Coding agents increasingly operate with web access and high privileges, creating a new attack surface where web fraud can directly target autonomous agents. Existing studies mainly focus on malicious links, prompt injection, and interface deception, while content-level web fraud against agents and the amplification of attacks through agent trust remain underexplored. In this paper, we present EtE (Earn-then-Exploit), a web fraud attack framework against coding agents, in which a malicious website first provides multiple rounds of genuinely verifiable assistance to establish trust and then introduces task-related instructions that cross intended operational boundaries. To systematically evaluate its effectiveness, we construct 56 organizational environments (8 task contexts x 7 attack consequences) and evaluate three representative coding agents. EtE achieves an average scene-level attack success rate of 71.4%, including 85.7% on Codex-GPT-5.5, 67.9% on ZCode-GLM-5.3-Flash, and 60.7% on QwenCode-Qwen3.7-Plus, significantly outperforming all injection-based baselines. More importantly, our experiments reveal that coding agents exhibit trust-like behavioral reliance on previously helpful Web sources, and that such accumulated credibility significantly increases their susceptibility to subsequent malicious instructions. Based on this finding, we propose action-authorization separation (AAS), which prevents web content from directly authorizing privileged actions and reduces the attack success rate to 0% while preserving 100% task completion, outperforming all evaluated defense baselines.

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