Crlf Injection in Koa with Dynamodb

Crlf Injection occurs when user-controlled data is inserted into HTTP headers without sanitization, allowing an attacker to inject newline characters (\r\n). In a Koa application that interacts with Amazon DynamoDB, this typically arises when data retrieved from or submitted to DynamoDB is reflected into HTTP response headers, such as custom headers, Location headers during redirects, or error messages that include user-controlled attribute values.

For example, consider a Koa route that reads an item from DynamoDB and sets a custom header based on an item attribute:

const Koa = require('koa');
const AWS = require('aws-sdk');
const app = new Koa();
const dynamo = new AWS.DynamoDB.DocumentClient();

app.use(async (ctx) => {
  const { id } = ctx.params;
  const data = await dynamo.get({ TableName: 'Users', Key: { id } }).promise();
  if (data.Item) {
    ctx.set('X-User-Status', data.Item.status); // Potential CRLF if status contains \r\n
    ctx.body = data.Item;
  } else {
    ctx.throw(404, 'Not found');
  }
});

If the status attribute in DynamoDB contains a string like active\r\nX-Injected: malicious, the ctx.set call will write that raw header value into the HTTP response, enabling header injection. This can lead to HTTP response splitting, session fixation, or cache poisoning. In a DynamoDB context, the risk is amplified when items are user-managed (e.g., user profiles or preferences) and are directly mapped into headers without validation.

The same issue can occur when constructing redirect URLs using DynamoDB data:

app.use(async (ctx) => {
  const { token } = ctx.query;
  const data = await dynamo.get({ TableName: 'Tokens', Key: { token } }).promise();
  if (data.Item && data.Item.redirectUrl) {
    ctx.redirect(data.Item.redirectUrl); // If redirectUrl contains \r\n, response splitting occurs
  }
});

Here, a malicious entry in DynamoDB with a redirectUrl like https://example.com\r\nSet-Cookie: session=attacker can inject additional headers or content. Because DynamoDB stores and serves raw strings, the onus is on the application layer to validate and sanitize any data before it is used in protocol-sensitive contexts like HTTP headers.

CRLF Injection in the Koa + DynamoDB combination is particularly dangerous in unauthenticated or low-privilege scan scenarios, where an attacker can read or write items to DynamoDB as part of the attack surface. This aligns with BOLA/IDOR and Improper Neutralization of Special Elements in HTTP Headers checks that middleBrick flags during a scan.

Remediation focuses on strict input validation, output encoding, and avoiding direct reflection of DynamoDB attributes into HTTP headers. When working with DynamoDB in Koa, treat all attribute values as untrusted, especially strings that may be set or modified by users.

1. Sanitize before setting headers

Never pass raw DynamoDB string attributes directly to ctx.set. Strip or reject carriage return (\r) and line feed (\n) characters:

function sanitizeHeader(value) {
  if (typeof value !== 'string') return value;
  return value.replace(/[\r\n]+/g, '');
}

app.use(async (ctx) => {
  const { id } = ctx.params;
  const data = await dynamo.get({ TableName: 'Users', Key: { id } }).promise();
  if (data.Item) {
    ctx.set('X-User-Status', sanitizeHeader(data.Item.status));
    ctx.body = data.Item;
  } else {
    ctx.throw(404, 'Not found');
  }
});

2. Validate URLs used in redirects

When using DynamoDB values in redirects, parse and validate the URL to ensure it is well-formed and does not contain newline characters. Use a URL parser and enforce a same-origin policy or strict allowlist:

const { URL } = require('url');

function isValidRedirectUrl(url) {
  try {
    const parsed = new URL(url);
    // Allow only specific origins
    return parsed.origin === 'https://yourapp.com';
  } catch (err) {
    return false;
  }
}

app.use(async (ctx) => {
  const { token } = ctx.query;
  const data = await dynamo.get({ TableName: 'Tokens', Key: { token } }).promise();
  if (data.Item && data.Item.redirectUrl && isValidRedirectUrl(data.Item.redirectUrl)) {
    ctx.redirect(data.Item.redirectUrl);
  } else {
    ctx.throw(400, 'Invalid redirect');
  }
});

3. Use middleware to reject dangerous characters globally

Add a lightweight Koa middleware that inspects any user-influenced data used in headers or redirects and rejects requests containing \r or \n:

app.use(async (ctx, next) => {
  const checkNoCrlf = (obj) => {
    for (const key of Object.keys(obj)) {
      const val = obj[key];
      if (typeof val === 'string' && /[\r\n]/.test(val)) {
        throw new Error('CRLF characters not allowed');
      }
    }
  };
  // Example: validate query, body, and params
  checkNoCrlf(ctx.query);
  checkNoCrlf(ctx.request.body || {});
  checkNoCrlf(ctx.params);
  await next();
});

These measures reduce the risk of CRLF Injection while preserving functionality. When integrating middleware and validation, middleBrick can help verify that your headers and redirects are properly hardened by running a scan and reviewing its findings and remediation guidance.

For teams using the CLI, you can run middlebrick scan <url> to test your endpoints; the dashboard allows you to track security scores over time, and the Pro plan supports continuous monitoring and CI/CD integration to catch regressions early.