Root access backdoor discovered in Chinese Tenda Wireless Routers

Craig Heffner has discovered a new backdoor in the Wireless Routers produced by China based networking device and equipment manufacturer Tenda Technology.

Just a week ago I wrote on a serious backdoor found by researcher Craig Heffner within the D-Link routers, today I’m here describing a new flaw discovered in the Wireless Routers produced by China based networking device and equipment manufacturer Tenda Technology (www.tenda.cn).

The intriguing analysis, titled “From China, with Love” is published on the blog “/dev/ttyS0” specialized on the embedded device hacking. D-Link is not the unique vendor that puts backdoors in its products, Heffner has unpacked the software framework update for the Tenda Technology device, and locating the httpd binary has found that the manufacturer is using GoAhead server.In reality he notes a modification in the server itself.

The researcher discovered that despite Tenda Technology routers are protected with security protocols WPS and WPA, an attacker just sending a specifically crafted UDP packet is able to block them. Craig Heffner has found a flaw in the httpd component, the MfgThread() function instantiates a service that listens for incoming messages containing commands to execute, this means that an attacker within a local network is able to send commands to the router that it executes with root privileges.

“The thread then goes into a recvfrom loop, reading up to 128 bytes from the socket. It expects each received UDP packet to be at least 14 bytes in length:””We can see that the thread is expecting a packet with the following structure:”

struct command_packet_t

{

char magic[10]; // 9 byte magic string (“w302r_mfg”), plus a NULL terminating byte

char command_byte;

char command_arg[117];

};

The Tenda Technology backdoor only listens on the LAN and it is not exploitable from the WAN, however hackers can exploit it over the wireless network if the victim has WPS enabled by default with no brute force rate limiting.

The service expects a packet starts with the string “w302r_mfg”, the code then compares the specified command byte against three ASCII characters (’1′, ‘x’, and ‘e’) associated with the following commands:

• ‘e’ – Responds with a pre-defined string, basically a ping test
• ’1′ – Intended to allow you to run iwpriv commands
• ‘x’ – Allows you to run any command, as root

Resuming the attacker just needs to run the command on UDP port 7329 to gain the root access, following the Telnet command used

echo -ne “w302r_mfgx00x/bin/busybox telnetd” | nc -q 5 -u 7329 192.168.0.1

Within the Telnet command is clearly visible the string “w302r_mfg” that is interpreted by the backdoor service. Some of the vulnerable routers are W302R and W330R as well as re-branded models including the Medialink MWN-WAPR150N.

An Nmap NSE script is available to test for the backdoored routers.