As reported at www.milw0rm.com there is a vulnerability in the http-server for the DD-WRT management GUI that can be used for execution of an exploit to gain control over the router.
They guys over at dd-wrt have fixed the issue and generated new builds of the latest DD-WRT version. You can temporarily download the these files from here until we did update the router database.
[UPDATE] The guys over at dd-wrt have integrated most of the fixed build files into the router database. You can check there if files for build 12533 are available for your router. If not (yet) please check the location mentioned above to obtain the files.