CWE-113: HTTP Response Splitting – A Comprehensive Guide for Penetration Testers

In today’s hyper-connected digital landscape, where online services are an essential part of business operations, security vulnerabilities remain a serious concern. One such vulnerability that has long been overlooked yet can lead to devastating attacks is HTTP Response Splitting (CWE-113). This issue, often found in web applications, can have dire consequences if exploited. For penetration testers, understanding CWE-113 is vital to safeguarding businesses from the risks associated with this vulnerability.

What is HTTP Response Splitting?
How HTTP Response Splitting Works
Common Attack Vectors
Business Impact of HTTP Response Splitting
Real-World Examples of HTTP Response Splitting Exploits
How to Detect HTTP Response Splitting
Mitigating HTTP Response Splitting: Best Practices for Developers
Penetration Testing for HTTP Response Splitting
Tools and Techniques for Identifying HTTP Response Splitting
The Role of Security Policies and Risk Management in Mitigation
Final Thoughts

1. What is HTTP Response Splitting?

HTTP Response Splitting is a web application vulnerability that occurs when an attacker is able to manipulate HTTP headers to split the response sent to the client. This manipulation exploits the way headers are processed by web servers and browsers, allowing attackers to inject malicious content into the response stream. The result can be a range of attacks, from cross-site scripting (XSS) to cache poisoning and web cache poisoning, all of which can disrupt business operations, damage brand reputation, and compromise sensitive data.

CWE-113, as defined by the Common Weakness Enumeration (CWE), refers specifically to improper handling of input data that is used to generate HTTP headers. In its simplest form, an HTTP response is a message sent by a server to a client (such as a web browser) after receiving a request. The response typically contains headers that provide information like the status of the request, content type, and length. An attacker can manipulate these headers to inject additional responses or modify the content.

2. How HTTP Response Splitting Works

HTTP Response Splitting typically occurs when the application fails to properly validate or sanitize user-supplied input. This could be anything from URL parameters to headers sent in HTTP requests. To better understand the mechanics, let’s break down how this vulnerability manifests:

Step 1: Malicious Input – The attacker sends a request to the server with specially crafted input that contains newline characters (e.g., \\n or \\r\\n). This input is often placed in HTTP headers, such as the Location or Set-Cookie headers.
Step 2: Splitting the Response – If the server is not properly sanitising the input, it processes the newline character as a new header. As a result, the server might send two separate HTTP responses in a single HTTP transaction. The first response is legitimate, but the second one can contain malicious data, such as scripts, redirects, or other harmful content.
Step 3: Exploitation – The attacker can now use the second HTTP response to inject harmful code or cause unwanted side effects. For instance, the malicious response could lead to a cross-site scripting (XSS) attack or a redirection to a malicious website.

Example:

Imagine a web application that accepts a user input for setting a redirect URL after login. If the input is not validated properly and includes a newline (\\n), the server could send two separate HTTP responses:

A legitimate redirect to the correct page.
An additional response that could inject an XSS script or malicious location header.

3. Common Attack Vectors

HTTP Response Splitting is particularly dangerous because it opens the door to various forms of attacks. Some of the most common attack vectors associated with CWE-113 include:

Cross-Site Scripting (XSS): By injecting a script into the response, attackers can execute malicious JavaScript in the context of another user’s browser. This can lead to the theft of session cookies, account hijacking, or other malicious activities.
Web Cache Poisoning: Attackers can manipulate HTTP headers to cache malicious content. When other users access the same resources, they are served poisoned content from the cache, bypassing security filters and potentially spreading malware.
HTTP Response Smuggling: In more advanced cases, HTTP Response Splitting can be used to perform response smuggling, where malicious payloads are hidden inside multiple HTTP responses. This can bypass security filters, cache systems, and proxies.
Redirection Attacks: Attackers can manipulate the Location header, causing users to be redirected to malicious websites. This can be used for phishing, spreading malware, or conducting social engineering attacks.
Session Fixation: By injecting headers like Set-Cookie, attackers can fix the session ID and hijack the user’s session after authentication.

4. Business Impact of HTTP Response Splitting

For penetration testers, it’s crucial to communicate the business impact of vulnerabilities like HTTP Response Splitting. If left unmitigated, the consequences can be severe:

Reputational Damage: If an attacker uses HTTP Response Splitting to inject malicious content, especially XSS, it can lead to users losing trust in the application, particularly in business-critical platforms such as banking, e-commerce, or healthcare applications.
Financial Loss: Exploits such as web cache poisoning can be used to steal sensitive data, conduct fraud, or cause financial disruptions. Similarly, redirection attacks could redirect customers to phishing websites, costing businesses significant financial damage.
Compliance Risks: Many industries, such as healthcare and finance, are subject to strict regulatory frameworks like GDPR, PCI DSS, or HIPAA. Exploiting HTTP Response Splitting could lead to violations of these regulations and hefty fines.
Loss of Intellectual Property: If HTTP Response Splitting is used to conduct session hijacking or data exfiltration attacks, critical business information can be stolen or exposed, leading to a loss of intellectual property and a competitive edge.
Operational Disruptions: Malicious HTTP responses can cause systems to become unreliable, leading to downtime or degraded service performance, ultimately affecting customer experience and trust.

