Understanding CRLF Injection Attacks
Written by Harry Wilson   
Friday, 03 May 2024

Recently a vulnerability was identified in the  Cisco Secure Client that could allow an unauthenticated, remote attacker to conduct a carriage return line feed (CRLF) injection attack against a user. CISCO responded promptly. What lessons can be drawn from this to help strengthen enterprise app, network and data security in general? 

In March, Cisco published an advisory over an injection vulnerability in its Cisco Secure Client – specifically in the SAML authentication process. Identified with the Cisco bug ID CSCwi37512 and the CVE identifier CVE-2024-20337, this security issue posed the risk of enabling a Client Carriage Return Line Feed (CRLF) injection attack via remote access without authentication. The devices affected are multiplatform, from Linux to Windows and MacOS.

The Cisco advisory notes that the vulnerability exists because of the insufficiency of user-supplied input validation. It creates the opportunity for attackers to deceive users into clicking a malicious link in the process of establishing a VPN session. If the exploit succeeds, the attacker gets to run an arbitrary script in the web browser. It can also lead to the attacker gaining access to sensitive data and valid SAML tokens.

Cisco has already patched this vulnerability. Software updates have been released, and users of the Cisco Secure Client are advised to immediately apply the update. However, beyond the application of the security update, it is also important to dissect the vulnerability and derive lessons that can help strengthen enterprise app, network and data security in general.

CRLF Threats Are Potent

Also referred to as an HTTP response splitting attack, CRLF injection involves the manipulation of the carriage return and line feed special characters to modify the HTTP header and cause various adverse outcomes. These include cross-site scripting, cookie injection, and web cache poisoning. The introduction of additional or anomalous CRLF sequences into the HTTP header allows attackers to manipulate the data that follows the HTTP response header.

Cross-site scripting (VSS) can result in a relatively minor adverse consequence like the defacement of a website. However, it can have severe consequences particularly when it comes to sensitive data access. The XSS attacks on the British Airways, the game Fortnite, and eBay several years back exposed customer data to bad actors. The XSS attack on the British Airways served as a precursor to a credit card skimming campaign that affected 380,000 transactions. In Fortnite's case, hackers compromised the data of more than 200 million accounts. Meanwhile, the eBay attack enabled threat actors to take over accounts and manipulate the listings of high-ticket products.

Cookie injection may sound less alarming, but it can actually lead to identity theft. The insertion of additional Set-Cookie header fields makes it possible for an attacker to change the settings in a browser and enable unauthorized access to user accounts. This is particularly problematic for those who synchronize their browsers across different devices.

On the other hand, web cache poisoning happens when a CRLF injection attack manages to modify the HTTP headers and cache control headers. It entails the caching of malicious content, which can mean the spread of malware or the execution of a harmful script. Notably, web cache poisoning is hard to detect and prevent, because conventional threat detection systems usually do not scan web applications and caching proxies in tandem.

The Most Important Solution Is a Software Update

As indicated in Cisco’s advisory, there are no workarounds for the CRLF injection vulnerability they revealed. To address this security flaw, the only solution is to apply the software update. That’s why it is crucial to be up-to-date with the latest vendor announcements regarding software updates and security patches. 

This security problem is a permanent defect in the software that will not go away or be adequately sealed by whatever measure. There are stopgap measures that can be implemented like the use of a web application firewall (WAF) to block anomalous CRLF sequences before they find their way to the applications. 

Specific rules set in the WAF can identify and prevent injection attempts. However, this solution is only temporary, as threat actors can easily find new flaws they can exploit if the software issue itself is not resolved. It ensures that the root cause of the vulnerability is thoroughly addressed.

Insufficient Input Validation Is a Key Problem

Even a sufficiently-staffed tech giant like Cisco encounters problems over the insufficiency of user-supplied input validation. Organizations need not chase a perfect or foolproof validation of inputs. However, it is advisable to always strive for the best possible scenarios. Here are key practices to bear in mind.

First, it is important to avoid utilizing user input directly in the HTTP stream. This might be easier to implement, but it can result in injection vulnerabilities, because it makes it easier for threat actors to introduce modificatory data that impacts behavior and structure in the HTTP stream. As a rule of thumb, user inputs should always be deemed as risk factors, hence they should go through sanitization and validation before they are used.

Next, CRLF characters have to be encoded when used in user inputs or eliminated altogether.  Without exploitable CRLF characters, CRLF injection attacks cannot happen. If they are necessary, like in cases where input fields have multiple lines, it is a must to encode them to make sure they do not create executable scripts.

In connection to caution needed in using CRLF characters, it is also advisable to avoid the newline character ( \n ) as much as possible. This character is used to force the repositioning of the cursor to the start of the next line, moving user inputs to a new line. This may sound like a benign operation, but it can be exploited by attackers to inject CRLF sequences into HTTP headers. It is a must to encode inputs (which removes all newline characters) before the inputs are sent to the HTTP headers.

Moreover, here’s an important safe practice that is not directly about input validation but it still involves the user interface: disabling unused headers in the web server. This step immediately lessens the potential CRLF injection attack surfaces. Headers are vital for HTTP responses because they present details about the responses. However, the headers in these responses may include unnecessary information like server-timing (information about server processing time), which unnecessarily discloses sensitive information to possible threats actors. They do not serve crucial functions, but they help attackers identify exploitable vulnerabilities.

Key Takeaways

Cisco is a trusted name in IT worldwide. It is not out of the ordinary to expect it to have higher standards when it comes to security, but the reality is that even a behemoth with ample resources can stumble upon security issues. Fortunately, tech giants tend to be relatively fast in releasing security patches or software updates. Users need to download and apply these updates as soon as possible and take all threats seriously, even if it is something not so common like CRLF injection. Additionally, it is crucial to observe security best practices to address vulnerabilities and prevent the exploitation of security holes embedded in software. 


More Information

Cisco Security

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Last Updated ( Sunday, 26 May 2024 )