  Backdoor attacks allow an attacker to embed a specific vulnerability in a
machine learning algorithm, activated when an attacker-chosen pattern is
presented, causing a specific misprediction. The need to identify backdoors in
biometric scenarios has led us to propose a novel technique with different
trade-offs. In this paper we propose to use model pairs on open-set
classification tasks for detecting backdoors. Using a simple linear operation
to project embeddings from a probe model's embedding space to a reference
model's embedding space, we can compare both embeddings and compute a
similarity score. We show that this score, can be an indicator for the presence
of a backdoor despite models being of different architectures, having been
trained independently and on different datasets. Additionally, we show that
backdoors can be detected even when both models are backdoored. The source code
is made available for reproducibility purposes.
