You don’t need to be the NSA to tap calls on Cisco’s SPA 300 and 500 IP phones: An authentication flaw allows potential attackers to do that by default.

An unpatched vulnerability in the firmware of the SPA 300 and 500 series IP phones, typically used by small businesses, could allow eavesdropping on calls.

“The vulnerability is due to improper authentication settings in the default configuration,” Cisco Systems said in a security advisory.

Unauthenticated remote attackers could send crafted XML requests to affected devices in order to exploit the flaw and remotely listen to audio streams or make phone calls through them, the company warned.

Cisco determined that phones running version 7.5.5 of the firmware are vulnerable, but said that later versions may also be affected. No patches are currently available, so phone owners are advised to take other safeguards.

Administrators should enable XML Execution authentication in the configuration settings of the affected devices and should make sure that only trusted users have access to the networks where the phones are installed, the company said. Solid firewall strategies and IP-based access control lists can also be used to protect the affected systems from external attacks.

According to Cisco, the fact that these systems are typically installed on internal networks, behind firewalls, may reduce the likelihood of a successful exploit, as attackers would first need to gain access to those networks.

However, it’s likely that some phones have been configured to be accessible from the Internet and it would be fairly easy for attackers to locate them, for example by using the specialized device search engine SHODAN.