Secret Scanner Hook

Hook Type

PreToolUse with Edit|Write matcher -- Intercepts file writes to scan for secrets.

Description

This hook scans the content of every file Claude is about to write or edit, checking for patterns that match common secret formats (API keys, tokens, passwords, private keys). If a secret is detected, the write is blocked before it reaches disk. This prevents accidental secret exposure in source code, configuration files, and documentation.

Patterns/Rules

Detected secret patterns:

• AWS access keys (AKIA[0-9A-Z]{16})
• AWS secret keys (40-character base64 strings)
• GitHub tokens (ghp_, gho_, ghu_, ghs_, ghr_)
• Generic API keys (api[_-]?key, apikey)
• Private keys (-----BEGIN (RSA|EC|OPENSSH) PRIVATE KEY-----)
• JWT tokens (eyJ[A-Za-z0-9-_]+\.eyJ[A-Za-z0-9-_]+)
• Database connection strings with passwords
• Generic high-entropy strings that look like secrets

Configuration

Hook Script (scripts/scan-secrets.sh)

Copy
#!/bin/bash
# scan-secrets.sh -- Scan for leaked secrets in file content

INPUT=$(cat)

# Get the content being written (new_string for Edit, content for Write)
CONTENT=$(echo "$INPUT" | jq -r '.tool_input.new_string // .tool_input.content // empty')
FILE_PATH=$(echo "$INPUT" | jq -r '.tool_input.file_path // empty')

# Skip binary files and lock files
if [[ "$FILE_PATH" =~ \.(png|jpg|gif|ico|woff|ttf|lock)$ ]]; then
  exit 0
fi

# Secret patterns (regex)
SECRET_PATTERNS=(
  'AKIA[0-9A-Z]{16}'                                    # AWS Access Key
  '[0-9a-zA-Z/+]{40}'                                   # AWS Secret Key (loose)
  'ghp_[0-9a-zA-Z]{36}'                                 # GitHub PAT
  'gho_[0-9a-zA-Z]{36}'                                 # GitHub OAuth
  'sk-[0-9a-zA-Z]{48}'                                  # OpenAI API Key
  'sk-ant-[0-9a-zA-Z-]{90,}'                            # Anthropic API Key
  '-----BEGIN (RSA |EC |OPENSSH )?PRIVATE KEY-----'     # Private Keys
  'eyJ[A-Za-z0-9-_]+\.eyJ[A-Za-z0-9-_]+'               # JWT Tokens
  'xox[bpars]-[0-9a-zA-Z-]+'                            # Slack Tokens
  'SG\.[0-9a-zA-Z-_]{22}\.[0-9a-zA-Z-_]{43}'           # SendGrid API Key
  'sk_live_[0-9a-zA-Z]{24,}'                            # Stripe Secret Key
)

for pattern in "${SECRET_PATTERNS[@]}"; do
  if echo "$CONTENT" | grep -qE "$pattern"; then
    MATCH=$(echo "$CONTENT" | grep -oE "$pattern" | head -1)
    # Redact most of the match for safety
    REDACTED="${MATCH:0:8}...REDACTED"
    echo "BLOCKED: Potential secret detected in $FILE_PATH" >&2
    echo "Pattern: $pattern" >&2
    echo "Match: $REDACTED" >&2
    echo "Use environment variables instead of hardcoding secrets." >&2
    exit 2
  fi
done

exit 0

Settings Configuration

Copy
{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "bash ./scripts/scan-secrets.sh"
          }
        ]
      }
    ]
  }
}

Action

Before any Edit or Write:

1. Extracts the content being written from the tool input
2. Runs regex patterns against the content
3. If a secret pattern matches: blocks the write, shows a redacted match, and suggests using environment variables
4. If no secrets found: allows the write to proceed