Server Side Template Injection in Restify with Dynamodb

Server Side Template Injection (SSTI) occurs when user-controlled data is merged into a template and then interpreted as code by a template engine. In a Restify service that builds responses using a templating engine and interpolates DynamoDB results, SSTI can arise when untrusted input is used to construct template variables or when the data shapes returned from DynamoDB are directly rendered without sanitization.

Consider a Restify endpoint that fetches a user profile from DynamoDB and passes the item to a Handlebars template. If the template includes helpers or expressions that evaluate strings as code, and the data from DynamoDB contains attacker-influenced values (for example, a user-controlled attribute like displayName), an attacker can inject template syntax. A malicious value such as {{this.constructor.constructor('return process')()}} can lead to arbitrary code execution on the server when the template engine evaluates it.

The combination of Restify, a Node.js server framework, and DynamoDB, a NoSQL database, introduces specific risk patterns. DynamoDB items often contain string fields that are later bound into templates. If an API stores or returns user-supplied content in attributes like bio or customMessage, and those attributes are rendered in a template without escaping, the attack surface expands. Moreover, if the application uses partial updates or expression-based path notation (e.g., updateExpression: "SET info = :info") and directly interpolates user input into the expression without validation, the injected data may affect how the template or downstream logic processes the item.

An example flow: a client sends a profile update with a field status containing {"value": "Hello {{this.constructor.constructor('return require')('child_process').execSync('id')}}"}. The Restify handler retrieves the item from DynamoDB and passes it to a Handlebars template. If the template does not autoescape status.value, the injected Handlebars code executes, potentially reading files or spawning processes. This demonstrates how improper handling of DynamoDB data within Restify templates can lead to Server Side Template Injection.

To detect this class of issue, scanners test template rendering paths with payloads that include framework-specific syntax, while also checking whether data from DynamoDB reaches the template layer unescaped. Findings typically highlight missing context-aware escaping and unsafe merging of user-influenced data into templates.

Remediation focuses on strict input validation, output encoding, and avoiding direct concatenation of user data into templates or DynamoDB expression values. Below are concrete, Restify-oriented code examples that demonstrate secure patterns.

1. Use a strict schema and validation before DynamoDB operations

Validate incoming data against a schema and sanitize strings before using them in update expressions or conditionals. This prevents malicious template syntax from entering DynamoDB and later being rendered.

const Joi = require('joi');
const schema = Joi.object({
  userId: Joi.string().alphanum().required(),
  status: Joi.string().max(200).allow('').pattern(/
\S/, 'must not contain control characters or template-like patterns')
});

function updateProfile(req, res, next) {
  const { error, value } = schema.validate(req.body);
  if (error) { return next(new restify.BadRequestError('Invalid input'));
  }

  const params = {
    TableName: 'profiles',
    Key: { userId: value.userId },
    UpdateExpression: 'SET #st = :st',
    ExpressionAttributeNames: { '#st': 'status' },
    ExpressionAttributeValues: { ':st': value.status },
    ReturnValues: 'UPDATED_NEW'
  };

  docClient.update(params, (err, data) => {
    if (err) { return next(new restify.InternalServerError(err.message)); }
    res.send(200, data.Attributes);
    return next();
  });
}

2. Autoescape template variables and avoid unsafe helpers

Configure your templating engine to autoescape by default and avoid custom helpers that evaluate strings as code. With Handlebars, use {{{ }}} only for trusted HTML and always escape user-controlled fields.

const handlebars = require('handlebars');
// Ensure autoescape is enabled for the runtime
const template = handlebars.compile('<div>{{escapeStatus status}}</div>');

// In your Restify handler
function showProfile(req, res, next) {
  const params = {
    TableName: 'profiles',
    Key: { userId: req.params.userId }
  };
  docClient.get(params, (err, data) => {
    if (err) { return next(new restify.InternalServerError(err.message)); }
    const item = data.Item || {};
    // Provide a safe helper that escapes output
    handlebars.registerHelper('escapeStatus', (val) => {
      return handlebars.Utils.escapeExpression(val || '');
    });
    const html = template(item);
    res.setHeader('Content-Type', 'text/html');
    res.send(200, html);
    return next();
  });
}

3. Use DynamoDB ConditionExpressions and Parameterized updates

Avoid building update expressions by string concatenation with user input. Use ExpressionAttributeValues for all variable data and reserve ExpressionAttributeNames for reserved words. This reduces the risk that attacker-controlled strings affect structure.

const params = {
  TableName: 'profiles',
  Key: { userId: 'u123' },
  UpdateExpression: 'SET #nm = :nm, #lv = :lv',
  ConditionExpression: 'attribute_exists(userId)',
  ExpressionAttributeNames: { '#nm': 'name', '#lv': 'level' },
  ExpressionAttributeValues: {
    ':nm': 'Jane Doe',
    ':lv': 5
  },
  ReturnValues: 'NONE'
};

docClient.update(params, (err, data) => {
  if (err) { console.error(err); }
});

4. Limit exposure of raw DynamoDB output in templates

Only pass the minimal transformed data to templates rather than the full DynamoDB item. This reduces the likelihood that unexpected fields containing executable template syntax reach the renderer.

function profileToViewModel(item) {
  return {
    userId: item.userId,
    displayName: item.displayName || '',
    createdAt: item.createdAt || null
    // Exclude raw user-supplied fields that may contain template syntax
  };
}

function getProfile(req, res, next) {
  const params = { TableName: 'profiles', Key: { userId: req.params.userId } };
  docClient.get(params, (err, data) => {
    if (err) { return next(new restify.InternalServerError(err.message)); }
    const vm = profileToViewModel(data.Item);
    res.render('profile', vm);
    return next();
  });
}