This post illustrates the path from diffing the firmware versions and finding the interesting files via reverse engineering the patch through to finally writing an exploit (a Metasploit module) for the MIPS-based DSL-Router series by AVM.
more here.....http://breaking.systems/blog/2014/04/avm-fritzbox-root-rce-from-patch-to-metasploit-module-i
more here.....http://breaking.systems/blog/2014/04/avm-fritzbox-root-rce-from-patch-to-metasploit-module-i