First, DSL router owners got an unwelcome Christmas present. Now, the same gift is back as an Easter egg. The same security researcher who originally discovered a backdoor in 24 models of wireless DSL routers has found that a patch intended to fix that problem doesn’t actually get rid of the backdoor—it just conceals it. And the nature of the “fix” suggests that the backdoor, which is part of the firmware for wireless DSL routers based on technology from the Taiwanese manufacturer Sercomm, was an intentional feature to begin with.

Back in December, Eloi Vanderbeken of Synacktiv Digital Security was visiting his family for the Christmas holiday, and for various reasons he had the need to gain administrative access to their Linksys WAG200G DSL gateway over Wi-Fi. He discovered that the device was listening on an undocumented Internet Protocol port number, and after analyzing the code in the firmware, he found that the port could be used to send administrative commands to the router without a password.

After Vanderbeken published his results, others confirmed that the same backdoor existed on other systems based on the same Sercomm modem, including home routers from Netgear, Cisco (both under the Cisco and Linksys brands), and Diamond. In January, Netgear and other vendors published a new version of the firmware that was supposed to close the back door.

However, that new firmware apparently only hid the backdoor rather than closing it. In a PowerPoint narrative posted on April 18, Vanderbeken disclosed that the “fixed” code concealed the same communications port he had originally found (port 32764) until a remote user employed a secret “knock”—sending a specially crafted network packet that reactivates the backdoor interface.

The packet structure used to open the backdoor, Vanderbeken said, is the same used by “an old Sercomm update tool”—a packet also used in code by Wilmer van der Gaast to "rootkit" another Netgear router. The packet’s payload, in the version of the backdoor discovered by Vanderbeken in the firmware posted by Netgear, is an MD5 hash of the router’s model number (DGN1000).

The nature of the change, which leverages the same code as was used in the old firmware to provide administrative access over the concealed port, suggests that the backdoor is an intentional feature of the firmware and not just a mistake made in coding. “It’s DELIBERATE,” Vanderbeken asserted in his presentation.

There are some limitations to the use of the backdoor. Because of the format of the packets—raw Ethernet packets, not Internet Protocol packets—they would need to be sent from within the local wireless LAN, or from the Internet service provider’s equipment. But they could be sent out from an ISP as a broadcast, essentially re-opening the backdoor on any customer’s router that had been patched.

Once the backdoor is switched back on, it listens for TCP/IP traffic just as the original firmware did, giving “root shell” access—allowing anyone to send commands to the router, including getting a “dump” of its entire configuration. It also allows a remote user to access features of the hardware—such as blinking the router’s lights.

Just how widely the old, new backdoor has been spread is unknown. Vanderbeken said that because each version of the firmware is customized to the manufacturer and model number, the checksum fingerprints for each will be different. While he’s provided a proof-of-concept attack for the DGN1000, the only way to find the vulnerability would be to extract the filesystem of the firmware and search for the code that listens for the packet, called “ft_tool”, or the command to reactivate the backdoor (scfgmgr –f ).

We attempted to reach Sercomm and Netgear for comment on the backdoor. Sercomm did not respond, and a Netgear spokesperson could not yet comment on the vulnerability. Ars will update this story as more details are made available by the device manufacturers.