The trust problem limits what rebels can do online. The scarcity of non-verbal cues in digital communication facilitates deceptive mimicry, which undermines the interpersonal trust required for sensitive transactions. Open-source data from jihadi discussion forums show that distrust there is very high and direct recruitment rare. General trust also declined during the observation period (2006-2011). As of 2012, forums are still in use, but primarily for low-stake activities such as propaganda-sharing and ideological debate. Confidence in the authenticity of propaganda remains relatively high, due to vetting institutions and hard-to-fake video formats. A modicum of interpersonal trust also remains, thanks to reputation systems and a few relatively reliable signs of trustworthiness involving time expenditure. The trust problem is an Achilles’ heel for terrorists online – but probably also for pro-democracy activists in authoritarian settings.