Title: DLP: Data-Driven Label-Poisoning Backdoor Attack. Abstract: Backdoor attacks, which aim to disrupt or paralyze classifiers on specific tasks, are becoming an emerging concern in several learning scenarios, e.g., Machine Learning as a Service (MLaaS). Various backdoor attacks have been introduced in the literature, including perturbation-based methods, which modify a subset of training data; and clean-sample methods, which relabel only a proportion of training samples. Indeed, clean-sample  attacks can be particularly stealthy since they never require modifying the samples at the training and test stages. However, the state-of-the-art clean-sample attack of relabelling training data based on their semantic meanings could be ineffective and inefficient in test performances due to heuristic selections of semantic patterns. In this work, we introduce a new type of clean-sample backdoor attack, named as DLP backdoor attack, allowing attackers to backdoor effectively, as measured by test performances, for an arbitrary backdoor sample size. The critical component of DLP is a data-driven backdoor scoring mechanism embedding in a multi-task formulation, which enables attackers to simultaneously perform well on the normal learning tasks and the backdoor tasks. Systematic empirical evaluations show the superior performance of the proposed DLP to state-of-the-art clean-sample attacks.