Server Side Template Injection in Hapi with Dynamodb

Server Side Template Injection (SSTI) occurs when user-controlled data is embedded into a template without proper sanitization, allowing an attacker to alter the template logic. In a Hapi application that renders responses using a template engine and retrieves data from Amazon DynamoDB, the combination can expose SSTI when dynamic values from DynamoDB are directly interpolated into templates.

Hapi supports multiple template engines via the Vision plugin (e.g., Handlebars, EJS, Pug). If a route handler fetches an item from DynamoDB and passes raw attribute values to the template context, and the template engine performs unsafe interpolation or evaluation, an attacker who can influence the DynamoDB content might inject template code. For example, if an application stores user-provided strings in DynamoDB and later renders them in a Handlebars template without escaping, a stored payload like {{this.constructor.constructor('return process')()}} could execute JavaScript in the rendering context.

The risk is amplified when templates are authored or edited with data from DynamoDB (e.g, dynamic partials or component names). An attacker who can write items into the table might store a malicious template snippet that gets included during rendering, leading to remote code execution on the server. Even non-stored (reflected) scenarios are possible if query parameters that influence which DynamoDB item is fetched also influence template selection or context without validation.

Because DynamoDB is a NoSQL database, data shapes can be flexible; an attacker may exploit unexpected attributes or nested objects to reach template-injection primitives. In a Hapi setup, if route validation does not strictly limit or escape values used in templates, SSTI becomes feasible. The impact can range from information disclosure to arbitrary code execution on the host, depending on the template engine’s features and runtime permissions.

Remediation focuses on strict input validation, output encoding, and avoiding dynamic template selection or unsafe interpolation of DynamoDB data. Use parameterized queries to DynamoDB and ensure retrieved data is sanitized before being passed to templates.

Example: Safe DynamoDB fetch and escaped Handlebars rendering in Hapi

const Hapi = require('@hapi/hapi');
const { DynamoDBClient, GetItemCommand } = require('@aws-sdk/client-dynamodb');
const { marshall, unmarshall } = require('@aws-sdk/util-dynamodb');

const client = new DynamoDBClient({ region: 'us-east-1' });

const init = async () => {
  const server = Hapi.server({ port: 3000, host: 'localhost' });

  server.views({
    engines: { html: { module: require('handlebars'), compileMode: 'sync' } },
    relativeTo: __dirname,
    path: './views',
    // Ensure template caching is enabled in production and validate template sources
  });

  server.route({
    method: 'GET',
    path: '/user/{id}',
    handler: async (request, h) => {
      const cmd = new GetItemCommand({
        TableName: 'Users',
        Key: marshall({ userId: { S: request.params.id } })
      });

      const { Item } = await client.send(cmd);
      if (!Item) {
        return h.response('User not found').code(404);
      }

      const user = unmarshall(Item);
      // Explicitly allowlist safe fields; never pass the raw DynamoDB item directly to templates
      const safeContext = {
        username: escapeHtml(user.username || ''),
        displayName: escapeHtml(user.displayName || ''),
        // Do not include raw HTML or user-controlled template fragments
      };

      return h.view('user-profile', safeContext);
    }
  });

  server.ext('onPreResponse', (request, h) => {
    // Centralized safety: ensure responses do not reflect unsafe data
    return h.continue;
  });

  return server;
};

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

init().catch((err) => {
  console.error(err);
  process.exit(1);
});

DynamoDB and template best practices

• Validate and sanitize all data from DynamoDB; treat database content as untrusted.
• Use strict schema definitions (e.g., with Joi) for items retrieved from DynamoDB before they enter the template context.
• Avoid dynamic template names or partials derived from DynamoDB fields. If you must use dynamic templates, use a strict allowlist and resolve paths on the server side only.
• Enable output encoding in your template engine (e.g., Handlebars automatically escapes HTML when you use {{variable}} in double-stache; avoid triple-stash {{{variable}}} unless absolutely necessary and content is trusted).
• Use the AWS SDK for JavaScript v3 and parameterized commands; never construct queries by string concatenation.

Compliance mappings

Findings related to SSTI in this stack map to OWASP API Top 10 (2023) A05:2021 — Security Misconfiguration and A01:2021 — Broken Access Control, and can affect compliance with PCI-DSS and SOC2 if untrusted data reaches presentation layers.