Server Side Template Injection in Restify with Hmac Signatures

Server Side Template Injection (SSTI) in Restify occurs when user-controlled data is interpolated into a template string that is later processed by a template engine, allowing an attacker to inject template directives. When Hmac Signatures are used to verify the integrity of requests (e.g., signing webhook payloads or API tokens), a common pattern is to include the signature or a derived value in the template context. If the signature value or related parameters are reflected into the template without proper escaping, an attacker can manipulate the input to alter the computed Hmac Signature or cause the template engine to execute unintended logic.

Consider a scenario where a Restify server uses Handlebars to render a response and includes an Hmac Signature in the template context for verification purposes. An attacker might supply a payload that modifies the template structure, such as injecting block helpers or variable overrides. Because the Hmac Signature is computed over the original request data, the attacker’s injected template directives do not change the signature itself, but they can cause the server to render unexpected content, bypass conditional checks, or expose sensitive data embedded in the template. This becomes particularly dangerous when the same context is used both for rendering and for signature validation, as confusion between the two can lead to logic flaws.

In practice, SSTI via Hmac Signatures in Restify often stems from unsafe usage of template engines combined with improper separation of data and logic. For example, if a developer passes raw user input into a template that also receives a signature-derived field, the engine may interpret injected strings as template commands. Even though the Hmac Signature remains intact on the server side, the rendered output can be altered in malicious ways, leading to information disclosure or unauthorized behavior. The risk is amplified when templates are loaded dynamically or when partial templates are selected based on user input, as this expands the attack surface for injection.

Real-world exploitation may not directly break the Hmac Signature verification, but it can subvert the application’s intended behavior. For instance, an attacker could force the template to include external resources, execute embedded expressions, or leak context variables that should remain hidden. Because Restify often serves as an API layer that returns rendered templates or JSON with embedded template logic, the impact of SSTI can extend beyond simple output manipulation to include authentication bypass or data exfiltration when combined with weak access controls.

To detect this class of issue, scanning should verify that user input is never directly embedded into templates that also handle Hmac Signatures, and that template engines operate in strict sandboxed modes. Tools like middleBrick can identify unsafe template usage patterns and flag contexts where signature-derived data and user input share the same rendering scope, helping developers maintain a clear boundary between integrity verification and presentation logic.

Remediation focuses on strict separation of data used for Hmac Signatures from data passed to templates, along with explicit escaping and context isolation. Always compute the Hmac Signature over a canonical representation of the request payload or headers, and keep the signature out of the template context entirely. If a signature must be included for client-side verification, ensure it is treated as an opaque string and never interpreted by the template engine.

Use a dedicated context object for templates that excludes any fields involved in signature generation. For example, in a Restify handler using the restify and handlebars packages, structure your code as follows:

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

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

server.post('/submit', (req, res, next) => {
  const { body, headers } = req;
  const secret = process.env.WEBHOOK_SECRET;

  // Compute Hmac Signature over the raw body
  const hmac = crypto.createHmac('sha256', secret);
  hmac.update(JSON.stringify(body));
  const signature = hmac.digest('hex');

  // Explicitly build a template-safe context
  const templateContext = {
    username: body.username ? Handlebars.escapeExpression(body.username) : '',
    amount: body.amount ? Handlebars.escapeExpression(String(body.amount)) : '',
    // Do NOT include signature in the template context
  };

  const templateSource = 'User {{username}} paid {{amount}}';
  const template = Handlebars.compile(templateSource);
  const html = template(templateContext);

  res.send({ html, receivedSignature: signature });
  return next();
});

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

This approach ensures that user input is HTML-escaped before inclusion, and the Hmac Signature is computed independently without being exposed to template interpolation. Avoid using dynamic template names or partials derived from user input, as this can reintroduce injection vectors even when signatures are handled correctly.

For JSON-based APIs that still need to convey signature information, return the signature in a separate field and do not merge it into objects that are passed to templating functions. If you use middleBrick’s CLI to scan endpoints that employ Hmac Signatures, it can highlight contexts where signature-related fields appear in template data structures, guiding you toward a safer architecture.

Finally, enforce strict Content-Type and input validation to reduce the likelihood of malicious payloads reaching your template layer. Regular scans with middleBrick, especially using the Pro plan for continuous monitoring, can help maintain separation between integrity checks and presentation logic as your codebase evolves.