CRLF injection

CRLF Injection Definition

CRLF Injection, also known as HTTP response splitting, refers to a web application security vulnerability that arises when an attacker inserts carriage return (CR) and line feed (LF) characters into input fields. These specific characters are used to mark the end of a line in HTTP headers. When maliciously inserted, they have the potential to manipulate the response sent by the server and perform unauthorized actions.

How CRLF Injection Works

CRLF Injection attacks take advantage of the way servers handle HTTP headers. In the process of sending back an HTTP response from the server to the client, the response is composed of multiple lines of text, with each line terminated by a carriage return (CR) and a line feed (LF) character. These characters serve to distinguish between the headers and the message body of the response.

Attacks targeting CRLF Injection involve exploitation of web application input fields like URL parameters or form inputs. Attackers intentionally insert CR and LF characters into these fields to inject additional headers into the server's response or modify existing ones. This manipulation paves the way for various types of attacks, including but not limited to cross-site scripting (XSS), session fixation, and cache poisoning.

To better understand the mechanics of CRLF Injection attacks, consider the following steps:

1. Identifying Input Fields: Attackers search for web application input fields, such as URL parameters or form inputs, where they can inject their malicious data.

2. Inserting CR and LF Characters: The attacker strategically inserts carriage return (CR) and line feed (LF) characters into the input fields. These characters are used to mark the end of a line in HTTP headers.

3. Manipulating Response Headers: As the server processes the malicious input, it interprets the CR and LF characters as line breaks, thus splitting the input into separate lines. This enables the attacker to inject additional headers into the server's response or modify existing ones.

4. Properties of Manipulated Headers: Attackers can manipulate various properties associated with the injected headers. For instance, they can modify the content, behavior, or location of the response. By setting the Location header, for example, they may redirect users to malicious websites. Additionally, they can conceal the true nature of the response by altering the Content-Type header. Unauthorized actions can also be performed by injecting arbitrary headers.

Examples of CRLF Injection Attacks

CRLF Injection attacks enable malicious activities to be carried out. Several examples of such attacks are:

1. Cross-Site Scripting (XSS): Through the injection of malicious headers that alter the Content-Type or Location headers, attackers trick the user's browser into executing arbitrary scripts. This can lead to the theft of sensitive information or the impersonation of the user.

2. Session Fixation: Attackers can inject headers that set the session ID to a specific value, allowing them to hijack the user's session after the user logs in. As a result, the attacker gains unauthorized access to the user's account and all associated privileges.

3. Cache Poisoning: CRLF Injection can be utilized to manipulate cache headers, paving the way for cache poisoning attacks. By injecting malicious headers, attackers can insert harmful content into caches, which is then served to unsuspecting users. Such attacks can result in the distribution of malicious software or the exposure of sensitive information to unintended recipients.

Prevention Tips

To mitigate the risk of CRLF Injection attacks, the following prevention tips should be considered:

1. Input Validation and Sanitization: Web applications should implement input validation and sanitization mechanisms to identify and block CRLF sequences. All user inputs, particularly those from URL parameters and form inputs, must be thoroughly validated. Implementing strict validation measures that only allow alphanumeric and whitelisted characters can significantly reduce the risk.

2. Web Application Firewalls (WAFs): Incorporate a Web Application Firewall (WAF) as part of your web application infrastructure. WAFs monitor incoming and outgoing HTTP traffic, and they can efficiently detect and prevent CRLF Injection attacks. By analyzing request and response headers, they flag any suspicious or malicious patterns, subsequently blocking or modifying the traffic as necessary.

3. Regular Updates: Ensure that web servers and application frameworks are kept up to date. Regularly update all components of the web application, including the servers, to patch known vulnerabilities that are related to CRLF Injection attacks. Stay informed about the latest security patches and updates released by software vendors, and promptly apply them to ensure the security of the systems.

Implementing these preventive measures can significantly reduce the risk of CRLF Injection attacks and safeguard the security of web applications and user data.

Related Terms

• Cross-Site Scripting (XSS): Another web application vulnerability where attackers inject malicious scripts into web pages viewed by other users.
• Session Fixation: A type of attack where an attacker sets a user's session ID to an explicit value, allowing them to hijack the session after the user logs in.
• Cache Poisoning: A hacking technique that allows attackers to insert and distribute malicious content into a cache to be served to other users.