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The subject is a challenge response mechanism used as an optional add-on to the face recognition part of the multi modal biometric authentication system "BioID". This mechanism greatly enhances security in regard to replay attacks. The user is required to look into a certain direction, which is randomly chosen by the system. By estimating the head pose, the system verifies the user's response to the direction challenge. The pose estimation is based on detection and subsequent tracking of suitable facial features. Experimental evaluations have shown the effectiveness of the approach against replay attacks.