Http response division: understanding, detecting and stopping a classic web exploitation
The HTTP response partition is a web application vulnerability that occurs when unvalidated user input HTTP response is included in the header. This defect allows an attacker to inject an additional HTTP header or manipulate the structure of an HTTP reaction. It often results in cross-site scripting (XSS), web cache poisoning, or redirect attacks.
While the HTTP response partition has been around since the early 2000s, it is relevant today due to poorly validated web applications and its appearance in APIs. Modern structures reduce the possibility of such weaknesses, but misunderstandings, old libraries, or custom implementation can still make applications weak.
This blog examines history, technical functioning, detection methods, attack scenarios, and prevention strategies for HTTP response division.
History of the HTTP Reaction Division
The vulgarity was first publicly documented by security researcher Amit Klein in 2004. He displayed that improper handling of carriage returns (CR) and line feed (LF) characters in HTTP header could be exploited to manipulate server reactions.
At that time, many applications took the data supplied by the user—such as the URL, cookies, or query parameters—and placed it in the HTTP response without direct hygiene. This made it easy to inject CRLF sequences (%0D%0A) for attackers and divided the response into two, effectively creating a completely new HTTP response that the attacker controlled.
The effect ranged from cache poisoning to frequent cross-site scripting. Although awareness and better framework default have reduced its frequency, modern attacks often target heritage systems, custom-made handling codes, or improperly configured APIs.
Understanding http reaction division
How does http work in this context
When a web server reacts to an HTTP request, it sends a position line, a set of headers, and alternately sends a message body. For example:
bash
HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 123
<html>...</html>
In HTTP Response Splitting, the attacker injects CR (\r) and LF (\n) characters into header values, which tricks the server into thinking the headers have ended and a new response is starting. This allows the attacker to insert arbitrary headers or even an entire fake HTTP body.
Example Scenario
A common example is manipulating the Location header used in HTTP redirection:
php
header("Location: " . $_GET['url']);
If the application fails to sanitize input and the attacker provides:
perl
http://example.com%0d%0aContent-Length:0%0d%0a%0d%0a<script>alert('XSS')</script>
The %0d%0a sequences represent CRLF. The browser interprets this as a split response, allowing injected HTML or JavaScript to execute.
How Attackers Exploit HTTP Response Splitting
1. Identify a Vulnerable Header
The attacker looks for application features that take user input and reflect it in an HTTP response header, such as:
- Redirect URLs
- Cookies
- Content-Disposition filenames
- Custom headers
2. Inject CRLF characters
Attode in the attacker request as %0d %0a, which decodes in Newline in the server header.
3. A malicious second reaction craft
The attacker combines arbitrary header and body content after CRLF injection, possibly providing malicious material to the victim.
Real-world attack results
- Web Cache poisoning—injecting a fake response that is cast by a proxy or CDN, serving other users malicious materials.
- Cross-site scripting (XSS)—adding client-side code execution, adding HTML or JavaScript to a fake response.
- Open Redirect Exploitation—Changing the location header to redirect victims on malicious sites.
- Information Disclosure—Manipulating headers to bypass security controls or leak sensitive server data.
How to detect http response division
1. Manual test
A security examiner can send a payload containing %0d%0a in input parameters that can end in HTTP header. For example:
ruby
GET /?url=test%0d%0aSet-Cookie:attacker=1 HTTP/1.1
Host: example.com
If the response contains a new header or structure, the application may be vulnerable.
2. Automatic scanner
Tools such as Burp Suite, OWASP ZAP, and Acunetix can detect the weaknesses of CRLF injections by analyzing the resulting HTTP response structure.
3. Log Analysis
Developers can review server logs for suspicious sequences like %0d%0a, which may indicate attempted injection attacks.
4. Code Review
Examining source code for instances where:
- User input is directly inserted into HTTP headers without sanitization.
- Custom header-handling functions are implemented instead of relying on framework defaults.
Prevention strategies
1. Input verification and hygiene
- Remove CR (\r) and LF (\n) characters from the user input before inclusion in the header.
- Reject any input with encoded CRLF sequences (%0d%0a).
2. Use Framework-Safe Functions
Modern web frameworks like Django, Express, Laravel, and Spring handle header encoding safely. Avoid custom header-writing code unless necessary.
3. Enforce Whitelisting
For redirect URLs or similar inputs, use a whitelist of allowed destinations instead of accepting arbitrary user input.
4. Output encoding
If the dynamic material is inserted into the header (eg, file names in the content explosion), encoded values prevent unexpected control characters.
5. Security test in CI/CD
Integrate vulnerable scanning in the build process so that issues of CRLF injection are caught before deployment.
conclusion
The HTTP reaction partition remains an important web security risk when the apps incorrectly induce incredible inputs in the app HTTP header. Although it was first exposed two decades ago, this vulnerability continues to revive in modern systems—often due to custom implementation, heritage code, or insufficient testing.
By combining safe coding practices, automatic scanning, and input verification, developers can prevent this attack from being a viable danger. Understanding and attacking its history ensures mechanics and the security professional attacker's playbook remain alert against one of the oldest tricks.
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