r/PromptEngineering: White-Text PDF Injection Bypasses Input-Only Prompt Filters — Developer Documents Near-Miss in Production RAG System

A developer reported that a user uploaded a contract PDF containing hidden white-text prompt injection in the footer; the model flagged and warned about it, but the organization's security stack watched only the user input field, not the document content channel, and fired no alerts. The post surfaces a structural gap common in RAG deployments: injection delivered via uploaded documents arrives through a different execution path than direct user messages and evades input-focused guardrails entirely. The thread is drawing active discussion of document-level scanning, chunked content validation, and the need to treat every retrieval artifact as an untrusted input surface.