Where injected instructions hide: the carriers of indirect prompt injection an agent reads

What it is

OWASP lists prompt injection as LLM01 in its 2025 Top 10 for LLM applications and separates direct injection (the user types the instruction) from indirect injection, where the model receives the instruction inside external content such as a website or a file. For an agent, indirect injection is the larger surface: every tool that returns text written by someone other than the user is a carrier.

Common carriers:

• Web pages: text hidden with CSS, white-on-white text, aria-label and alt attributes, HTML comments, <noscript> blocks — invisible in a browser, present in the extracted text.
• Documents: PDF text layers, speaker notes, document properties, tracked changes, spreadsheet cells outside the visible range.
• Repositories: README files, code comments, issue and pull-request bodies, commit messages, test fixtures, configuration files an agent opens while coding.
• Messages: e-mail bodies and headers, calendar invitations, chat messages, support tickets.
• Tool output: API error strings, search snippets, file names and directory listings, results from third-party MCP servers.
• Images and audio for multimodal models: text rendered into an image, instructions in a transcript.

Why it matters

The model cannot reliably tell an instruction from a quotation of one. Whatever the carrier, the text lands in the same context window as the user's request. An agent that can act — send, write, delete, fetch — turns a successful injection into an action taken with the user's authority.

How to apply

• Keep an explicit list of which tools return third-party text and treat all of it as quoted data.
• Extract text the way a browser renders it when you can, and note hidden-text removal as a heuristic, not a defence.
• Record the source of every passage in the run log so an incident can be traced back to its carrier.
• After reading third-party content, require a fresh confirmation from the user before any action the user did not already ask for.
• In red-team exercises, plant a harmless marker instruction in each carrier type your agent reads and check whether it is followed.

Pitfalls

• Assuming a trusted site cannot carry an injection: user-generated sections (comments, reviews, wiki edits) inherit none of the site's trust.
• Filtering only the visible text of a page while the model receives the raw extraction.
• Treating your own repository as trusted when it contains issue text, vendored code or fixtures written by others.