Your Guide to CRLF Injection: Meaning, Types, and Prevention Tips

Quick Summary: Different combinations of CRLF enable web servers to determine the beginning or end of headers. While it is a useful function for managing HTTP streams, it can become a security challenge if not used with care and attackers can exploit it to fulfill their malicious objectives. So, it becomes crucial to learn more about it, how it can be a security risk, its types, and more information to make the right decisions to protect your applications and systems. This article provides all the information you need.

CRLF has long been used in computing for text-based formatting. It provides a neat and structured appearance of texts. The use of CRLF in HTTP header and other software code signifies an End of Marker (EOL) that denotes the end of a statement, code unit, or file.

There is a crucial role of CRLF in computing, especially in the functioning of HTTP requests and responses. However, there is also a potential for security issues in the form of CRLF injection. It is a kind of security risk when attackers inject sequences of CRLF to change the behavior of HTTP streams.

It is a sort of injection attack, just like an SQL injection, which is the most common security challenge for web apps, as per Statista. However, it targets HTTP headers instead of databases. Attackers can leverage it to manipulate HTTP headers to meet their malicious agenda. They can either steal sensitive data or use it to launch further attacks.

Keep reading this blog to learn more about this security threat and to make a prudent defense strategy.

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What is CRLF Injection?

CRLF injection is a cyberattack technique in which an attacker manipulates the Carriage Return and Line Feed characters present in the HTTP header of a web application. An attacker can take advantage of this attack vector to cause severe damage.

Besides, the attacker can also launch phishing attacks, web cache poisoning, cross-site scripting, and other types of security threats. So, there are a lot of security challenges with CRLF injection, especially when attackers can inject malware into the victim’s system or gain access to sensitive data.

CRLF (Carriage Return Line Feed) denotes control characters that are used to signify the termination of a line. It is a combination of Carriage Return (CR) and Line Feed (LF) controllers. They are widely used control characters across programming languages and operating systems.

CR is marked by ‘\r’ and LF is marked by ‘\n’ characters with 13 and 10 as ASCII codes, respectively.

Types of Security Threats Caused by CRLF Injection Attack

CRLF not only allows attackers to manipulate an application’s behavior, but it also allows them to launch other serious cyber risks. The following are the different types of cyber threats that occur due to CRLF injection.

Cookie Poisoning

This is a type of security risk in which an attacker attempts to modify the value stored in cookies. Since most of the time cookies are in plain text and without any safeguard, they are vulnerable to easy modification with the use of document.cookie in JavaScript. Cookie poisoning encompasses other cookie-related attacks as well.

Cross-Site Scripting

Another critical security risk that arises with CRLF injection is cross-site scripting (XSS). In this kind of attack tactic, an attacker can insert malicious scripts in web pages that others can view and redirect them to untrusted sites. It can allow attackers to steal sensitive data. An attacker can manipulate HTTP headers to insert malicious scripts in the case of a CRLF attack.

Web Cache Poisoning

In this attack method, an attacker can cache malicious content by modifying the cache control headers through HTTP headers with the injection of CRLF sequences. As a result, users who access the cached content are affected by this threat. It causes various security risks for the victims.

Phishing Attacks

Phishing is another type of cyberattack that occurs due to the insertion of CRLF sequences in HTTP headers. The attacker may force users to redirect to other sites controlled by attackers by manipulating HTTP headers. He can even replace the content in the header. It can result in phishing attacks that can redirect users to fake bank websites or another duplicate domain to extract sensitive data.

Understanding the Different Types of CRLF Injection Attack

There are different techniques that attackers use to carry out a Carriage Return and Line Feed attack. The following are the different types of CRLF injection attacks.

HTTP Response Splitting

When an attacker manipulates an HTTP response header by injecting a CRLF sequence into it, the result is HTTP response splitting. With this kind of attack method, an attacker can split the response into two different responses. Besides, the attacker can also add arbitrary headers. It occurs when there is no proper input sanitization and user-supplied input is inserted directly into the response header.

Memcached Injection

In this type of CRLF attack, the attacker manipulates the values to be stored in Memcached and other related caching systems. The attacker inserts CRLF characters into the values to corrupt or modify the data. It affects the behavior of the application. For example, an attacker can manipulate data in Memcached to cause the application to corrupt.

