DyePack: Provably Flagging Test Set Contamination in LLMs Using
  Backdoors
Authors: name: Yize Cheng; name: Wenxiao Wang; name: Mazda Moayeri; name: Soheil Feizi
Published: 2025-05-29 02:22:14

Abstract: Open benchmarks are essential for evaluating and advancing large language
models, offering reproducibility and transparency. However, their accessibility
makes them likely targets of test set contamination. In this work, we introduce
DyePack, a framework that leverages backdoor attacks to identify models that
used benchmark test sets during training, without requiring access to the loss,
logits, or any internal details of the model. Like how banks mix dye packs with
their money to mark robbers, DyePack mixes backdoor samples with the test data
to flag models that trained on it. We propose a principled design incorporating
multiple backdoors with stochastic targets, enabling exact false positive rate
(FPR) computation when flagging every model. This provably prevents false
accusations while providing strong evidence for every detected case of
contamination. We evaluate DyePack on five models across three datasets,
covering both multiple-choice and open-ended generation tasks. For
multiple-choice questions, it successfully detects all contaminated models with
guaranteed FPRs as low as 0.000073% on MMLU-Pro and 0.000017% on Big-Bench-Hard
using eight backdoors. For open-ended generation tasks, it generalizes well and
identifies all contaminated models on Alpaca with a guaranteed false positive
rate of just 0.127% using six backdoors.