CRLF Injection

In the HTTP protocol, the CR-LF sequence is always used to terminate a line.

A CRLF Injection attack occurs when a user manages to submit a CRLF into an application. This is most commonly done by modifying an HTTP parameter or URL.

Exploitation

HTTP Response Splitting

Manipulating HTTP headers by injecting CRLF characters.

One could craft a malicious URL containing CRLF characters to add extra headers or modify existing ones. Consider the following vulnerable code:

<?php
$new_url = $_GET["url"];
header("Location: " . $new_url);
?>

An attacker could exploit this code by appending CRLF characters and injecting a custom header like:

http://remote.com/index.php?url=http://remote.com/%0D%0AContent-Length:%200%0D%0A%0D%0AHTTP/1.1%20200%20OK%0D%0A%0D%0AeSomeContent

This vulnerability could lead to other attacks such as; XSS, Cache Poisoning, Page Hijacking

Log Forging

The bad actor injects CRLF characters into application logs, creating fake log entries or modifying existing ones.

Bypassing sanity checks

Some applications perform poorly written checks on the user input. However, most of these checks are not checking for CRLF sequences.

For example: There's a web app that checks for the parameter role. If the parameter is present it will assign a session variable called ROLE. With this, a user is able to access the dashboard of the given role.

GET /get_profile.php?role=admin

Here the check would be performed which would result in an error.

GET /get_profile.php?role%0d%0a=admin

However, with the new request the role parameter has passed the check.