The company plans to release firmware updates to remove the undocumented feature by the end of the month

Cisco Systems promised to issue firmware updates removing a backdoor from a wireless access point and two of its routers later this month. The undocumented feature could allow unauthenticated remote attackers to gain administrative access to the devices.

The vulnerability was discovered over the Christmas holiday on a Linksys WAG200G router by a security researcher named Eloi Vanderbeken. He found that the device had a service listening on port 32764 TCP, and that connecting to it allowed a remote user to send unauthenticated commands to the device and reset the administrative password.

It was later reported by other users that the same backdoor was present in multiple devices from Cisco, Netgear, Belkin and other manufacturers. On many devices this undocumented interface can only be accessed from the local or wireless network, but on some devices it is also accessible from the Internet.

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Cisco identified the vulnerability in its WAP4410N Wireless-N Access Point, WRVS4400N Wireless-N Gigabit Security Router and RVS4000 4-port Gigabit Security Router. The company is no longer responsible for Linksys routers, as it sold that consumer division to Belkin early last year.

The vulnerability is caused by a testing interface that can be accessed from the LAN side on the WRVS4400N and RVS4000 routers and also the wireless network on the WAP4410N wireless access point device.

“An attacker could exploit this vulnerability by accessing the affected device from the LAN-side interface and issuing arbitrary commands in the underlying operating system,” Cisco said in an advisory published Friday. “An exploit could allow the attacker to access user credentials for the administrator account of the device, and read the device configuration. The exploit can also allow the attacker to issue arbitrary commands on the device with escalated privileges.”

The company noted that there are no known workarounds that could mitigate this vulnerability in the absence of a firmware update.

The SANS Internet Storm Center, a cyber threat monitoring organization, warned at the beginning of the month that it detected probes for port 32764 TCP on the Internet, most likely targeting this vulnerability.

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