A third party manages access to a web application I’m supposed to begin using. While accessible from the Internet, they whitelist IP addresses, so it rejects the login credentials if not coming from a whitelisted address.

I provided my external IP address to their support team but the application was not letting me in, so I called their help desk. A support technician said that my IP address was 10.x.x.x, a private IP address. I’m not on the same network as this application, so I did a quick check and realized they are reading my internal IP address from my X-FORWARDED-FOR (XFF) header (yes, my employer exposes this).

I explain to him that the application is incorrectly reading my external (connection) IP address and is instead reading my internal IP address from my XFF header. I also explain that it’s not a good idea to add a private IP address to their whitelist as it somewhat defeats the point as anyone can assign that IP address within their network and expose it via an XFF header.

After talking to numerous support personnel, I came to the conclusion that not a single support person on their team understands basic networking and private IP address ranges.

I finally just said, “Fine. Go ahead and add my internal IP address but keep in mind it will change a lot.”

He then proceeded to “explain” to me how my IP address is assigned by my ISP and should change very infrequently. I explained to him that the IP address their application is reading is actually assigned by DHCP inside my network, but I was clearly wasting my breath.

  • 12
    Make a "responsible disclosure" to someone higher up than support monkeys. If you get no joy, post full PoC publicly.
  • 3
    Yes, you should spoof that IP range and let them know how secure they access control is
  • 10
    On the other hand, start by trying to get your it to not expose internal ips.

    Its a security weakness that will make it easier for an aggressor.

    And once that is done they have to change to your external ip.
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