Few years ago, DefenseCode discovered a remotely exploitable preauth Format String vulnerability in Broadcom UPnP implementation used in popular routers. Vendors were notified and advisory was published. Broadcom fixed the vulnerability in their UPnP implementation and some router vendors did it also.
Vulnerability was initially discovered on Cisco Linksys (now Belkin) WRT54GL routers, but as stated before, vulnerable UPnP implementation was used by many vendors. Back in the days, Cisco fixed the vulnerability, but we are not sure about all other router vendors and models because there are too many of them.
When the vulnerability was initially discovered, Rapid7 also discovered various overflows in other popular UPnP implementations, and published a paper about it. When they did the research, there were approximately 15 Million devices with vulnerable Broadcom UPnP implementation discovered on the Internet, probably many more in the Intranets. We have written a paper about detailed exploitation steps for now fixed Broadcom UPnP Format String vulnerability, but never published it due to the severity of the bug. Now, few years later, we feel comfortable to release a full research paper with vulnerability details and exploitation steps for discovered Format String vulnerability.
Big issue with routers is that they are rarely updated by users with new firmware and there could be still a lot of vulnerable routers on the Internet and in the Intranets.
Full research paper on discovery and exploitation of the Broadcom UPnP Format String vulnerability can be read here.