5. Real-World Examples of HTTP Response Splitting Exploits

To illustrate the severity of HTTP Response Splitting, let’s examine a real-world example:

Example 1: A Financial Institution’s Customer Portal

A customer portal for a financial institution had a flaw in its login process. The site allowed users to input a custom redirect URL after a successful login. Due to insufficient input validation, attackers were able to inject newline characters into the redirect URL, leading to two separate HTTP responses. The attacker used this vulnerability to inject a malicious XSS payload, stealing session cookies and gaining unauthorised access to customer accounts.

Example 2: E-Commerce Platform Cache Poisoning

An e-commerce website used caching for storing product information. Attackers exploited HTTP Response Splitting by sending malformed URLs to the server, which caused the server to send two responses. The first response was legitimate, but the second response poisoned the cache with malicious JavaScript. This resulted in customers being served with malicious content when they accessed the product pages, leading to a widespread attack.

6. How to Detect HTTP Response Splitting

As a penetration tester, detecting HTTP Response Splitting requires a combination of manual and automated techniques. Here are a few methods:

Input Validation Checks: During testing, ensure that all user inputs, including URL parameters, headers, and form fields, are properly sanitized and validated before being processed.
Response Analysis: Look for abnormal HTTP responses, such as multiple HTTP/1.1 200 OK headers or unexpected headers like Location or Set-Cookie. These may indicate that the response has been split.
Fuzz Testing: Use automated fuzzing tools to send a variety of inputs, including newline characters and unusual symbols, to identify potential weaknesses in the server’s input handling.
Traffic Analysis: Review HTTP traffic for signs of malicious redirections or responses that do not match the intended behaviour. Tools like Burp Suite or OWASP ZAP can help identify such issues.

7. Mitigating HTTP Response Splitting: Best Practices for Developers

Mitigating CWE-113 requires a proactive approach. Developers and businesses can adopt several best practices to prevent HTTP Response Splitting:

Sanitise User Input: Ensure that user input is thoroughly validated and sanitized, especially for parameters used in HTTP headers. Avoid allowing newline characters (\\n or \\r\\n) or other control characters in user input.
Use Secure HTTP Headers: When setting HTTP headers, such as Location, Set-Cookie, or Content-Type, ensure they are well-formed and do not include user input without proper encoding.
Implement Proper Error Handling: Ensure the server responds with appropriate error codes and does not expose sensitive information in error messages. This can prevent attackers from gaining insights into potential attack vectors.
Apply Security Headers: Implement security headers such as Strict-Transport-Security (HSTS) and Content-Security-Policy (CSP) to reduce the impact of successful attacks.
Input Length Limits: Apply reasonable length limits for URL and header values to reduce the risk of splitting or excessive header manipulation.

8. Penetration Testing for HTTP Response Splitting

Penetration testers must evaluate web applications for the presence of HTTP Response Splitting. This includes testing all user input points and simulating attacks that exploit the vulnerability. Tools like Burp Suite, OWASP ZAP, and Nikto can be used to automate and streamline this process. Manual testing, combined with these tools, provides a comprehensive approach to identifying and exploiting this vulnerability.

9. Tools and Techniques for Identifying HTTP Response Splitting

Some of the most effective tools for detecting HTTP Response Splitting include:

Burp Suite: Burp Suite’s scanner and Intruder functionality can help identify issues related to HTTP headers, while its Interceptor allows manual inspection of HTTP responses.
OWASP ZAP: ZAP is a versatile open-source security testing tool that can be configured to look for specific vulnerabilities, including HTTP Response Splitting.
Nikto: A web scanner that can help identify a range of vulnerabilities, including improper HTTP header processing.

10. The Role of Security Policies and Risk Management in Mitigation

Organisations can reduce the risk of HTTP Response Splitting by incorporating robust security policies and risk management practices. These should include regular security audits, secure coding standards, and penetration testing as part of the software development lifecycle (SDLC). By addressing security issues early in the process, organisations can significantly reduce their exposure to this and other vulnerabilities.

CWE-113: HTTP Response Splitting is included in the SANS Top 25 Most Dangerous Software Errors because it poses a significant threat to the security of web applications, leading to a range of potentially devastating attacks. Here are several key reasons why this vulnerability has earned its place in the list:

1. Wide Range of Exploitable Attack Vectors

HTTP Response Splitting can be leveraged in various malicious ways, making it a versatile vulnerability. This ability to cause multiple types of attacks contributes to its high risk and importance. Some of the potential exploitations include:

Cross-Site Scripting (XSS): Attackers can inject harmful JavaScript or other payloads into HTTP responses, leading to the theft of sensitive data such as session cookies or login credentials.
Web Cache Poisoning: Malicious actors can poison web caches, causing subsequent users to receive malicious or outdated content from the cache. This can affect large numbers of users, bypassing normal application security measures.
Redirection Attacks: Malicious redirects can be injected through the response splitting, sending users to phishing sites or sites hosting malware.
Session Fixation: By manipulating cookies, attackers may be able to set a session ID, leading to session hijacking.

