Server Side Template Injection in Loopback with Basic Auth

Server Side Template Injection (SSTI) in Loopback combined with Basic Auth can expose user identity and application logic, increasing the risk of unauthorized access and data exposure. Basic Auth sends credentials in an easily decoded base64 header, and when endpoints reflect user-supplied input into templates, attackers can achieve template injection that reveals internal variables or control rendering paths. A middleBrick scan flags this combination under Authentication and BFLA/Privilege Escalation checks, noting that unauthenticated probing can identify template injection points even when Basic Auth is present.

In Loopback, templates are often used for dynamic responses, emails, or error messages. If user-controlled data is interpolated without sanitization into a template engine (e.g., Handlebars or lodash templates) and that engine supports arbitrary code execution, an attacker can inject payloads that read server-side files or trigger remote code execution. Because Basic Auth does not inherently protect template rendering paths, the presence of credentials in headers may lead developers to assume safety while the endpoint remains vulnerable. A payload such as {{&constructor.constructor('return process')()}} in an injected template can expose Node.js globals, demonstrating how SSTI can bypass authentication assumptions.

During a black-box scan, middleBrick tests unauthenticated attack surfaces and then re-tests with Basic Auth credentials to see whether authentication changes template behavior. Findings often include missing output encoding, unsafe reflection of request parameters into views, and overly permissive template contexts. These map to OWASP API Top 10 A03:2023 — Injection and A07:2021 — Identification and Authentication Failures. Remediation focuses on context-aware output encoding, strict input validation, and avoiding dynamic template execution, which the dashboard presents with severity, guidance, and references to CVE patterns such as CVE-2021-21342-like injection chains.

To secure Loopback endpoints using Basic Auth, enforce strict validation and avoid direct interpolation of user input into templates. Use context-aware escaping for the template engine in use, and ensure credentials are verified before rendering any dynamic content. The following examples show a secure approach with explicit authentication middleware and safe rendering.

Example: Secure Basic Auth setup with input sanitization

const loopback = require('loopback');
const boot = require('loopback-boot');
const bodyParser = require('body-parser');
const crypto = require('crypto');

const app = loopback();
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));

// Basic Auth middleware with constant-time comparison
app.use((req, res, next) => {
  const authHeader = req.headers.authorization || '';
  const match = authHeader.match(/^Basic\s+(\S+)$/);
  if (!match) { return res.status(401).send('Unauthorized'); }
  const buffer = Buffer.from(match[1], 'base64');
  const [user, pass] = buffer.toString().split(':');
  // In production, use a slow compare and a user store
  const expectedUser = 'admin';
  const expectedPassHash = '5e884898da28047151d0e56f8dc6292773603d0d6aabbdd62a11ef721d1542d8'; // SHA-256 of 'password'
  const passHash = crypto.createHash('sha256').update(pass).digest('hex');
  if (user !== expectedUser || !slowEquals(passHash, expectedPassHash)) {
    return res.status(401).send('Unauthorized');
  }
  next();
});

function slowEquals(a, b) {
  if (a.length !== b.length) return false;
  let result = 0;
  for (let i = 0; i < a.length; i++) {
    result |= a.charCodeAt(i) ^ b.charCodeAt(i);
  }
  return result === 0;
}

// Safe endpoint: no dynamic template execution, explicit context
app.get('/profile', (req, res) => {
  // Assume user data from a secure source
  const userData = { name: 'Alice', role: 'admin' };
  // Use a template engine with autoescape or render with a strict context
  const html = `
Name: ${escapeHtml(userData.name)}
Role: ${escapeHtml(userData.role)}
`;
  res.set('Content-Type', 'text/html').send(html);
});

function escapeHtml(str) {
  return str.replace(/[&<>"']/g, (s) => ({'&':'&','<':'<','>':'>','"':'"',"'":'''}[s]));
}

app.start();

Key remediation practices

• Never pass raw request parameters directly to template engines; use explicit view models.
• Apply context-specific escaping (HTML, JS, URL) based on where data is rendered.
• Use HTTP-only, Secure cookies for session tokens if moving away from Basic Auth for sensitive endpoints.
• Integrate middleBrick CLI to scan from terminal with middlebrick scan <url> and include scans in CI/CD via the GitHub Action to fail builds if risk scores drop below your chosen threshold.
• For continuous assurance, the Pro plan supports continuous monitoring and can trigger alerts via Slack/Teams when findings appear, while the MCP Server lets you scan APIs directly from AI coding assistants within your IDE.