Crlf Injection in Restify with Dynamodb

Crlf Injection occurs when user-controlled data is reflected in HTTP headers without sanitization, allowing an attacker to inject newline sequences (\r\n) to split headers and inject new ones. In a Restify service that uses Amazon DynamoDB as a backend, this risk is amplified when response metadata from DynamoDB is inadvertently passed into HTTP headers.

Consider a typical Restify handler that retrieves an item from DynamoDB and returns selected fields in custom response headers for client-side routing or debugging. If the item attributes (such as a user-supplied X-Custom-Reason) are not validated, an attacker can provide a value like injected\r\nX-Admin: true. When Restify forwards this value into a response header, the injected CRLF sequence creates a new header, potentially enabling HTTP response splitting, cache poisoning, or header manipulation.

The DynamoDB layer does not introduce CRLF characters by itself, but if your application stores or retrieves user-controlled strings in DynamoDB and then reflects them into HTTP headers via Restify, the data path becomes vulnerable. For example, scanning with middleBrick against such an endpoint would flag the lack of input validation and improper header handling as a BFLA/Privilege Escalation and Unsafe Consumption finding, noting that unauthenticated attackers can manipulate header boundaries without requiring credentials.

Because Restify does not sanitize data before setting headers, and DynamoDB may store attacker-controlled strings, the combination creates a practical injection surface. middleBrick’s checks include Input Validation and Unsafe Consumption, which test for missing canonicalization and header injection, highlighting how an unauthenticated attacker might chain CRLF injection with other techniques to escalate impact.

Remediation focuses on two areas: sanitizing data before it is stored in DynamoDB or before it is reflected into HTTP headers, and ensuring Restify does not directly forward untrusted strings into headers.

First, validate and sanitize any user input that will be stored in DynamoDB if it may later be used in a header context. Use a strict allowlist for header-safe values (e.g., alphanumeric and a limited set of punctuation) and reject or transform disallowed characters such as carriage return (\r) and line feed (\n). This prevents malicious payloads from being persisted and later reflected.

Second, when constructing HTTP responses in Restify, explicitly set headers using safe, pre-sanitized values. Do not pass raw DynamoDB attribute values into header setters. Below is a concrete example using the AWS SDK for JavaScript v3 and Restify that demonstrates secure handling.

const restify = require('restify');
const { DynamoDBClient, GetItemCommand } = require('@aws-sdk/client-dynamodb');

const server = restify.createServer();
const db = new DynamoDBClient({ region: 'us-east-1' });

server.get('/items/:id', async (req, res, next) => {
  const cmd = new GetItemCommand({
    TableName: 'Items',
    Key: { id: { S: req.params.id } },
  });

  try {
    const { Item } = await db.send(cmd);
    if (!Item) return next();

    // Safe: explicitly map known fields; do not forward raw user data into headers
    const safeReason = Item.reason?.S || '';
    const sanitizedReason = safeReason.replace(/[\r\n]+/g, '');

    // Set headers only with sanitized values
    res.setHeader('X-Item-Reason', sanitizedReason);
    res.send({ id: Item.id?.S, reason: sanitizedReason });
    return next();
  } catch (err) {
    return next(err);
  }
});

server.listen(8080, () => console.log('Listening on port 8080'));

In this pattern, the code retrieves an item from DynamoDB, explicitly extracts the reason attribute, removes any CRLF characters, and then sets the header using the cleaned value. This ensures that even if DynamoDB contains injected newline sequences, they are neutralized before reaching the HTTP layer. middleBrick’s scans will show improved Input Validation and Unsafe Consumption findings when such canonicalization and output encoding practices are in place.

Additionally, consider applying a global normalization layer in Restify to reject or strip newline characters across all user-supplied inputs, and enforce schema validation on DynamoDB-stored data to restrict header-bound fields to safe character sets. These measures reduce the attack surface without requiring changes to DynamoDB’s core behavior.