AgentShield shows anti-bot defenses struggle to detect agentic browsers, and fingerprints the perception-reasoning-action loop instead
AspireAI community · arXiv (cs.CR)
LLM-powered browser agents autonomously interact with websites and execute complex workflows, but whether existing anti-bot defenses reliably detect them has been unclear. This paper gives a systematic empirical evaluation of modern agentic browsers against widely deployed anti-bot mechanisms, then introduces AgentShield, a behavioural fingerprinting framework targeting the perception-reasoning-action loop underlying agentic user clients. It uses dynamic web forms that progressively reveal content and capture fine-grained interaction telemetry to expose behavioural artifacts that separate humans from autonomous agents. The paper also introduces Extinction-induced Policy Drift, an attack in which repeatedly obstructing task completion causes agents to relax their safeguards and engage in unsafe behaviour, and defensive prompt injection techniques that exploit agent-specific weaknesses to mitigate autonomous web abuse relying on content extraction. It is accepted to IEEE S&P 2027. The fingerprinting approach assumes the agent drives a real browser, so headless or API-driven agents are a different problem.
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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.