diff --git a/src/content/content.js b/src/content/content.js
index 14bded1..7981a2b 100644
--- a/src/content/content.js
+++ b/src/content/content.js
@@ -253,19 +253,30 @@
   }
 
   // ============================================================
-  // Combined substitution: smart patterns first, then explicit
+  // Combined substitution: smart patterns + explicit + secret scan
   // ============================================================
   function substituteAll(text) {
     const allReplacements = [];
 
-    // Smart patterns (broad catches)
+    // 1. Smart patterns (broad catches)
     const smart = smartSubstitute(text, identity);
     allReplacements.push(...smart.replacements);
 
-    // Explicit mappings (specific overrides)
+    // 2. Explicit mappings (specific overrides)
     const explicit = substitute(smart.text, mappings);
     allReplacements.push(...explicit.replacements);
 
+    // 3. Secret scanner (API keys, tokens, SSNs, credit cards, etc.)
+    if (settings.secretScanning !== false) {
+      const secrets = scanAndRedactSecrets(explicit.text);
+      allReplacements.push(...secrets.redactions);
+      return {
+        text: secrets.text,
+        replacements: allReplacements,
+        modified: allReplacements.length > 0,
+      };
+    }
+
     return {
       text: explicit.text,
       replacements: allReplacements,
@@ -273,6 +284,84 @@
     };
   }
 
