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The search for Galactic PeVatrons - astrophysical accelerators of cosmic raysto PeV energies - has entered a new phase in recent years with the discovery ofthe first Ultra-High-Energy (UHE, $E>100$ TeV) gamma-ray sources by the HAWCand LHAASO experiments. Establishing whether the emission is leptonic orhadronic in nature, however, requires multiwavelength data and modellingstudies. Among the currently known UHE sources, LHAASO J2108+5157 is anenigmatic source without clear association to a plausible accelerator, yetspatially coincident with molecular clouds. We investigate the scenario of amolecular cloud illuminated by cosmic rays accelerated in a nearby supernovaremnant (SNR) as an explanation for LHAASO J2108+5157. We aim to constrain therequired properties of the SNR as well as which of the clouds identified in thevicinity is the most likely association. We use a model for cosmic rayacceleration in SNRs, their transport through the interstellar medium andsubsequent interaction with molecular material, to predict the correspondinggamma-ray emission. The parameter space of SNR properties is explored to findthe most plausible parameter combination that can account for the gamma-rayspectrum of LHAASO J2108+5157. In the case that a SNR is illuminating thecloud, we find that it must be young ($<10$ kyr) and located within $40-60$ pcof the cloud. A SN scenario with a low Sedov time is preferred, with a maximumproton energy of 3 PeV assumed. No SNRs matching these properties are currentlyknown, although an as yet undetected SNR remains feasible. The galactic CR seais insufficient to solely account for the observed flux, such that a PeVatronaccelerator must be present in the vicinity.

LHAASO J2108+5157 as a Molecular Cloud Illuminated by a Supernova Remnant