The vulnerability’s potential to support such diverse attacks means it has a far-reaching impact on organisations, making it a priority for security professionals to address.

2. Prevalence and Ease of Exploitation

One of the reasons HTTP Response Splitting is in the SANS Top 25 is its widespread nature and the relative ease with which it can be exploited. The vulnerability typically arises due to a failure in proper input validation or sanitisation. As such, it is common in web applications that do not rigorously check user-supplied input, especially in HTTP headers.

Attackers can exploit the flaw with simple tools like Burp Suite, OWASP ZAP, or even manual testing, making it easy for them to test and trigger this vulnerability in web applications. This accessibility to attackers increases its threat level and underscores the need for developers and penetration testers to prioritise addressing this weakness.

3. Impact on Web Application Security

HTTP Response Splitting directly impacts critical aspects of web application security:

Data Integrity: The ability to inject content into HTTP responses can result in data corruption or injection of malicious code, undermining the integrity of the application’s response to users.
Bypassing Security Filters: Attackers can use response splitting to bypass filters and protections, such as web application firewalls (WAFs) or proxy servers. This allows malicious content to reach the user, even if the server employs basic security mechanisms.
Impact on User Trust: When users are redirected to malicious sites or see unexpected content, their trust in the application is eroded, leading to reputational damage for businesses.

4. Difficulty in Detection and Mitigation

HTTP Response Splitting often remains undetected because it may not manifest in a way that immediately alerts security teams. The server may still respond correctly in the majority of cases, with attackers exploiting the vulnerability on specific conditions or hidden within regular application flows.

Moreover, mitigating the vulnerability requires a combination of proper input sanitisation, secure coding practices, and thorough testing, which many applications fail to implement adequately. This makes it a persistent and insidious issue in many environments.

5. Potential for Cascading Security Issues

Exploiting HTTP Response Splitting can lead to cascading failures in security. A small oversight in input validation can open the door to various other vulnerabilities, including but not limited to:

Cache poisoning, which can propagate attacks to multiple users without requiring any further effort from the attacker.
Cross-Site Request Forgery (CSRF), when combined with session-based attacks.
Information disclosure due to poorly handled headers or improper error responses.

This interconnectedness with other vulnerabilities increases the overall risk and potential damage to organisations.

6. Compliance and Legal Concerns

Many organisations must adhere to strict compliance standards, such as the General Data Protection Regulation (GDPR), HIPAA, or PCI DSS. Failure to protect against HTTP Response Splitting may lead to security breaches that expose personal or financial data, resulting in legal consequences and regulatory fines.

For instance, if attackers leverage this vulnerability to exfiltrate personal data or access sensitive financial information, the company could face significant legal repercussions for failing to secure user data in accordance with the law.

7. Cumulative Risk in Complex Applications

As web applications become more complex, the possibility of HTTP Response Splitting increases. Web applications often involve multiple components, such as APIs, microservices, and third-party integrations, all of which may inadvertently contribute to vulnerabilities like CWE-113. In these complex environments, attackers can exploit small weaknesses to escalate their attack, causing even larger breaches. This multi-faceted risk escalates the importance of addressing HTTP Response Splitting in comprehensive security testing.

11. Final Thoughts

HTTP Response Splitting (CWE-113) may seem like a niche vulnerability, but its potential impact on business operations cannot be understated. Penetration testers play a crucial role in identifying and mitigating this threat by understanding its mechanics, detecting its signs, and employing best practices for remediation. Through careful testing and proactive security measures, businesses can prevent the devastating consequences that often arise from this vulnerability.

By embracing a comprehensive security approach, from prevention to detection, penetration testers ensure that web applications remain secure, safeguarding not just technical infrastructure but also the business itself.

The inclusion of CWE-113: HTTP Response Splitting in the SANS Top 25 Most Dangerous Software Errors is well justified, given the wide range of attacks it enables, its prevalence in real-world applications, and the difficulty in detecting and mitigating it. As a vulnerability that directly affects the integrity and security of web applications, it has the potential to compromise sensitive user data, damage organisational reputation, and lead to significant financial losses. For penetration testers, developers, and security professionals, understanding and addressing this vulnerability is critical to maintaining the security and trustworthiness of web applications.

By incorporating robust input validation, secure coding practices, and comprehensive testing procedures, organisations can significantly reduce the risk of HTTP Response Splitting, safeguarding both their users and their business from the far-reaching consequences of this vulnerability.

Table of Contents