Backdoors in custom software applications

[Sebastian Schinzel <ssc () seecurity org>]

Hi all,

I am looking for ideas how intentional backdoors in real software applications may look like.

Wikipedia already provides a good list of backdoors that were found in software applications: 
http://en.wikipedia.org/wiki/Backdoor_(computing)

Has anyone encountered backdoors during code audits, penetration tests, data breaches? 
Could you share some details of how the backdoor looked like? I am really interested in
a technical and abstract description of the backdoor (e.g. informal descriptions or pseudo-code).
Anonymized and off-list replies are also very welcome.

Thanks,
Sebastian 
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