A digital microfluidic biochip (DMFB) is an emerging technology that enables miniaturized analysis systems for point-of-care clinical diagnostics, DNA sequencing, and environmental monitoring. A DMFB reduces the rate of sample and reagent consumption, and automates the analysis of assays. In this paper, we highlight the security vulnerabilities of DMFBs by identifying two potential attacks on a DMFB that performs enzymatic glucose assay on serum. In the first attack, the attacker adjusts the concentration of the glucose sample and thereby modifies the final result. In the second attack, the calibration curve of the assay operation is maliciously modified in order to make it deviate from the nominal/golden calibration curve. We demonstrate these attacks using a digital microluidics synthesis simulator. The results show that the attacks are stealthy as they do not result in any noticeable change in the DMFB synthesis.