"On a whim I downloaded firmware v1.13 for the DIR-100 revA," Heffner said, referring to a widely used version of D-Link's router firmware. After using a firmware analysis tool known as Binwalk, "soon I had the firmware's Web server (/bin/webs) loaded into IDA," he added, referring to an interactive disassembler -- IDA -- which is used to reverse-engineer code.
Perusing the code, Heffner found a variable called "alpha_auth_check" -- the "alpha" apparently refers to D-Link spinoff Alpha Networks, while "auth" means "authentication" -- that returns a value of "1" if the user has been authenticated. After a bit of exploratory work, Heffner found that if he changed a browser's HTTP user-agent string to "xmlset_roodkcableoj28840ybtide" then "you can access the Web interface without any authentication and view/change the device settings."
[ Don't be a victim. See 15 Signs Pointing To A Data Breach. ]
Heffner doesn't appear to be the first person to have recovered this string from inside the D-Link firmware. "A quick Google for the 'xmlset_roodkcableoj28840ybtide' string turns up only a single Russian forum post from a few years ago, which notes that this is an 'interesting line' inside the /bin/webs binary," he said. "I'd have to agree."
A spokesman for D-Link didn't immediately respond to an emailed query about whether it had verified the vulnerability detailed by Heffner, and if so, how the company planned to notify and issue updates to consumers who own a vulnerable device.
Based on a search made with the embedded Web server search engine Shodan, the vulnerable firmware appears to be used by at least seven routers sold by D-Link (DIR-100, DI-524, DI-524UP, DI-604S, DI-604UP, DI-604+, TM-G5240) as well as two Planex routers (BRL-04UR, BRL-04CW), according to Heffner. "Several people have reported ... that some versions of the DIR-615 are also affected, including those distributed by Virgin Mobile," he said in a later update to his blog post. "I have not yet verified this, but it seems quite reasonable."
What's the count of the total number of networked D-Link devices that are affected by the vulnerability? That remains to be seen, although Robert David Graham, CEO of Errata Security, said Monday that he was "scanning the entire Internet for the D-Link 'xmlset_roodkcableoj28840ybtide' backdoor" using his Masscan tool, which can be used to scan the Internet for certain variables, which in this case would be fingerprints of the vulnerable firmware. Graham said he hoped to be able to report on the results of his scan by Monday afternoon.
What's the risk from an attacker gaining full access to a router? For starters, the attacker could eavesdrop on the network by loading the router with custom firmware designed to send a copy of all data flowing to or from the device to an attacker-controlled server. Likewise, the router could be configured to automatically launch distributed denial-of-service attacks against designated sites.
As Heffner's facility with reverse-engineering device firmware suggests, this isn't his first foray into hacking networking equipment. At this past summer's Black Hat conference in Las Vegas, for example, the researcher highlighted how consumer-grade as well as enterprise-class networked surveillance cameras from the likes of D-Link, Trendnet, Cisco, IQInvision, Alinking and 3SVision were vulnerable to zero-day flaws that would allow an attacker to freeze or modify their video streams. That research was the latest in a long line of vulnerability reports involving Internet protocol (IP) cameras.