Dragonfly is an open source P2P-based file distribution and image acceleration system. Prior to 2.1.0, The Manager disables TLS certificate verification in HTTP clients. The clients are not configurable, so users have no way to re-enable the verification. A Manager processes dozens of preheat jobs. An adversary performs a network-level Man-in-the-Middle attack, providing invalid data to the Manager. The Manager preheats with the wrong data, which later causes a denial of service and file integrity problems. This vulnerability is fixed in 2.1.0.