HTTP Header Injection

It is similar to HTTP response splitting, but it involves inserting malicious headers. It means the attacker tries to manipulate the header section of an HTTP request or response by inserting a malicious one. For instance, the attacker can insert a Set-Cookie header that compels the browser to store a cookie stored by an attacker.

Log-based CRLF Injection

In this type of CRLF injection, the attacker targets the log file. Log files store events and data related to software behavior and usage. An attacker can use a CRLF attack to compromise the integrity of log files. As a result, it will be hard to identify the malicious activities. The attacker can include misleading information in the log file to carry out malicious activities without raising a doubt.

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How Can You Detect CRLF Injection Vulnerability?

CRLF injection is a critical security risk that can allow attackers to gain unauthorized access to your data or manipulate your system to execute malicious code. Hence, identifying CRLF vulnerability is essential to mitigate the risks associated with it. You can detect CRLF injection vulnerability using the steps below.

• You can leverage penetration testing to perform attacker-like attacks to check for potential vulnerabilities, including CRLF injection. Penetration testing is done by ethical hackers with the permission of the concerned organizations.
• Another method is automated vulnerability assessment. It helps detect vulnerabilities by automatically scanning applications or systems. It requires a dynamic application security testing tool. This kind of tool scans applications at runtime and performs simulated attacks to discover security weaknesses like OWASP Top 10.

Tips to Prevent CRLF Injection Attack

Preventing CRLF injection vulnerability requires you to address the common security mistakes. These common mistakes lead to weaknesses in the systems or applications that allow attackers to easily manipulate data or cause serious attacks like XSS, cache poisoning, and more.

Remove Unnecessary Headers

HTTP responses include different types of headers that carry information about the response. Not all of these headers are useful, and you can safely turn off any of them if they are not necessary. You can use this method to avoid security breaches by removing potential security loopholes. These unnecessary headers might have weak spots that attackers can exploit to steal data or carry out severe attacks.

Ensure Input Sanitization

Unsanitized input is a major reason for the CRLF security issue. It occurs when user-supplied input is directly used in HTTP streams. Attackers can take advantage of this security weakness to trick servers into executing requests consisting of manipulated HTTP headers.

Hence, user inputs must be sanitized before being used in the headers. Sanitizing user inputs might seem a trivial thing, but there are huge consequences if you do not do it. If the data is not sanitized and it is inserted in HTTP headers directly, this can lead to data theft, unauthorized access, and risks.

Encode/Remove CRLF

Encoding the CRLF characters is another way to prevent CRLF attacks. In this case, the characters can be transformed into a format that a server doesn’t interpret as line breaks. So, even when an attacker is able to insert these characters into the header, they are safely ignored by the server instead of executing with the request. As a result, the attacker will not be able to modify the HTTP headers.

Another way attackers can tamper with HTTP headers is by using newline characters. These characters denote the end of a line, and an attacker can use them to modify the HTTP stream’s structure. This risk can be mitigated by removing the newline characters from user input before they are used in the HTTP headers.

Remove CRLF in Logging

Logging user data is a widely used activity in organizations that help in tracking issues and debugging. Moreover, this can also become an entry point for various vulnerabilities, including CRLF, which can allow attackers to steal data or manipulate application/system behavior.

Hence, you should be careful about CRLF sequences when logging in. It’s best to remove CRLF sequences before logging to keep it secure from potential risks. It eliminates the possibility of these sequences being logged that could be further used in exploiting headers.

Security Assessment

Identifying and addressing the CRLF and similar vulnerabilities is essential to keep these threats at bay. It requires regular security audits to evaluate applications, systems, and networks for potential vulnerabilities or loopholes.

With a comprehensive security analysis, you can discover various security risks in web apps, APIs, networks, and other digital assets. It will provide a deep insight into the threat landscape to address security issues on time before they become a risk for your organization.

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In a Nutshell

Although CRLF injection attacks are not quite dangerous on their own, they can pave the way for more critical security risks. So, it becomes essential to prevent this threat and avoid data breaches. While there are different measures to avoid this threat, including input sanitization, removing unnecessary headers, and more, a thorough security assessment is crucial to get ultimate protection against it.

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