+  // ============================================================
+  // Secret Scanner (inline for page world)
+  // Detects API keys, tokens, passwords, SSNs, credit cards, etc.
+  // ============================================================
+  const SECRET_PATTERNS = [
+    // OpenAI
+    { name: 'OpenAI Key', re: /\bsk-[A-Za-z0-9]{20,}\b/g, to: '[REDACTED-OPENAI-KEY]' },
+    { name: 'OpenAI Project Key', re: /\bsk-proj-[A-Za-z0-9_-]{20,}\b/g, to: '[REDACTED-OPENAI-KEY]' },
+    // Anthropic
+    { name: 'Anthropic Key', re: /\bsk-ant-[A-Za-z0-9_-]{20,}\b/g, to: '[REDACTED-ANTHROPIC-KEY]' },
+    // Google
+    { name: 'Google API Key', re: /\bAIza[A-Za-z0-9_-]{35}\b/g, to: '[REDACTED-GOOGLE-KEY]' },
+    // AWS
+    { name: 'AWS Access Key', re: /\bAKIA[A-Z0-9]{16}\b/g, to: '[REDACTED-AWS-KEY]' },
+    // GitHub
+    { name: 'GitHub Token', re: /\b(?:ghp|gho|ghu|ghs|ghr)_[A-Za-z0-9]{36,}\b/g, to: '[REDACTED-GITHUB-TOKEN]' },
+    // GitLab
+    { name: 'GitLab Token', re: /\bglpat-[A-Za-z0-9_-]{20,}\b/g, to: '[REDACTED-GITLAB-TOKEN]' },
+    // Slack
+    { name: 'Slack Token', re: /\bxox[bpras]-[A-Za-z0-9-]{10,}\b/g, to: '[REDACTED-SLACK-TOKEN]' },
+    // Stripe
+    { name: 'Stripe Key', re: /\b[sr]k_(?:test|live)_[A-Za-z0-9]{20,}\b/g, to: '[REDACTED-STRIPE-KEY]' },
+    // SendGrid
+    { name: 'SendGrid Key', re: /\bSG\.[A-Za-z0-9_-]{22}\.[A-Za-z0-9_-]{43}\b/g, to: '[REDACTED-SENDGRID-KEY]' },
+    // Bearer tokens
+    { name: 'Bearer Token', re: /\bBearer\s+[A-Za-z0-9_\-./+=]{20,}\b/g, to: 'Bearer [REDACTED]' },
+    // Private keys
+    { name: 'Private Key', re: /-----BEGIN (?:RSA |EC |DSA |OPENSSH )?PRIVATE KEY-----[\s\S]*?-----END (?:RSA |EC |DSA |OPENSSH )?PRIVATE KEY-----/g, to: '[REDACTED-PRIVATE-KEY]' },
+    // Generic key=value assignments
+    { name: 'API Key Assignment', re: /\b(?:api[_-]?key|apikey|api[_-]?secret|api[_-]?token)\s*[:=]\s*['"]?([A-Za-z0-9_\-./+=]{16,})['"]?/gi,
+      fn: (m) => m.replace(/[:=]\s*['"]?[A-Za-z0-9_\-./+=]{16,}['"]?/, '=[REDACTED]') },
+    { name: 'Password/Secret Assignment', re: /\b(?:password|passwd|pwd|secret|token|auth[_-]?token|access[_-]?token)\s*[:=]\s*['"]?([^\s'"]{8,})['"]?/gi,
+      fn: (m) => m.replace(/[:=]\s*['"]?[^\s'"]{8,}['"]?/, '=[REDACTED]') },
+    // Connection strings with credentials
+    { name: 'Connection String', re: /\b(?:mongodb|postgres|mysql|redis|amqp):\/\/[^\s"']+/gi,
+      fn: (m) => m.replace(/:\/\/([^:]+):([^@]+)@/, '://REDACTED:REDACTED@') },
+    // SSN
+    { name: 'SSN', re: /\b\d{3}-\d{2}-\d{4}\b/g, to: '[REDACTED-SSN]' },
+    // Credit cards (Visa, MC, Amex, Discover)
+    { name: 'Credit Card', re: /\b(?:4\d{3}|5[1-5]\d{2}|3[47]\d{2}|6(?:011|5\d{2}))[- ]?\d{4}[- ]?\d{4}[- ]?\d{4}\b/g, to: '[REDACTED-CARD]' },
+  ];
+
+  function scanAndRedactSecrets(text) {
+    const redactions = [];
+    let result = text;
+
+    for (const pat of SECRET_PATTERNS) {
+      pat.re.lastIndex = 0;
+      const matches = [];
+      let m;
+
+      while ((m = pat.re.exec(result)) !== null) {
+        matches.push({ index: m.index, value: m[0] });
+      }
+
+      if (matches.length === 0) continue;
+
+      // Replace from end to preserve indices
+      for (let i = matches.length - 1; i >= 0; i--) {
+        const match = matches[i];
+        const replacement = pat.fn ? pat.fn(match.value) : pat.to;
+        redactions.push({
+          original: match.value.slice(0, 8) + '...',  // Don't log the full secret
+          replaced: replacement,
+          category: 'secret',
+          pattern: pat.name,
+        });
+        result =
+          result.slice(0, match.index) +
+          replacement +
+          result.slice(match.index + match.value.length);
+      }
+    }
+
+    redactions.reverse();
+    return { text: result, redactions };
+  }
+
   // ============================================================
   // Notify content script of substitutions (for badge + logging)
   // ============================================================
diff --git a/src/lib/secret-scanner.js b/src/lib/secret-scanner.js
new file mode 100644
index 0000000..89758f0
--- /dev/null
