Server Side Template Injection in Loopback with Dynamodb

Server Side Template Injection (SSTI) in Loopback applications that interact with DynamoDB can occur when user-controlled input is reflected into a template expression that is later evaluated. Loopback, a Node.js framework, supports dynamic views and expression-based templates in some integrations. If a developer passes data from a DynamoDB query result directly into a template without sanitization, an attacker may be able to inject template code that executes on the server.

Consider a scenario where an endpoint retrieves a user record from DynamoDB and renders it using a templating engine such as Handlebars or an expression language that supports lookup or function calls. A typical vulnerable pattern is constructing a response object from a DynamoDB item and passing it to a template that uses interpolation like {{name}}. If an attacker can control a field stored in DynamoDB — for example, by registering or updating a profile with a specially crafted value — that value may contain template syntax such as {{this.constructor.constructor('return process')()}}. When the template is rendered, the injected expression executes in the server-side context, potentially exposing environment variables or runtime information.

The DynamoDB-specific aspect arises from the way data is stored and retrieved. Items in DynamoDB are schemaless JSON-like structures, which means user input can contain deeply nested objects or arrays. If a Loopback application deserializes these items and feeds them into a templating layer without validation, nested properties can be leveraged to traverse the prototype chain or invoke methods. For instance, an attacker might store a value like { "__proto__": { "polluted": true } } or a more complex expression that relies on JavaScript object coercion when the template engine resolves references.

Additionally, the interaction between Loopback's model layer and DynamoDB can amplify risk. If a developer uses Loopback models to map DynamoDB results to JavaScript objects and then exposes those objects through a template, the mapping may inadvertently expose internal methods or constructor references. This can enable an attacker to reach Node.js built-in modules via constructor chains, a common pattern in SSTI payloads. Real-world techniques such as CVE-classic prototype pollution or code execution patterns are relevant here, even if the specific vulnerability is in the templating logic rather than the database driver.

Because middleBrick scans the unauthenticated attack surface and runs checks in parallel, it can detect template injection risks by analyzing endpoint behavior and OpenAPI specifications. The scanner evaluates input validation and data exposure controls, helping identify places where DynamoDB-sourced data reaches client-facing templates without sufficient sanitization or strict schema enforcement.

To prevent SSTI when using DynamoDB in Loopback, ensure that data retrieved from DynamoDB is never directly interpolated into templates. Instead, use strict schema validation, output encoding, and safe rendering patterns. Below are concrete code examples that demonstrate secure handling.

1. Validate and sanitize DynamoDB responses before rendering

Use a validation library to enforce a strict shape for items coming from DynamoDB. This prevents unexpected fields or nested expressions from reaching the template layer.

const Ajv = require('ajv');
const ajv = new Ajv();

const userSchema = {
  type: 'object',
  required: ['userId', 'email'],
  properties: {
    userId: { type: 'string' },
    email: { type: 'string', format: 'email' },
    name: { type: 'string' },
    // Explicitly deny fields that could carry template content
  },
  additionalProperties: false,
};

const validateUser = ajv.compile(userSchema);

// After fetching from DynamoDB
const isValid = validateUser(dynamoItem);
if (!isValid) {
  throw new Error('Invalid user data');
}
// Use validated data only

2. Use safe template rendering with explicit data binding

Choose a template engine that does not evaluate arbitrary JavaScript and pass only the minimal required data. For Handlebars, disable helpers and partials that could enable dynamic evaluation.

const Handlebars = require('handlebars');

// Compile with strict mode and no helpers
const template = Handlebars.compile('Name: {{name}}', { noEscape: true });

// Provide only whitelifted fields
const safeData = {
  name: dynamoItem.name || 'Anonymous',
};

const output = template(safeData);
console.log(output); // Safe: Name: John Doe

3. Avoid passing entire DynamoDB items to views

Map DynamoDB results to simple view models that contain only the fields needed for rendering. This reduces the attack surface and prevents leakage of internal metadata or unexpected properties.

function toViewModel(item) {
  return {
    userId: item.userId?.S || '',
    email: item.email?.S || '',
    name: item.name?.S || '',
  };
}

// In a controller
const userItem = await dynamodb.get({ TableName: 'Users', Key: { userId: { S: '123' } } }).promise();
const viewModel = toViewModel(userItem);
res.render('profile', { user: viewModel });

4. Enforce input validation on data stored in DynamoDB

Ensure that any user-supplied content written to DynamoDB is sanitized at write time. Reject or encode values that contain template-like patterns if they are not expected.

function sanitizeForStorage(value) {
  if (typeof value !== 'string') return value;
  // Basic neutralization of Handlebars-like patterns
  return value.replace(/{{[\s\S]*?}}/g, '[REDACTED_TEMPLATE]');
}

const cleanName = sanitizeForStorage(rawName);
await dynamodb.put({ TableName: 'Users', Item: { userId: { S: '123' }, name: { S: cleanName } } }).promise();

5. Use middleware to restrict prototype pollution

Add runtime protections that prevent modification of Object.prototype, which is often leveraged in SSTI and injection chains.

if (process.env.NODE_ENV === 'production') {
  const prohibitedKeys = ['__proto__', 'constructor', 'prototype'];
  const originalHas = Reflect.has;
  Reflect.has = function(target, property) {
    if (prohibitedKeys.includes(String(property))) {
      return false;
    }
    return originalHas(target, property);
  };
}