den Ast absägen, auf dem man sitzt

Etymology
lit. 'to saw off the branch on which one is sitting'.

Phrase

 * 1)  to undermine one's own position; to act in a way that will eventually make one lose something that one is dependent on

Usage notes

 * This idiom is often used to convey the meaning of . However, the German saying is broader as it is not specific to causing harm to a benefactor. There is also the literal equivalent, though this is somewhat less common.