+++ b/src/lib/secret-scanner.js
@@ -0,0 +1,243 @@
+/**
+ * Silent Send - Secret Scanner
+ *
+ * Detects common secret/credential patterns in text and either
+ * warns or auto-redacts them. This catches things the identity-based
+ * smart patterns can't: API keys, tokens, passwords, SSNs, credit
+ * cards, private keys, connection strings, etc.
+ *
+ * Each pattern has:
+ *   - name: human-readable label
+ *   - regex: detection pattern
+ *   - redact: replacement string (or function)
+ *   - severity: 'critical' (always redact) or 'warning' (flag but allow)
+ */
+
+const SECRET_PATTERNS = [
+  // --- API Keys ---
+  {
+    name: 'OpenAI API Key',
+    regex: /\bsk-[A-Za-z0-9]{20,}\b/g,
+    redact: '[REDACTED-OPENAI-KEY]',
+    severity: 'critical',
+  },
+  {
+    name: 'OpenAI Project Key',
+    regex: /\bsk-proj-[A-Za-z0-9_-]{20,}\b/g,
+    redact: '[REDACTED-OPENAI-PROJECT-KEY]',
+    severity: 'critical',
+  },
+  {
+    name: 'Anthropic API Key',
+    regex: /\bsk-ant-[A-Za-z0-9_-]{20,}\b/g,
+    redact: '[REDACTED-ANTHROPIC-KEY]',
+    severity: 'critical',
+  },
+  {
+    name: 'Google API Key',
+    regex: /\bAIza[A-Za-z0-9_-]{35}\b/g,
+    redact: '[REDACTED-GOOGLE-KEY]',
+    severity: 'critical',
+  },
+  {
+    name: 'AWS Access Key',
+    regex: /\bAKIA[A-Z0-9]{16}\b/g,
+    redact: '[REDACTED-AWS-KEY]',
+    severity: 'critical',
+  },
+  {
+    name: 'AWS Secret Key',
+    regex: /\b[A-Za-z0-9/+=]{40}\b(?=.*aws|.*secret)/gi,
+    redact: '[REDACTED-AWS-SECRET]',
+    severity: 'critical',
+  },
+  {
+    name: 'GitHub Token',
+    regex: /\b(ghp|gho|ghu|ghs|ghr)_[A-Za-z0-9]{36,}\b/g,
+    redact: '[REDACTED-GITHUB-TOKEN]',
+    severity: 'critical',
+  },
+  {
+    name: 'GitLab Token',
+    regex: /\bglpat-[A-Za-z0-9_-]{20,}\b/g,
+    redact: '[REDACTED-GITLAB-TOKEN]',
+    severity: 'critical',
+  },
+  {
+    name: 'Slack Token',
+    regex: /\bxox[bpras]-[A-Za-z0-9-]{10,}\b/g,
+    redact: '[REDACTED-SLACK-TOKEN]',
+    severity: 'critical',
+  },
+  {
+    name: 'Stripe Key',
+    regex: /\b[sr]k_(test|live)_[A-Za-z0-9]{20,}\b/g,
+    redact: '[REDACTED-STRIPE-KEY]',
+    severity: 'critical',
+  },
+  {
+    name: 'Twilio Key',
+    regex: /\bSK[a-f0-9]{32}\b/g,
+    redact: '[REDACTED-TWILIO-KEY]',
+    severity: 'critical',
+  },
+  {
+    name: 'SendGrid Key',
+    regex: /\bSG\.[A-Za-z0-9_-]{22}\.[A-Za-z0-9_-]{43}\b/g,
+    redact: '[REDACTED-SENDGRID-KEY]',
+    severity: 'critical',
+  },
+  {
+    name: 'Heroku API Key',
+    regex: /\b[a-f0-9]{8}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{12}\b/g,
+    redact: null, // UUIDs are too common — flag but don't auto-redact
+    severity: 'warning',
+  },
+  {
+    name: 'Generic API Key',
+    regex: /\b(?:api[_-]?key|apikey|api[_-]?secret|api[_-]?token)\s*[:=]\s*['"]?([A-Za-z0-9_\-./+=]{16,})['"]?/gi,
+    redact: (match) => match.replace(/[:=]\s*['"]?[A-Za-z0-9_\-./+=]{16,}['"]?/, '=[REDACTED]'),
+    severity: 'critical',
+  },
+  {
+    name: 'Generic Secret/Password Assignment',
+    regex: /\b(?:password|passwd|pwd|secret|token|auth[_-]?token|access[_-]?token|bearer)\s*[:=]\s*['"]?([^\s'"]{8,})['"]?/gi,
+    redact: (match) => match.replace(/[:=]\s*['"]?[^\s'"]{8,}['"]?/, '=[REDACTED]'),
+    severity: 'critical',
+  },
+  {
+    name: 'Bearer Token',
+    regex: /\bBearer\s+[A-Za-z0-9_\-./+=]{20,}\b/g,
+    redact: 'Bearer [REDACTED]',
+    severity: 'critical',
+  },
+
+  // --- Private Keys ---
+  {
+    name: 'Private Key Block',
+    regex: /-----BEGIN (?:RSA |EC |DSA |OPENSSH )?PRIVATE KEY-----[\s\S]*?-----END (?:RSA |EC |DSA |OPENSSH )?PRIVATE KEY-----/g,
+    redact: '[REDACTED-PRIVATE-KEY]',
+    severity: 'critical',
+  },
+
+  // --- Connection Strings ---
+  {
+    name: 'Database Connection String',
+    regex: /\b(?:mongodb|postgres|mysql|redis|amqp):\/\/[^\s"']+/gi,
+    redact: (match) => {
+      try {
+        const url = new URL(match);
+        if (url.password) url.password = 'REDACTED';
+        if (url.username) url.username = 'REDACTED';
+        return url.toString();
+      } catch {
+        return '[REDACTED-CONNECTION-STRING]';
+      }
+    },
