I maintain an alternative firmware for the Speedport series and can confirm the flaw in the following firmwares:
Speedport W500V Firmware T-Com 1.30
Speedport 500V Firmware T-Com 1.31

Speedport W500V Firmware mod500
Speedport 500V Firmware mod500
Targa WR500 VoIP Firmware mod500

I'm 99% certain that the original firmware for the Targa is also concerned.

CORRECTION
White the description of the security flaw is correct, please notice:
Installing a new firmware does NOT change the system password!