Even leaving aside the question of whether the FBI actually compromised Tor, or just did something way more mundane like infect the defendant's PC with malware, the linked court document is really interesting.
Not only does the FBI decline to say how they determined what IP address the defendant used to access the Tor hidden service, but they're also trying to hide the fact that the defendant asked to see that information by requesting the court label the defendant's court filing itself a "highly sensitive document"? And the court granted that request? Is that normal? It seems really bizarre to me, but I'm not a lawyer.
If becomes widely known that the government can de-cloak Tor users, that will change the behaviour of their targets and so hurt their surveillance, so it's fairly common that the government wants to hide this fact. (same with stingrays, for example).
But just wanting to keep it secret is not enough. So they will claim that this has national security implications, saying some targets are terrorists. And courts defer very heavily to the government in this area, so the FBI might be successful.
Because just knowing that there is a break is enough to tip someone off, the FBI can and courts likely would classify a request for details as well as the details themselves.
>If becomes widely known that the government can de-cloak Tor users, that will change the behaviour of their targets and so hurt their surveillance, so it's fairly common that the government wants to hide this fact. (same with stingrays, for example).
As was discussed verbally at Defcon, a huge chunk of the exit nodes are either in the US or EU. Same for guards.
(The whole GCHQ vs several EU countries trying to do intel in parallel without a shared intelligence agency thing is perpetually amusing.)
Not only does the FBI decline to say how they determined what IP address the defendant used to access the Tor hidden service, but they're also trying to hide the fact that the defendant asked to see that information by requesting the court label the defendant's court filing itself a "highly sensitive document"? And the court granted that request? Is that normal? It seems really bizarre to me, but I'm not a lawyer.