We study the infrared photodissociation dynamics of a single hydrogen fluoride (HF) molecule in a single-mode cavity vacuum, and compare it with the case of strong cw laser driving. We show that in the absence of additional IR sources, a single cavity mode can efficiently dissociate a polar diatomic molecule prepared in the ground vibrational level. We predict dissociation probabilities of up to 20%, for a vacuum field that is resonant with the fundamental vibration frequency at the onset of the ultrastrong coupling regime. In contrast, similar dissociation rates can only be achieved in free space using resonant laser fields with intensities not smaller than 1014 W/cm2. Our work highlights the fundamental differences that can be expected for reactive dynamical processes inside infrared cavities.