013: GITHUB_OUTPUT Multi-Line Value Injection

Problem

The fix agent workflow passes user-controlled values (e.g., /fix-agent comment body) to $GITHUB_OUTPUT using:

bash

echo "instruction=$INSTRUCTION" >> "$GITHUB_OUTPUT"

If $INSTRUCTION contains newlines, an attacker can inject arbitrary output variables by crafting a multi-line comment.

Use heredoc-style EOF delimiters for multi-line values:

bash

cat <<'EOF' >> "$GITHUB_OUTPUT"
instruction<<SCRIPT_EOF
$INSTRUCTION
SCRIPT_EOF
EOF

This is the GitHub-recommended pattern for multi-line output values.