The security researcher who identified an admin backdoor in a range of routers last year has found that Netgear's patches don't adequately address the security issue.
In December, Eloi Vanderbeken discovered that he could get a remote root shell on his Linksys WAG200G router without authenticating when connecting to the device over TCP port 32764. The same backdoor feature was later confirmed in 24 router models sold under the Netgear, Cisco Systems, Linksys (now owned by Belkin) and Diamond brands and it was believed to have its origins in firmware code developed by Sercomm, a Taiwan-based manufacturer of wireless and broadband equipment that serves as hardware partner for multiple networking vendors.
Vanderbeken, a researcher at Paris-based IT security firm Synacktiv, recently downloaded and analyzed a firmware update released by Netgear for its DGN1000 Wireless router and DSL modem that was supposed to have fixed the issue. He found that the backdoor is no longer directly accessible over port 32764 TCP, but that it can be reactivated by sending raw Ethernet packets to the device with protocol type 0x8888 and the MD5 signature of the router model (DGN1000) as the payload.
"The 0x8888 ethertype and packet structure is used in an old Sercomm update tool," the researcher said in a report of his findings.
Once the device receives such a packet, the backdoor is reactivated and starts listening on port 32764 again, Vanderbeken said Tuesday via email.
The vulnerability is now harder to exploit because an attacker can only send the required raw packets from the same local area network as the targeted device or from one hop away, like its ISP. However, the backdoor is essentially still there, according to the researcher.
This mechanism involving specifically crafted raw Ethernet packets does not exist in the older firmware, he said. "It was added on purpose, to reactivate the backdoor. This is 100 percent intentional."
In addition to giving attackers the ability to execute commands with root access -- the highest privilege on Linux systems -- the backdoor also allows flashing the router LEDs by sending special messages to the device.
"I don't know if it was Netgear, Sercomm or both who decided to keep the backdoor open but it is deliberate," the researcher said. "I don't know why. There is no legitimate reason to do so."
There is a physical button that can be used to reset the router to factory defaults if needed in technical support cases and the setup wizard doesn't require the backdoor for remote configuration either, Vanderbeken said. "It's used internally by some services to update the configuration but it is never used from outside the router. There is no reason to keep it open."
The vulnerability was properly patched in at least one device that had previously been identified as affected -- the Cisco WAP4410N wireless access point. A firmware update for that device causes the backdoor to listen only to the loopback address 127.0.0.1, meaning it can't be accessed remotely, according to the researcher.
The new mechanism for reactivating the backdoor might be present in other previously affected routers that received firmware patches for the vulnerability, but so far the researcher has only tested the update for the Netgear DGN1000.
"Sercomm is the manufacturer of the routers found vulnerable to the port 32764 backdoor," the researcher said. "It's almost certain that they introduced the backdoor and the new service to reactivate it."
Netgear is in the process of issuing firmware updates for products that contained the backdoor: DG834G, DG384B, DGN3500B, DGN2000B, DGND330Bv2, DM111Pv2, DM111PSPv2, JNR3210, DGN3500, DGN1000 v1, DGN1000 v2 and DGN1000B. Updates had already been released for the last four models.
"All but two -- DM111PSPv2 and JNR3210 -- of these products are no longer being manufactured," the company said in an emailed statement.
"We are currently working with our technology partner, Sercomm, to investigate the assertions that the firmware patch did not remove the vulnerability," the company said. "If we find that any issues exist, we will make the necessary firmware changes and update the installed base."
Sercomm did not immediately respond to a request for comment.