Crlf Injection in Loopback (Javascript)

Crlf Injection occurs when user-controlled data is embedded into HTTP headers without proper sanitization, allowing an attacker to inject CRLF sequences (\r\n) that break header parsing and enable header injection or response splitting. In Loopback applications written in JavaScript, this typically arises when dynamic values from request parameters, query strings, or headers are passed directly into functions that set headers or construct HTTP responses.

Loopback is a Node.js framework that encourages building APIs with JavaScript/TypeScript. Because JavaScript does not enforce strict header validation, developers must explicitly validate and sanitize any value used in header operations. A common pattern that introduces the risk is using res.set() or res.header() with user input:

app.controller('UserController', function(User) {
  User.getUsername = function(req, res, next) {
    const username = req.query.name;
    res.set('X-Username', username);
    res.send({message: 'ok'});
  };
});

In this example, if an attacker supplies name as attacker\r\nSet-Cookie: session=evil, the header becomes malformed and the injected Set-Cookie header may be interpreted by some HTTP clients or intermediaries. This is a classic Crlf Injection vector. The risk is compounded when Loopback applications compose redirects or generate custom Location headers without validation:

app.controller('RedirectController', function(req, res) {
  const target = req.query.to;
  res.redirect(302, target);
});

An attacker can provide http://example.com\r\nSet-Cookie: token=stolen as the to parameter, causing the response to include an unexpected Set-Cookie header. Although browsers typically mitigate the impact by not following injected headers in redirects, the presence of unvalidated user input in header construction remains a security weakness.

Because middleBrick tests unauthenticated attack surfaces across 12 security checks including Input Validation and Data Exposure, it flags such patterns as high-risk when user-controlled data reaches response headers. The scanner also cross-references OpenAPI/Swagger specs (2.0, 3.0, 3.1) with full $ref resolution to detect whether header definitions expose endpoints to injection risks. These findings map to OWASP API Top 10 and help prioritize remediation.

To remediate Crlf Injection in Loopback JavaScript applications, ensure that any data used in HTTP headers is strictly validated and sanitized. The simplest approach is to disallow CR (\r) and LF (\n) characters in user input before using it in headers, and to avoid directly passing request parameters into header-setting functions.

Use a validation helper to strip or reject dangerous characters. For example:

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

Apply this function before setting headers:

app.controller('UserController', function(User) {
  User.getUsername = function(req, res, next) {
    const username = safeHeaderValue(req.query.name);
    res.set('X-Username', username);
    res.send({message: 'ok'});
  };
});

When handling redirects, validate and constrain the target to a known set of allowed domains or paths rather than using raw user input:

app.controller('RedirectController', function(req, res) {
  const allowedDomains = ['myapp.com', 'app.myapp.com'];
  const url = new URL(req.query.to);
  if (!allowedDomains.includes(url.hostname)) {
    return res.status(400).send({error: 'Invalid redirect target'});
  }
  res.redirect(302, url.toString());
});

For more robust protection, adopt a whitelist approach for header names and values, and centralize header-setting logic to ensure consistent sanitization across controllers. middleBrick’s Pro plan supports continuous monitoring, so these fixes can be validated on a configurable schedule with alerts sent via Slack/Teams if regressions occur. The GitHub Action can also fail builds when risk scores drop below your defined threshold, integrating security into your CI/CD pipeline.

Finally, if you use the MCP Server, you can scan APIs directly from your AI coding assistant within the IDE, catching header-injection patterns early during development. These practices reduce the likelihood of Crlf Injection while maintaining the flexibility of JavaScript-based Loopback applications.