A vulnerability researcher named Craig Heffner has found out that some of the D-Link ethernet and wireless broadband routers can be logged into via their web interface using a certain text string as a browser user agent.
It is being guessed that this particular backdoor was created and left intentionally by the software developers at D-Link as a feature that may come handy some day.
However the knowledge of this backdoor can lead to unauthorized access and allows anyone to change the routers settings, putting all the internet traffic made through the router in a hacker's hands.
Whatever the legal consequences of this flaw that D-Link may have to face by angry clients, but at least for now it is utmost important for you to see whether you are using one of the routers from D-Link that has been identified as vulnerable to this backdoor access and test it yourself, and remove the router from your network until D-Link comes up with a security patch for its firmware to eliminate the flaw.
You can find a list of the D-Link routers on the researcher Craig Heffner's blog, on this link as well as on some other articles that have been written about this incident on this link.