Server Side Template Injection in Mongodb

Server Side Template Injection (SSTI) in Mongodb environments typically occurs when user input is improperly interpolated into template strings that are later executed as database queries or commands. This vulnerability is particularly dangerous in Mongodb because the database's flexible document structure and JavaScript engine can execute arbitrary code when template injection succeeds.

The most common manifestation occurs in application code where template engines like Handlebars, EJS, or Pug are used to construct Mongodb queries. Consider this vulnerable pattern:

const template = require('handlebars').compile('db.collection.find({ name: "{{name}}" })');
const query = template({ name: req.query.name });
db.eval(query); // Executes arbitrary JavaScript!

In this example, an attacker can inject Handlebars expressions that execute arbitrary JavaScript when the template is compiled. A payload like {{this.constructor.constructor('return process')().mainModule.require('child_process').execSync(`cat /etc/passwd`)}} would execute system commands.

Mongodb-specific SSTI often appears in aggregation pipelines where template strings construct complex operations. The aggregation framework's $where operator is particularly dangerous as it accepts JavaScript functions:

const pipelineTemplate = `[{ $match: { $where: "{{condition}}" } }]`;
const condition = req.query.condition;
const pipeline = JSON.parse(pipelineTemplate.replace('{{condition}}', condition));
db.collection.aggregate(pipeline);

An attacker could inject return this.password.includes('admin') or worse, return db.eval('malicious code'). The $where operator bypasses index usage and executes JavaScript for every document, making it both a performance and security anti-pattern.

Another Mongodb-specific vector is through db.eval() which executes JavaScript on the database server. Template injection here can lead to complete database compromise:

const jsCode = `function() { return ${userInput}; }`;
db.eval(jsCode); // Executes arbitrary JavaScript on the database

Even more subtle are injection points in Mongodb's $function aggregation operator or when using Mongoose middleware with template-derived query parameters.

Detecting SSTI in Mongodb requires understanding both the template engine being used and how templates interact with database operations. Static analysis tools often miss these vulnerabilities because the dangerous code path involves runtime template compilation.

Dynamic scanning with middleBrick specifically tests for SSTI by injecting template expressions into all user input fields and monitoring for execution. The scanner tests 27 regex patterns covering common template engines (Handlebars, EJS, Pug, Mustache) and their expression syntax. For Mongodb contexts, it specifically probes:

• Aggregation pipeline construction with template-derived $where conditions
• Template-based query builders that generate Mongodb queries
• JavaScript evaluation contexts (db.eval, $function)
• Property access chains that could traverse to dangerous objects

The scanner executes payloads like {{this.constructor.constructor('return process')().mainModule.require('os').hostname()}} and monitors responses for indicators of successful execution, such as:

• Application errors containing template engine artifacts
• Unexpected database behavior or query failures
• Response times indicating JavaScript execution
• System information leakage in error messages

For Mongodb specifically, middleBrick analyzes the aggregation pipeline structure when templates are involved. It looks for patterns where user input might construct $where clauses or JavaScript functions. The scanner also tests for NoSQL injection patterns that often accompany SSTI, as attackers frequently combine both techniques.

Code review should focus on these Mongodb-specific patterns:

// Vulnerable: template-derived aggregation pipeline
const pipeline = JSON.parse(templateString.replace('{{userInput}}', userInput));
db.collection.aggregate(pipeline);

// Vulnerable: dynamic $where construction
const whereClause = `this.${field}.toString() === "${value}"`;
db.collection.find({ $where: whereClause });

Runtime detection should include monitoring for unusual JavaScript execution patterns in Mongodb logs and application error messages containing template syntax.

Remediating SSTI in Mongodb applications requires eliminating template-based query construction and implementing proper input validation. The primary defense is to avoid any template engine usage for database operations.

Instead of template-based query construction, use parameterized queries and Mongodb's native query builders:

// Vulnerable pattern
const template = `db.collection.find({ name: "{{name}}" })`;
const query = template.replace('{{name}}', userInput);

// Secure pattern
const query = { name: userInput };
db.collection.find(query); // Uses parameterized query

For aggregation pipelines, construct them programmatically rather than through templates:

// Vulnerable
const pipeline = `[{"$match": {"$where": "${userInput}"}}]`;

// Secure
const pipeline = [
  { $match: { fieldName: userInput } }
];
db.collection.aggregate(pipeline);

Never use db.eval() or $function with user input. These operators are deprecated in modern Mongodb versions and should be completely avoided:

// Remove all instances of db.eval
// Replace with proper query operators
db.eval('dangerous code'); // ❌ REMOVE THIS

Implement strict input validation and sanitization. For Mongodb specifically, validate that user input matches expected patterns before using it in queries:

function validateMongodbInput(input, allowedPattern) {
  // Use a whitelist approach
  const regex = new RegExp(allowedPattern);
  if (!regex.test(input)) {
    throw new Error('Invalid input format');
  }
  return input;
}

// Usage
const safeInput = validateMongodbInput(userInput, '^[a-zA-Z0-9_]+$');
db.collection.find({ name: safeInput });

Enable Mongodb's built-in security features:

// Enable audit logging to detect suspicious operations
db.adminCommand({setParameter: 1, auditAuthorizationSuccess: true});

// Use role-based access control
db.createRole({
  role: 'applicationRole',
  privileges: [
    { resource: { db: 'myapp', collection: 'users' }, actions: ['find'] }
  ],
});

For applications that must use template engines for other purposes, ensure templates containing database operations are never exposed to user input. Use separate template contexts for presentation and data access layers.

Consider using Object Document Mapping (ODM) libraries like Mongoose with strict schema validation, which provides an additional layer of protection against injection attacks by enforcing data types and structure.