A backdoor hidden in a third-party model survives your own fine-tuning, and reinforcement learning can make it stronger, from 20 to 74 percent attack success
AspireAI community · arXiv (cs.CR) - EMNLP 2026 Findings
Developers build agents by adapting other people's models, which creates a supply-chain path where an attacker ships a model carrying hidden behavior that fires on a specific input pattern. This work asks whether such backdoors survive a developer's own supervised fine-tuning and task-level reinforcement learning, and finds that benign SFT substantially reduces attack success while subsequent RL often preserves what is left and sometimes increases it, with backdoor strength and gradient compatibility with benign training identified as the factors that favor survival. PersistBD deliberately hardens an already-backdoored model before release, raising attack success from 20 percent to 74 percent after SFT and from 20 percent to 76 percent after SFT-RL on Qwen2.5-Coder-7B while keeping benign task performance comparable, and the code is released. The limitation: results come from software-engineering agents and one backdoored model family, and the paper demonstrates attacker capability rather than a working defense.
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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.