Users of the popular Vonage voice over IP (VOIP) service could be vulnerable to a wide variety of attacks, security vendor Sipera Systems warned earlier this week.

Researchers at Sipera's Viper research lab, which focuses on VOIP security, issued four threat advisories about Vonage on Wednesday.

One of the most serious warnings deals with the Motorola phone adapter used to link telephones to the Vonage service. The adapter doesn't have a way to authenticate incoming Session Initiation Protocol (SIP) call requests, which means that the bad guys can make an attack with a simple phone call, Sipera says.

"When the Vonage customer receives a phone call, the SIP message provides information about the person calling," Sipera states in its advisory. "However, the Vonage Motorola phone adapter does not authenticate the INVITE received from the server. Consequently, an attacker or spammer can send a SIP message directly to a Vonage customer, from IP address spoofed to be the server’s IP address."

Disguised as the SIP server, an attacker can execute a variety of exploits, including information theft, Sipera says. In a separate warning, Sipera also said this vulnerability might be used to launch a denial of service attack on a Vonage user.

Vonage users should authenticate incoming SIP requests before accepting them, the researchers say.

In another "high severity" alert, Sipera warns that attackers can eavesdrop on Vonage calls without much difficulty. Vonage traffic is often sent over the Internet using unencrypted RTP packets, which can be captured and reconstructed by a third party, the researchers note.

"The availability of several free tools to reconstruct media from captured RTP packets further increases the threat," Sipera cautions."

Sipera also issued a "medium severity" warning that a weak authentication vulnerability in Vonage’s SIP server may allow an attacker to send spoofed REGISTER messages to the server. This may make it possible for legitimate users' registration sessions to be sniffed, copied, and replayed, the security vendor says.

"These vulnerabilities create serious privacy and service availability issues for users," says Krishna Kurapati, Sipera founder/CTO and head of Sipera VIPER Lab. "Vonage and [other VOIP service] customers can no longer assume that their VOIP providers are automatically securing their services, but they should demand best security practices be followed as a condition of becoming a customer."

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