On 20th of February, the Russian server got a small patch (which is scheduled to hit the EU server around 26.2. or so). This patch contains some small engine fixes, but also several Wargaming programs. These programs scan your client - much like the World of Warcraft software does - for illegal mods, possibly even for bots. Obviously, if it finds any, you'll be banned. Of course if you don't use any, you have nothing to fear.
What mods are banned:

- "laser sights" (red line pointing from the barrel)
- defoliants (leaf removers)
- 3D hitskins (which make tanks transparent and show the modules inside) - please note regular hitskins are NOT banned
- tracer changes/hacks
- free camera mods

Please note: some reputable mod sites (drongolab) have been offering them lately. That does NOT mean they are okay.