+    severity: 'critical',
+  },
+
+  // --- PII Patterns ---
+  {
+    name: 'US Social Security Number',
+    regex: /\b\d{3}-\d{2}-\d{4}\b/g,
+    redact: '[REDACTED-SSN]',
+    severity: 'critical',
+  },
+  {
+    name: 'Credit Card Number',
+    regex: /\b(?:4\d{3}|5[1-5]\d{2}|3[47]\d{2}|6(?:011|5\d{2}))[- ]?\d{4}[- ]?\d{4}[- ]?\d{4}\b/g,
+    redact: '[REDACTED-CARD]',
+    severity: 'critical',
+  },
+
+  // --- Generic Long Hex/Base64 Strings ---
+  // Catches things that look like secrets but don't match known prefixes
+  {
+    name: 'Long Hex String (possible secret)',
+    regex: /\b[a-f0-9]{40,}\b/gi,
+    redact: null, // Too many false positives — warn only
+    severity: 'warning',
+  },
+];
+
+const SecretScanner = {
+  /**
+   * Scan text for secrets. Returns list of findings.
+   */
+  scan(text) {
+    const findings = [];
+
+    for (const pattern of SECRET_PATTERNS) {
+      // Reset regex lastIndex
+      pattern.regex.lastIndex = 0;
+      let match;
+
+      while ((match = pattern.regex.exec(text)) !== null) {
+        findings.push({
+          name: pattern.name,
+          value: match[0],
+          index: match.index,
+          length: match[0].length,
+          severity: pattern.severity,
+          redactTo: typeof pattern.redact === 'function'
+            ? pattern.redact(match[0])
+            : pattern.redact,
+        });
+      }
+    }
+
+    // Deduplicate overlapping matches (keep the most specific / longest)
+    findings.sort((a, b) => a.index - b.index || b.length - a.length);
+    const deduped = [];
+    let lastEnd = -1;
+    for (const f of findings) {
+      if (f.index >= lastEnd) {
+        deduped.push(f);
+        lastEnd = f.index + f.length;
+      }
+    }
+
+    return deduped;
+  },
+
+  /**
+   * Redact all critical secrets in text. Warnings are not auto-redacted.
+   * Returns { text, redactions[] }
+   */
+  redact(text) {
+    const findings = this.scan(text);
+    const redactions = [];
+    let result = text;
+
+    // Process from end to preserve indices
+    const critical = findings
+      .filter(f => f.severity === 'critical' && f.redactTo)
+      .reverse();
+
+    for (const f of critical) {
+      result = result.slice(0, f.index) + f.redactTo + result.slice(f.index + f.length);
+      redactions.push({
+        original: f.value,
+        replaced: f.redactTo,
+        category: 'secret',
+        pattern: f.name,
+      });
+    }
+
+    // Reverse so they're in order
+    redactions.reverse();
+
+    return {
+      text: result,
+      redactions,
+      warnings: findings.filter(f => f.severity === 'warning'),
+    };
+  },
+};
+
+if (typeof globalThis !== 'undefined') {
+  globalThis.SecretScanner = SecretScanner;
+}
+
+export default SecretScanner;
diff --git a/src/lib/storage.js b/src/lib/storage.js
index 57bb2b3..d047ac1 100644
--- a/src/lib/storage.js
+++ b/src/lib/storage.js
@@ -18,6 +18,7 @@ const DEFAULT_SETTINGS = {
   enabled: true,
   showHighlights: false,
   revealMode: false,
+  secretScanning: true,
   maxLogEntries: 200,
   customDomains: [],
   categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'general'],
diff --git a/src/options/options.html b/src/options/options.html
index 43a2bc3..ceafb41 100644
--- a/src/options/options.html
+++ b/src/options/options.html
@@ -24,6 +24,16 @@
           <span class="toggle-slider"></span>
         </label>
       </div>
+      <div class="setting-row">
+        <div>
+          <label>Secret scanning</label>
+          <p class="setting-desc">Auto-detect and redact API keys, tokens, passwords, SSNs, credit card numbers</p>
+        </div>
+        <label class="toggle">
+          <input type="checkbox" id="secretScanning" checked>
+          <span class="toggle-slider"></span>
+        </label>
+      </div>
       <div class="setting-row">
         <div>
           <label>Max log entries</label>
diff --git a/src/options/options.js b/src/options/options.js
index 83f04d9..30674cc 100644
--- a/src/options/options.js
+++ b/src/options/options.js
@@ -11,6 +11,7 @@ document.addEventListener('DOMContentLoaded', async () => {
 
