Fix instruction file contains text a human reader cannot see in GitHub Copilot

An agent instruction file holds zero-width characters, a bidirectional override, or Unicode tag characters. The model reads the bytes; the reviewer reads the rendering. When those disagree, review is not review. The finding renders the run as escaped codepoints and never reproduces it.

high likely GitHub Copilot CWE-838 / ASI ASI01

The vulnerable pattern in GitHub Copilot

This finding comes from the GitHub Copilot fixture in the owlwarden test suite. The model reads these 1 character(s); a reviewer reading this file sees nothing there. An instruction hidden this way is an instruction nobody approved.

The corrected configuration

Strip them from `.github/copilot-instructions.md`. That file is prepended to every request in the repository, so anything hidden in it is hidden in every completion.

// .github/copilot-instructions.md - visible characters only

If you are not using GitHub Copilot

Delete the invisible characters. Then find out how they got in: a paste from a web page is the innocent explanation, and a commit that added them alone is not.

Check your own repository

npx owlwarden scan
npx owlwarden explain agent-instructions-hidden-text

Runs on your machine. No account, no telemetry, no network unless you ask. In CI, SARIF uploads to code scanning and the exit code is the gate.

Other GitHub Copilot checks

Rules with a tested GitHub Copilot example.

• agent-instructions-directive medium Instruction file tells the agent to bypass its own controls

agent-instructions-hidden-text for every agent host / All rules / owlwarden