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Security Researcher Compromises Cisco VoIP Phones With Vulnerability

Grad student demonstrates how phones can be turned into listening devices by attackers

A researcher has demonstrated how Cisco Voice-over-IP (VoIP) phones can be hijacked and turned into listening devices.

At the Amphion Forum this month, Columbia University grad student Ang Cui demonstrated how networked printers and phones can be abused by attackers. The forum, held in San Francisco, is produced by Mocana, which makes security software for non-PC devices that connect to the Internet.

"The attack I demonstrated is caused by the multiple vulnerabilities within the syscall interface of the CNU [Cisco Native Unix] kernel," Cui tells Dark Reading. "It is caused by the lack of input validation at the syscall interface, which allows arbitrary modification of kernel memory from userland, as well as arbitrary code execution within the kernel. This, in turn, allows the attacker to become root, gain control over the DSP [Digital Signal Processor], buttons, and LEDs on the phone. The attack I demonstrated patches the existing kernel and DSP in order to carry out stealthy mic exfiltration."

As part of the demonstration, Cui inserted and removed a small external circuit board from the phone's Ethernet port -- a move he asserted could be accomplished by someone left alone inside a corporate office for a few seconds. He then used his own smartphone to capture every word spoken near the VoIP phone, even though it was still "on-hook."

By patching the VoIP phone's software with his own code, he was able to turn the Off-Hook Switch into what he refers to as a "funtenna."

The issue can be exploited remotely as well, explains Cui, where a likely method of exploiting the kernel is by using an arbitrary execution bug on the phone's surface.

"Typically, exploitation of a nonprivileged process will give the attacker limited access to the phone," he says. "But in this case, any arbitrary code execution bug can be used to exploit the vulnerable syscalls, giving the attacker kernel-level access. I've identified several third-party libraries within the Cisco phone that have known exploitable vulnerabilities."

"The exploitability of these vulnerable third-party libraries will be discussed at 29C3 [29th Chaos Communication Congress]," he says.

In response to his findings, Cisco says that workarounds and a software patch are available to address the issue, and that successful exploitation requires physical access to the device serial port or a combination of remote authentication privileges and nondefault settings.

"The company maintains a very open relationship with the security community, and we view this as vital to helping protect our customers’ networks," the company said in a statement, adding that "a formal release note for customers was issued on November 2nd (bug id: CSCuc83860)."

"As for mitigation, I encourage everyone to patch to the latest [not publicly] available firmware ASAP," Cui says. "I looked today and found that the firmware images are not available for download. I believe it is still only available upon request through TAC.

"At the end of the day, I'd like to see actual mechanical switches that control the various input/output devices on IP phones," he adds. "Looking past this specific vulnerability, I believe there is a real need for actual host-based defense on embedded systems. The Software Symbiote technology developed at Columbia University is designed to protect embedded devices against this kind of attack."

The vulnerability Cui demonstrated was based on work he did during the past year on ‘Project Gunman v2’, where a laser printer firmware update could be compromised with additional -- and potentially malicious -- code. The compromised printer could then be used to launch other attacks on the internal network.

"The VoIP phone vulnerability demonstrated at the Amphion Forum was a stark reminder of the need to address the device security mess," says Kurt Stammberger, vice president of market development at Mocana and chair of the Amphion Forum, in a statement. "The sad fact is that most devices connected to corporate networks, like printers and VoIP phones, are almost totally unsecured."

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