Physical Unclonable Functions (PUFs) provide means to gen-erate chip individual keys, especially for low-cost applications such as theInternet of Things (IoT). They are intrinsically robust against reverseengineering, and more cost-effective than non-volatile memory (NVM).For several PUF primitives, countermeasures have been proposed to mit-igate side-channel weaknesses. However, most mitigation techniques re-quire substantial design effort and/or complexity overhead, which can-not be tolerated in low-cost IoT scenarios. In this paper, we first ana-lyze side-channel vulnerabilities of the Loop PUF, an area efficient PUFimplementation with a configurable delay path based on a single ringoscillator (RO). We provide side-channel analysis (SCA) results frompower and electromagnetic measurements. We confirm that oscillationfrequencies are easily observable and distinguishable, breaking the se-curity of unprotected Loop PUF implementations. Second, we presenta low-cost countermeasure based on temporal masking to thwart SCAthat requires only one bit of randomness per PUF response bit. The ran-domness is extracted from the PUF itself creating aself-secured PUF.The concept is highly effective regarding security, low complexity, andlow design constraints making it ideal for applications like IoT. Finally,we discuss trade-offs of side-channel resistance, reliability, and latency aswell as the transfer of the countermeasure to other RO-based PUFs