similar attacks against VoIP systems based open source IAX 2.0, which also uses MD5 for authentication. iSec Partners discussed ways to launch an IAX denial-of-service attack; force IAX phone to hang up or be placed on hold or reject calls. iSec has released half a dozen tools for public use to test whether H.323 and IAX VoIP networks are vulnerable to the types of attacks Dwivedi and Lackey described.