   // Apply settings to UI
   $('#showHighlights').checked = settings.showHighlights || false;
+  $('#secretScanning').checked = settings.secretScanning !== false;
   $('#maxLogEntries').value = settings.maxLogEntries || 200;
 
   renderMappings();
@@ -28,6 +29,10 @@ document.addEventListener('DOMContentLoaded', async () => {
     await Storage.saveSettings({ showHighlights: e.target.checked });
   });
 
+  $('#secretScanning').addEventListener('change', async (e) => {
+    await Storage.saveSettings({ secretScanning: e.target.checked });
+  });
+
   $('#maxLogEntries').addEventListener('change', async (e) => {
     await Storage.saveSettings({ maxLogEntries: parseInt(e.target.value, 10) || 200 });
   });
diff --git a/src/popup/popup.js b/src/popup/popup.js
index d49eafe..8d3cbb3 100644
--- a/src/popup/popup.js
+++ b/src/popup/popup.js
@@ -1,5 +1,6 @@
 import SubstitutionEngine from '../lib/substitution-engine.js';
 import SmartPatterns from '../lib/smart-patterns.js';
+import SecretScanner from '../lib/secret-scanner.js';
 import Storage from '../lib/storage.js';
 import api from '../lib/browser-polyfill.js';
 
@@ -331,31 +332,50 @@ function renderTestDiff() {
 
   const smartResult = SmartPatterns.substitute(input, identity);
   const explicitResult = SubstitutionEngine.substitute(smartResult.text, mappings);
+  const secretResult = SecretScanner.redact(explicitResult.text);
 
-  const allReplacements = [...smartResult.replacements, ...explicitResult.replacements];
-  const finalText = explicitResult.text;
+  const allReplacements = [
+    ...smartResult.replacements,
+    ...explicitResult.replacements,
+    ...secretResult.redactions,
+  ];
+  const finalText = secretResult.text;
 
-  if (finalText === input) {
+  if (finalText === input && secretResult.warnings.length === 0) {
     output.textContent = input;
     stats.textContent = 'No substitutions detected';
     return;
   }
 
   let html = escapeHtml(finalText);
-  for (const r of allReplacements) {
+
+  // Highlight identity + explicit substitutions in green
+  for (const r of [...smartResult.replacements, ...explicitResult.replacements]) {
     const escapedReplaced = escapeHtml(r.replaced);
     html = html.replace(
       escapedReplaced,
       `<span class="sub-highlight" title="Was: ${escapeHtml(r.original)} [${r.pattern || r.category}]">${escapedReplaced}</span>`
     );
   }
+  // Highlight secret redactions in red
+  for (const r of secretResult.redactions) {
+    const escapedReplaced = escapeHtml(r.replaced);
+    html = html.replace(
+      escapedReplaced,
+      `<span class="sub-highlight" style="background:#fee2e2;color:#dc2626" title="${escapeHtml(r.pattern)}">${escapedReplaced}</span>`
+    );
+  }
   output.innerHTML = html;
 
   const smartCount = smartResult.replacements.length;
   const explicitCount = explicitResult.replacements.length;
+  const secretCount = secretResult.redactions.length;
+  const warnCount = secretResult.warnings.length;
   const parts = [];
   if (smartCount > 0) parts.push(`${smartCount} smart`);
   if (explicitCount > 0) parts.push(`${explicitCount} explicit`);
+  if (secretCount > 0) parts.push(`${secretCount} secrets redacted`);
+  if (warnCount > 0) parts.push(`${warnCount} warnings`);
   stats.textContent = `${allReplacements.length} substitution${allReplacements.length !== 1 ? 's' : ''} (${parts.join(', ')})`;
 }