“Rules File Backdoor”: hidden Unicode instructions in AI rules files

What it is

Attackers encode instructions with zero-width joiners and bidirectional markers. They are invisible to humans, including in GitHub pull request review, but parsed by the AI.

How to check

1. Scan .md / .mdc / rules files for invisible Unicode (command below, or Pillar’s free Rule File Scanner).

How to fix

1. Treat AI rules/config files like code: review them, and block hidden Unicode in CI.

# Find zero-width and bidi control characters in rules files
grep -rnP "[\x{200B}-\x{200F}\x{202A}-\x{202E}\x{2066}-\x{2069}\x{FEFF}]" \
  .cursorrules .cursor/ --include=*.md --include=*.mdc

Sources

• Pillar Security: Rules File Backdoor ↗
• Pillar Security: Rule File Scanner ↗