this post was submitted on 18 Mar 2026
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...oh.
How long before that data gets sold?
-1 year
The data is indeed encrypted, but both you and the VPN provider have the keys - that's why they advertise no-logs policies, because they have access to the data you send, such as which website you're attempting to visit.
Can a VPN provider do man in the middle attacks if they wanted to? Like sniff my /api/login calls and get my password? My gut tells me yes but I don't know enough to be sure, I feel.
Generally not. Anything with authentication would be using HTTPS encryption. So there will be two layers of encryption: the VPN encryption and the web site's HTTPS encryption. The VPN provider can't replace the HTTPS encryption because your browser would identify it as being encrypted with the wrong certificate and it would block the connection.
Although...given that they control the browser, too, I suppose they could code it to remove those safeguards, but that would not go unnoticed for long.
When you use a VPN, it basically replaces your ISP as the intermediary who can snoop all your traffic, so the real question is who do you trust more: your ISP or Mozilla?
What I was thinking was that the VPN would be able to sniff the key exchange hand shake thing that HTTPS does in order to setup the connection, and use the data during the handshake to basically recreate the cryptographic key used for the connection later. So then it'd be able to just decrypt all traffic and reencrypt it again after passing through. Seamlessly to both ends.
Is that a thing in theory?
The SSL/TLS encryption used on the web is designed to prevent that type of attack. It's called "asymmetric encryption", because a different key is used to encrypt and decrypt data. So each side has both a private and public key. You can broadcast your public key, and then anyone can use it to encrypt a message that only YOU can decrypt, with your private key. The private key never goes over the network, so the VPN provider can't intercept messages meant for you.
It's possible to spoof the server's certificate and make your browser think it's talking to a real server when it's not. However, this is difficult, because certificates follow a chain of trust, so they'd either need to compromise one of the certificate authorities (plausible for state-level actors), or compromise your web browser/OS so that it trusts a fake authority (definitely possible for a browser/OS vendor, but again, it would be noticed, especially in an open-source product like Firefox).
There are lot of enterprise products out there that perform this kind of "attack" so that businesses can monitor their network traffic. They do this by requiring adding their certificate authority to the list of trusted certs in the OS. This is often called "deep packet inspection". If you try to use these products without adding their certs, your browser will throw a fit every time you load a site because it will see that the certificate doesn't match.
Thank you for finally making how ssl/tls works click.
Interesting, okay. Just to be clear, if the middle man, i.e. the VPN, were to listen in on the data exchanged during the TLS handshake, wouldn't it be able to recreate both parties' key pairs simply using the same algorithm? If it has all the pieces, so to speak. Or is the tech resistant to that as well?
This would require the server's certificate, which the VPN wouldn't (generally) have access to.
The key part of this is that server certificates are trusted and verifiable. To the extent that's true, the process is secure against man-in-the-middle attacks. That's because the client browser can confirm that the server's certificate is legitimate, and encrypt its own public key with that server's public key, so nobody in the middle can read it.
If a man in the middle replaced the encryption keys with their own, then the client would know about it, because it can verify the server's encryption key and of course it knows its own private key, which has never been transmitted over the wire.
You can read more details on the process here: https://www.ssl.com/article/ssl-tls-handshake-ensuring-secure-online-interactions/
Of course nothing is 100% secure. God only knows how many third parties (like government intelligence agencies) have compromised major certificate authorities. If you have access a certificate authority's private keys, then you can spoof basically anything.
assume the VPN provider is adversarial
now re-run your analysis
This is true regardless. HTTPS encryption keeps a man in the middle from seeing your URL. They just get the domain name, which is a lot, but it isn't your credentials.
They don't know the URL but they know the IP address so ... yes, they also can get the URL
IP addresses do not map to URLs.
They might map to domains, but not necessarily.
You cannot reliably get the URL from an IP, there's no direct mapping, especially with shared hosting.
When you connect to a secure https site, then no. When you connect to unsecured http, then yes.
Firefox has its own cert store for https, and could theoretically impersonste other websites by adding their own and then sniff your traffic. But thats not how vpn providers typically work, and thered be huge backlash if they were to do that unprompted.
We had a proxy server at work that would route all internet traffic and scan for viruses, blocked urls or other traffic patterns, depending on your network rules. It did work on https and SSL traffic, because you had to accept the cert from the proxy server in your browser. So your traffic was encrypted between proxy and webserver, and proxy and your computer, but unencrypted on the proxy server itself. It would be similar with a VPN. Plus, if you control the browser you could just ship the required certs with the update...
So a VPN could basically sniff the Diffie-Hellman keys used during the exchange, recreate the key that browser and server use for HTTPS, and then decrypt all traffic sent through the VPN? Is that correct? And basically the same goes for any ISP or whatever else that's acting as a node?
No, not at all. You have 2 encrypted connections A to B and B to C, where B is the proxy server. The proxy server decrypts AB, sees the plaintext traffic to check against rules, then reencrypts the traffic with his own key and forwards it to B to C. Your browser on C sees the proxy servers cert for BC, and the website and proxy handle out a different cert AB. No encryption or cert is broken during the process.
So if they were going to do an attack like this, they wouldn't do anything like the DH attack you're talking about, they'd have a custom CA in the browser's SSL root store. That root cert means they can generate a certificate for any website you visit, and that custom root cert would be how they decrypt your traffic.
Afaik there isn't a current attack on proper DH key pairings, but you can't block the custom certificate path at the browser level without some serious server side work/client side JS to validate
If you have to install any closed-source software to use the VPN, the answer is oh hell yes, they can install a root cert. If they are clever they can remove it when you disconnect, so it will not be noticed by most people.
Even if they require no proprietary install, by definition the VPN knows every IP address you connect to. Even if you use DoH. Even if you use Quad9 DNS. The VPN knows you visited midwestsluts.com
If you want privacy, either spin up your own selfhost OpenVPN, or use the Tor nodes myself and other volunteers pay for, to make free for you to use.
Tor node operators can't tell what site you are visiting (if they run an exit node they can see the site -- but don't know your IP; if they run a Guard/proxy node they can see your IP, but can't tell anything about what sites you visit or what data you get)
Joke's on them, because half the web is behind CloudFlare now.
What if the gov has access to both tor nodes, cloudfare infrastructure and mozilla servers?