Your Mailbox Is Mine: Prompt Injection Attacks Against Real-World LLM Email Agents

Large Language Model (LLM) email agents have emerged as pivotal autonomous assistants, serving as a critical root of trust for digital identity by managing sensitive communications and authentication workflows. Despite their importance, the prompt injection (PI) resilience of the real-world LLM email agent ecosystem remains poorly understood. Existing assessments largely rely on simulated environments or fragmented production case studies, while existing PI attacks mainly follow an instruction-takeover strategy that directly competes with the system prompt and the user's task. Our evaluation shows that such attacks are insufficient in email-agent settings: existing template-based prompt injection attack baselines achieve Attack Success Rates (ASRs) of only 4.58%-8.13%. In this paper, we propose Email-Specific Prompt Injection (ESPI), a new attack paradigm that manipulates how email agents interpret mailbox operational context. By combining Email Protocol State Masquerade and Camouflage Logic Chain, ESPI reframes attacker-desired mailbox operations as necessary remediation steps under forged email-operational states. We further develop ESPInspector, an automated black-box attack pipeline for analyzing and evaluating real-world email agents. Across 480 controlled attack trials, ESPI achieves 73.54% ASR, substantially outperforming all baselines. Further, ESPI successfully hijacks all 63 evaluated applications across 870 black-box instances, requiring only 1.84-1.89 attempts on average for the first successful hijack. Our responsible disclosure receives formal risk acknowledgment from 22 vendors and the assignment of 16 CVE IDs. Our research provides the first holistic map of security gaps in the real-world email agent ecosystems and highlights the urgent necessity for robust, state-verified security enhancement solutions.