Server Side Template Injection in Restify with Api Keys

Server Side Template Injection (SSTI) occurs when an attacker can inject template expressions that are evaluated on the server. In Restify, this can arise when user-controlled data—such as an API key value, key identifier, or related metadata—is incorporated into server-side templates without proper escaping or validation. When Api Keys are handled by Restify plugins or custom handlers that render responses using a templating engine, any unchecked input may become a vector for SSTI.

Consider a scenario where an endpoint returns plugin or diagnostic information and uses a template to format output. If an API key is accepted via query or header and directly interpolated into the template, an attacker may provide a payload designed to break out of the intended context. For example, a Restify plugin that logs key metadata into a Handlebars template could expose the application to SSTI if the key value is not sanitized. The injected template code could execute unintended logic, read application state, or leak sensitive configuration stored in template context variables.

Because middleBrick scans the unauthenticated attack surface, it can detect endpoints where Api Key parameters appear in inputs that reach template rendering paths. Findings may highlight unsafe usage of user data in templates and map the issue to OWASP API Top 10:2023 A05 (Broken Function Level Authorization) and A03 (Injection), as well as relevant PCI-DSS and SOC2 controls. The scanner does not assume authentication; it probes with and without keys to observe whether key-related inputs affect template behavior, helping you identify exposure before an attacker does.

To mitigate SSTI when handling Api Keys in Restify, ensure that any user-controlled data used in templates is escaped according to the rules of the templating engine, and avoid passing raw key values into template contexts. Prefer passing minimal, sanitized metadata rather than the raw key, and validate key formats before use.

Example 1: Safe usage with Handlebars in Restify, explicitly escaping key-derived strings.

const restify = require('restify');
const Handlebars = require('handlebars');

const server = restify.createServer();
server.use(restify.plugins.bodyParser());

server.get('/status', (req, res, next) => {
  const apiKeyInput = req.query.key || '';
  // Validate expected key format before use
  const isValidKey = /^[A-F0-9]{32}$/.test(apiKeyInput);
  const safeContext = {
    keyStatus: isValidKey ? 'valid' : 'invalid',
    // Explicitly escape any user-controlled strings used in templates
    keyFragment: isValidKey ? Handlebars.escapeExpression(apiKeyInput.slice(0, 4)) : '****'
  };
  const templateSource = 'Key status: {{keyStatus}}, fragment: {{keyFragment}}';
  const template = Handlebars.compile(templateSource);
  res.send(template(safeContext));
  return next();
});

server.listen(8080, () => {
  console.log('Server running on port 8080');
});

Example 2: Avoiding template interpolation of raw Api Key values in a custom Restify plugin.

const restify = require('restify');

function keyInfoPlugin(options, callback) {
  return function (req, res, next) {
    const providedKey = req.headers['x-api-key'] || '';
    // Do not inject raw key into template context; use safe metadata
    const safeMetadata = {
      keyLength: providedKey.length,
      hasPrefix: providedKey.startsWith('ak_')
    };
    // Render with a precompiled template that does not include raw key
    const template = restify.plugins.handlebars.compile('Key length: {{keyLength}}, prefix present: {{hasPrefix}}');
    res.send(template(safeMetadata));
    return next();
  };
}

const server = restify.createServer();
server.use(restify.plugins.bodyParser());
server.use(keyInfoPlugin);
server.get('/info', (req, res, next) => {
  res.send({ message: 'Key metadata shown safely' });
  return next();
});

server.listen(8081, () => {
  console.log('Server running on port 8081');
});

Additional recommendations: review your templates to ensure no automatic variable resolution is enabled, and adopt a principle of least privilege for any contextual data passed to rendering functions. If you use OpenAPI specs, ensure that parameters related to Api Keys are clearly defined and that generated client SDKs do not inadvertently expose raw keys in logs or templates. middleBrick can help by scanning your endpoints to detect when Api Key inputs reach template-rendering paths and by mapping findings to recognized security frameworks.