Detecting Whether an LLM Has Been Backdoored

As language models are deployed in high-stakes domains, adversaries may poison training data to implant backdoors: hidden triggers that covertly manipulate model behavior at inference time. In this work, we formalize the affordances which a defender has and, to evaluate whether defenders can identify backdoors under these affordances, construct a benchmark for backdoor-detection algorithms. This benchmark spans attack mechanisms and objectives, including an adversarial backdoor explicitly designed to evade detection. We use this benchmark to evaluate a suite of backdoor-elicitation hypotheses. We find that while some techniques can flag poisoned models, none reliably surface backdoors. Indeed, hunting for backdoors in poisoned models is likely to surface jailbreaks instead. Finally, we show that backdoor-related activation vectors are consistently different from the vectors which account for undesirable behaviors without triggers. We release our benchmark to motivate the interpretability community to develop stronger algorithms for eliciting backdoors. We make all code available at: https://anonymous.4open.science/r/SPARBackdoor-464B/