iSec Partners has detailed half a dozen ways to hack into VoIP phone systems that use the H.323 and Inter Asterisk eXchange protocols.

Himanshu Dwivedi, principal partner at iSec, and Zane Lackey, security analyst there, also released exploit tools to back up their claims about the weaknesses in H.323 and IAX. Their presentation was made at the Black Hat conference in Las Vegas.

The researchers said they concentrated on H.323 and IAX phones 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 H.323 equipment from Cisco, Avaya and Polycom could be affected by the types of attacks he described.

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."

iSec 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 on the open-source IAX 2.0, which also uses MD5 for authentication. iSec discussed ways to launch an IAX denial-of-service attack; force IAX phones to hang up, be placed on hold or reject calls. iSec has released half a dozen tools to test whether H.323 and IAX VoIP networks are vulnerable to these attacks.

Barrie Dempster, senior security consultant at NGS Software, who also gave a presentation on the topic of VoIP security at Black Hat, said that the iSec research is valuable. But he added that if these exploits are possible based on network sniffing, the first order of concern should be that an attacker is sniffing the network.

Dempster said his opinion overall about VOIP systems is that "the security of VoIP phones is not very good." Attacks for spoofing and intercepting, as well as toll fraud, have been known for years.

"VoIP brings problems," Dempster said. "It's as bad as email, instant messaging clients and web browsers." The irony about VoIP and security, he noted, is that traditional telephony networks look good by comparison.