During a technique for authenticating a user using a portable electronic device (such as a smartphone) while preventing successful 2D media attacks and virtual-camera attacks, the portable electronic device provides an instruction to a user to move the portable electronic device relative to the user's face. Then, using an imaging sensor in the portable electronic device, the portable electronic device acquires, a series of images while the portable electronic device is at different positions or orientations relative to the user's face, where the series of images include at least two images relative to a symmetry axis of the user's face (such as the midline). Moreover, the portable electronic device determines a change in a sign of a nose angle in at least the two images. Next, the portable electronic device authenticates the user based on at least one of the series of images using a facial-recognition technique.