Shoulder surfing attacks are an unfortunate consequence of entering passwords or PINs into computers, smartphones, PoS terminals, and ATMs. Such attacks generally involve observing the victim’s input device. This paper studies leakage of user secrets (passwords and PINs) based on observations of output devices (screens or projectors) that provide “helpful” feedback to users in the form of masking characters, each corresponding to a keystroke. To this end, we developed a new attack called Secret Information Leakage from Keystroke Timing Videos (SILK-TV). Our attack extracts inter-keystroke timing information from videos of password masking characters displayed when users type their password on a computer, or their PIN at an ATM or PoS. We conducted several studies in various envisaged attack scenarios. Results indicate that, while in some cases leakage is minor, it is quite substantial in others. By leveraging inter-keystroke timings, SILK-TV recovers 8-character alphanumeric passwords in as little as 19 attempts. However, When guessing PINs, SILK-TV yields no substantial speedup compared to brute force. Our results strongly indicate that secure password masking GUIs must consider the information leakage identified in this paper.