Crlf Injection in Loopback with Dynamodb

Crlf Injection occurs when user-controlled data is written into HTTP headers without validation, enabling injection of extra carriage return (CR, \r) and line feed (\n) sequences. In a Loopback application that uses DynamoDB as a persistence layer, this typically arises when data stored in DynamoDB is later rendered into HTTP response headers, such as custom headers, Location, or Set-Cookie. If an attacker can inject \r\n into a DynamoDB attribute that is later read by Loopback and used in header values, they can split the response headers from the body and inject additional headers or body content.

Consider a scenario where user profile information (e.g., displayName) is stored in DynamoDB and later used in a JSON response that also sets a custom header. An attacker might register or update their profile with a display name containing CRLF sequences (e.g., John\r\nX-Internal: injected). When Loopback retrieves this item from DynamoDB and includes displayName in a header, the injected CRLF causes the header to be duplicated or new headers to be appended, potentially leading to header smuggling, session fixation, or reflected injection.

The DynamoDB data model exacerbates the risk when string attributes are used verbatim in headers without normalization or encoding. Because DynamoDB does not enforce header-safe character sets, developers must explicitly sanitize and validate any data retrieved from DynamoDB before it reaches the HTTP layer. In Loopback, this often means applying strict validation in model hooks, remote methods, or controller logic before using DynamoDB-stored values in res.set() or res.header(). The combination of a flexible NoSQL schema in DynamoDB and dynamic header assignment in Loopback creates a pathway for CRLF Injection when input validation and output encoding are omitted.

To prevent CRLF Injection when using DynamoDB with Loopback, ensure that any data stored in or retrieved from DynamoDB is sanitized before being used in HTTP headers. Below are concrete code examples illustrating safe practices.

1. Validate and sanitize input before saving to DynamoDB

Use Loopback validators to reject or escape CRLF characters in string properties. This prevents malicious payloads from being persisted.

module.exports = function(YourModel) {
  YourModel.validatesFormatOf('displayName', {with: /^[^\r\n]+$/, message: 'Display name must not contain carriage returns or line feeds'});
};

2. Encode data when setting headers from DynamoDB results

When constructing headers from DynamoDB items, explicitly remove or encode CRLF characters. For example, when setting a custom header with a user-supplied value:

const safeValue = (rawValue || '').replace(/[\r\n]/g, '');
res.set('X-User-Display', safeValue);
// Or use a whitelist approach for safer characters
res.set('X-User-Display', safeValue.replace(/[^\w\-\s]/g, ''));

3. Use a serialization layer for responses that include DynamoDB data

Create a transform function that processes DynamoDB items before they are sent in responses, ensuring headers and body fields are safe.

function sanitizeForHttpResponse(item) {
  return {
    id: item.id,
    displayName: (item.displayName || '').replace(/[\r\n]/g, ''),
    email: (item.email || '').replace(/[\r\n]/g, '')
  };
}

// In a controller or remote method
YourModel.findById(id, (err, instance) => {
  if (err) return next(err);
  const safe = sanitizeForHttpResponse(instance);
  res.set('X-User-Display', safe.displayName);
  res.json(safe);
});

4. Apply header-specific validation in Loopback middleware

Add an operation hook or middleware to validate outgoing headers. This provides a centralized safeguard against CRLF Injection across all endpoints that use DynamoDB data in headers.

app.middleware('response:phaseOne').add((ctx, next) => {
  const headers = ctx.res.getHeaders();
  for (const [key, value] of Object.entries(headers)) {
    if (typeof value === 'string' && /[\r\n]/.test(value)) {
      throw new Error('Unsafe header value detected: ' + key);
    }
  }
  return next();
});

5. Prefer parameterized APIs over raw header manipulation

Where possible, use Loopback’s built-in response methods that abstract header handling, and avoid directly setting headers with untrusted DynamoDB data. If headers are necessary, rely on standard, well-audited header-setting functions that enforce safe character sets.