this post was submitted on 08 May 2024
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Eh, there's 20 different ways to detect DHCP Option 121 fuckery and once you know it's happening its fairly trivial to stop. Any VPN client worth its salt will be updated in 60 days or less to fix this and existing VPN clients can be hardened against TunnelVision with some fairly simple scripting.
It's a serious vulnerability but it's hardly the unfixable world ender that the media has made it out to be.
Good to know. Got any specific sources for the scripting, or should I just search for something like "option 121 mitigation?"
I don't know if there's any pre-built scripting out there (yet) for this but it's relatively straight forward in Windows to use powershell and either look in the registry for the assigned dhcp options ( HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Dhcp\Parameters\Options) or check the routing table for illogical routes.
Assuming that you aren't using split tunneling you could also have powershell check your external IP address for the expected result.
Another possibility is to grab the dhcp test tool from Github, run it in non-interactive mode and then parse it's output. Something I find VERY interesting is that Andrey Baranov specifically added Option 121 to that tool in March of 2023!
With any of those it's a matter of what you want to have happen when you detect the problem such as warning the user and disconnecting the vpn or attempting to mitigate the problem by reconfiguring the routing table.
I should point out that Option 121 is a legit thing and it does have valid uses so you can't assume something nefarious just because it's being used.
I'll probably be scripting up a remediation over the next few days, I'll try and remember to come back and share what I did.
I'd appreciate it! It's interesting, if nothing else.
I'll have to read more about Option 121, because multiple sources said that Option 121 is used for "legitimate things," and yet Android doesn't implement it. Makes me wonder why Android doesn't have it, but other implementations do.