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CRLF Injection

CRLF Injection


CRLF injection is a vulnerability where an attacker manages to inject a CRLF sequence (carriage return and line feed) into the response, allowing them to manipulate the response body and/or headers.

CRLF injection attacks include:

  • HTTP Response Splitting

An attack where an attacker inserts CRLF sequences into user input, aiming to manipulate the HTTP response generated by the web application. This can lead to various consequences, such as injecting malicious content or crafting misleading responses.

  • HTTP Header Injection

Involves injecting CRLF sequences into HTTP headers. This attack can lead to security issues, allowing an attacker to add or modify headers, potentially leading to cache poisoning, session fixation, or other forms of web application compromise.

  • Memcache Injection

A type of CRLF attack that targets systems utilizing Memcached, a distributed memory caching system. Attackers inject CRLF sequences to manipulate the content stored in Memcached, possibly leading to cache poisoning, information disclosure, or denial of service.

  • Server-Side Request Forgery:

A broader attack that sometimes involves CRLF injection. In this scenario, an attacker tricks the server into making requests to internal resources by injecting malicious input containing CRLF sequences. The attacker may exploit this to access sensitive information, pivot through internal systems, or perform unauthorized actions on behalf of the server.

import flask
from flask import request
from flask import make_response

app = flask.Flask(__name__)

def index():
    header = request.args.get("header")
    username = request.args.get("username")
    resp = make_response("Hello: %s" % username)
    resp.headers.set(header, username)
    return resp"", port=8080)
GET /?header=GET%20/?name=A:a%0a%0dA:d%0d%0d%0a%0dInjected-Header&username=abc HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0
Referrer: http://localhost/
HTTP/1.1 200 OK
Server: Werkzeug/2.3.7 Python/3.10.12
Date: Tue, 09 Jan 2024 11:03:05 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 10
GET /?name=A:a


Injected-Header: abc
Connection: close

Hello: abc


  • Upgrade to the latest software version: CRLF injection usually impacts the webserver or the reverse proxy itself, therefore it's advised to keep it up to date.
  • Avoid setting header name from user input: Allowing users to control http header names can lead to several security issues including CRLF injection.
  • User input sanitization: in some cases, it might be possible to achieve CRLF injection if the web application concatenates user input into response headers or cookies, therefore, user input should be sanitized from special characters.
GET /?page=login%0D%0ACustom-Header:%20vulnerable HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0
Referrer: http://localhost/
HTTP/1.1 200 OK
Date: Wed, 05 Jan 2024 12:00:00 GMT
Server: Apache/2.4.58 (Unix)
Content-Length: 1234
Content-Type: text/html; charset=UTF-8
Set-Cookie: page=login%0D%0ACustom-Header:%20vulnerable



  • GDPR:
    • ART_25
    • ART_32
    • REQ_6_4
    • REQ_6_5