Header Injection in Buffalo with Basic Auth

Header Injection occurs when user-controlled data is reflected into HTTP response headers without validation or sanitization. In the Buffalo web framework, this commonly arises when developers construct headers using string concatenation or interpolation with values from request parameters, headers, or cookies. When Basic Authentication is used, the Authorization header is typically parsed to extract credentials; if the application then reuses parts of that header or related user input to set other headers, it can lead to injection of additional header lines.

Basic Auth credentials are base64-encoded but not encrypted; the header follows the format Authorization: Basic <base64>. If a Buffalo handler decodes this value and uses the username or password in constructing another header (for example, a custom X-User header) without proper validation, an attacker can inject newline characters (\r\n) to append malicious headers like X-Admin: true or to split headers and inject a second HTTP response line. This can enable HTTP response splitting, cache poisoning, or the leakage of sensitive information via crafted responses.

The risk is amplified when the application relies on default middleware configurations that do not strictly validate header values. For instance, if a developer writes code that takes the decoded username and assigns it directly to a custom header, newline characters in the username (which are technically disallowed in Basic Auth credentials but may be accepted by lenient parsers) can break header structure. Because the scan testing performed by middleBrick tests unauthenticated attack surfaces and includes header injection checks among its 12 parallel security checks, such weaknesses can be detected before they are exploited in production.

Real-world attack patterns mirror classic OWASP API Top 10 injection risks. For example, an attacker might send a request with an Authorization header containing alice\r\nX-Injected: malicious as part of the base64 payload, aiming to have the server reflect the injected header downstream. While frameworks like Buffalo do not automatically sanitize these values, the presence of such input in header construction logic creates a path for manipulation. middleBrick’s LLM/AI Security checks do not apply here, but its standard authentication and input validation checks can surface the improper use of user-influenced header data in Buffalo applications.

Concrete mitigation in Buffalo centers on strict validation and avoiding the use of any user-influenced input in header names or values. Never concatenate or interpolate request-derived data into headers. If you must pass identity information, use server-side session state or cryptographically signed tokens instead of reflecting raw credentials into headers. MiddleBrick’s dashboard can help track these issues over time, and its CLI tool allows you to integrate scans into your workflow to verify that header construction remains safe after changes.

To remediate Header Injection when using Basic Auth in Buffalo, ensure that decoded credentials are never directly used to construct response headers. Instead, treat the username and password as opaque authentication tokens and store authorization state server-side or in a secure token. Below are concrete code examples showing unsafe patterns and their secure alternatives.

Unsafe Example

The following pattern is vulnerable because it decodes the Basic Auth header and uses the username to set a custom header without validation:

// UNSAFE: directly using Basic Auth username in a custom header
let authHeader = request.Header.get("Authorization");
if (authHeader && authHeader.startsWith("Basic ")) {
  let decoded = base64.decode(authHeader.substring(6));
  let parts = decoded.split(":");
  let username = parts[0];
  let password = parts[1];
  // Vulnerable: username may contain newline characters
  response.setHeader("X-Username", username);
  response.send("Authenticated as " + username);
}

An attacker can supply a username such as alice\r\nX-Admin: true, which after base64 encoding and decoding will cause the server to send two separate headers, leading to response splitting.

Secure Example

Use the credentials only for authentication and store minimal, non-user-derived state server-side:

// SECURE: authenticate without reflecting user input into headers
let authHeader = request.Header.get("Authorization");
if (authHeader && authHeader.startsWith("Basic ")) {
  let decoded = base64.decode(authHeader.substring(6));
  let parts = decoded.split(":");
  let username = parts[0];
  let password = parts[1];
  if (isValidUser(username, password)) {
    // Do not set headers from username; use a session ID or token
    let sessionId = createSession(username);
    response.setHeader("Set-Cookie", "session=" + sessionId + "; HttpOnly; Secure");
    response.redirect("/dashboard");
  } else {
    response.status(401).send("Unauthorized");
  }
}

If you must pass identity information to downstream handlers, encode it in a signed token or use Buffalo’s built-in session mechanisms rather than custom headers. Additionally, enforce newline rejection on any user input that could ever reach header construction logic, even if not used directly as a header value.

Validation Helper

Add a validation helper to reject newlines and carriage returns in any user-controlled strings that might be used in security-sensitive contexts:

function containsNewline(str) {
  return str.includes('\r') || str.includes('\n');
}
// Use before any header or redirection logic
if (containsNewline(username)) {
  response.status(400).send("Invalid input");
  return;
}

By following these patterns, you reduce the risk of Header Injection and ensure that Basic Auth remains a straightforward authentication mechanism rather than an attack vector. middleBrick’s scans can validate these controls by checking for improper header manipulation and injection-prone code patterns.