CLZero: A project for fuzzing HTTP/1.1 CL.0 Request Smuggling Attack Vectors

Fuzzing with CLZero is a dedicated project designed for testing and identifying vulnerabilities in HTTP/1.1 CL.0 Request Smuggling Attack Vectors. This tool focuses on systematically probing and analyzing potential weaknesses, offering a valuable resource for enhancing the security of web applications.


usage: [-h] [-url URL] [-file FILE] [-index INDEX] [-verbose] [-no-color] [-resume] [-skipread] [-quiet] [-lb] [-config CONFIG] [-method METHOD]

CLZero by Moopinger

optional arguments:
  -h, --help      show this help message and exit
  -url URL        (-u), Single target URL.
  -file FILE      (-f), Files containing multiple targets.
  -index INDEX    (-i), Index start point when using a file list. Default is first line.
  -verbose        (-v), Enable verbose output.
  -no-color       Disable colors in HTTP Status
  -resume         Resume scan from last index place.
  -skipread       Skip the read response on smuggle requests, recommended. This will save a lot of time between requests. Ideal for targets with standard HTTP traffic.
  -quiet          (-q), Disable output. Only successful payloads will be written to ./payloads/
  -lb             Last byte sync method for least request latency. Due to the nature of the request, it cannot guarantee that the smuggle request will be processed first. Ideal for targets with a high
                  amount of traffic, and you do not mind sending multiple requests.
  -config CONFIG  (-c) Config file to load, see ./configs/ to create custom payloads
  -method METHOD  (-m) Method to use when sending the smuggle request. Default: POST

Regular ( is with some debugging enabled):

Skipping read on the Smuggle request ensures less time between requests:

More Speed with last-byte sync

What’s that… you need more speed? “She’s at max captain!” Just kidding. If the website has high amounts of traffic this will be your best bet. I have adapted the HTTP/1.1 last-byte sync race condition (No, not the cool one you heard about at Defcon this year, that requires HTTP/2). This saves even more time between the smuggle request and probe. However, there is a catch… We cannot guarantee which will be processed first by the server (The smuggle-gadget or the probe). Such is the nature of race conditions I guess. However if you don’t mind testing the same technique more than once on a high traffic site, this will be your best bet. And it should trigger within two or three requests. Strangely I have had better success with this than the skip-read technique.

single target attack:

  • python3 -u -c configs/ -skipread

  • python3 -u -c configs/ -lb

Multi target attack:

  • python3 -l urls.txt -c configs/ -skipread

  • python3 -l urls.txt -c configs/ -lb


git clone
cd CLZero
pip3 install -r requirements.txt

Download CLZero

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