Security researchers have discovered what they believe may be a government attempt to covertly wiretap an instant messaging service in Germany — an attempt that was blown because the potential intercepting authorities failed to reissue a TLS certificate.
The suspected man-in-the-middle attack was identified when the administrator of jabber.ru, the largest Russian XMPP service, received a notification that one of the servers’ certificates had expired.
However, jabber.ru found no expired certificates on the server — as explained in a blog post by ValdikSS, a pseudonymous anti-censorship researcher based in Russia who collaborated on the investigation.
The expired certificate was instead discovered on a single port being used by the service to establish an encrypted Transport Layer Security (TLS) connection with users. Before it had expired, it would have allowed someone to decrypt the traffic being exchanged over the service.
The wiretap is believed to have lasted for up to 6 months, from April 18 through to October 19, although the researchers were only able to confirm 90 days of actual interception. “All jabber.ru and xmpp.ru communications between these dates should be assumed compromised,” wrote ValdikSS.
“Given the nature of the interception, the attacker have been able to execute any action as if it is executed from the authorized account, without knowing the account password. This means that the attacker could download account's roster, lifetime unencrypted server-side message history, send new messages or alter them in real time,” they added.
The researchers said they do not believe that the servers were hacked by criminals, but were reconfigured to facilitate the wiretapping as a result of a government request. “We believe this is lawful interception Hetzner and Linode were forced to setup,” ValdikSS wrote, referencing the hosting providers in Germany.
You can run your own private CA and sign things you access with it.
Another option is short lived certificates. If your certificate is only valid for 3 months and the certificate is compromised only the traffic in the certificate validity period is effected.
I believe that is the reason Let's Encrypt limits certificates to 3 months and google is pushing CA to stop issuing multi year certificates.
I'm trying to think of a way to make it obvious that there's another 'trusted' cert being served by a man in the middle. If I sign and trust my own, then no one else would have one from that chain.
However, the problem is still there. If someone MITM me, they can serve a Let's Encrypt cert and I'd trust it. If I don't trust any but my own cert then I can't really browse the web.
This is specifically what HSTS and certificate pinning does.
When those features are enabled your app or browser is told remember this certificate fingerprint for this domain and throw an error if a different cert is used.
There is also a draft standard where you put your details of your certificate in a DNS record so if a new chain is used a cert warning is thrown. But I don't remember the specifics.