The flaw was discovered and its exploitability proved with a PoC by Tactical Network Solutions’ security researcher Craig Heffner. D-Link confirmed the existence of the problem a few weeks later.
“Various D-Link routers allow administrative web actions if the HTTP request contains a specific User-Agent string. This backdoor allows an attacker to bypass password authentication and access the router's administrative web interface,” D-Link explained in a security advisory.
Attacks exploiting the vulnerability were also been seen in the wild. At the time, the company had advised users not to enable or to disable the Remote Management feature as it would allow attackers to perform the exploit, and advised them not to fall for unsolicited emails that warn them about the vulnerability and promt them to action.
Vulnerable devices include D-Link DIR-100, DIR-120, DI-624S, DI-524UP, DI-604S, DI-604UP, DI-604+, and TM-G5240 routers; Planex BRL-04R, BRL-04UR, and BRL-04CW routers; and Alpha Networks routers.
The patched firmware has been offered for some of them, but not all.
Users are advised to closely follow the firmware install guide provided within the ZIP firmware package. As these devices are pretty old, exchanging them for newer ones might also not be a bad idea.