Root access backdoor discovered in Chinese Tenda Wireless Routers

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