ML Security PapersMUZZLE: Adaptive Agentic Red-Teaming of Web Agents Against Indirect Prompt Injection Attacks
Georgios Syros 1, Evan Rose 1, Brian Grinstead 2, Christoph Kerschbaumer 2, William Robertson 1, Cristina Nita-Rotaru 1, Alina Oprea 1
Published on arXiv
2602.09222
Prompt Injection
OWASP LLM Top 10 — LLM01
Excessive Agency
OWASP LLM Top 10 — LLM08
Key Finding
MUZZLE automatically discovers 37 new indirect prompt injection attacks across 4 web applications with 10 adversarial objectives, including cross-application attacks and a novel agent-tailored phishing strategy.
MUZZLE
Novel technique introduced
Large language model (LLM) based web agents are increasingly deployed to automate complex online tasks by directly interacting with web sites and performing actions on users' behalf. While these agents offer powerful capabilities, their design exposes them to indirect prompt injection attacks embedded in untrusted web content, enabling adversaries to hijack agent behavior and violate user intent. Despite growing awareness of this threat, existing evaluations rely on fixed attack templates, manually selected injection surfaces, or narrowly scoped scenarios, limiting their ability to capture realistic, adaptive attacks encountered in practice. We present MUZZLE, an automated agentic framework for evaluating the security of web agents against indirect prompt injection attacks. MUZZLE utilizes the agent's trajectories to automatically identify high-salience injection surfaces, and adaptively generate context-aware malicious instructions that target violations of confidentiality, integrity, and availability. Unlike prior approaches, MUZZLE adapts its attack strategy based on the agent's observed execution trajectory and iteratively refines attacks using feedback from failed executions. We evaluate MUZZLE across diverse web applications, user tasks, and agent configurations, demonstrating its ability to automatically and adaptively assess the security of web agents with minimal human intervention. Our results show that MUZZLE effectively discovers 37 new attacks on 4 web applications with 10 adversarial objectives that violate confidentiality, availability, or privacy properties. MUZZLE also identifies novel attack strategies, including 2 cross-application prompt injection attacks and an agent-tailored phishing scenario.
Key Contributions
- Automated trajectory-based identification of high-salience injection surfaces in web agent executions
- Adaptive, iterative attack generation that refines malicious prompts using feedback from failed injection attempts
- Empirical discovery of 37 novel attacks across 4 web apps, including 2 cross-application prompt injection attacks and an agent-tailored phishing scenario