Ubuntu – Do I need to be concerned about browser security

does firefox under ubuntu has something similar to activeX, in terms of security vulnerability?

‘ActiveX’ can be considered in two parts, the object model and the installation method. Firefox has something similar—and cross-platform compatible, Ubuntu or other—for both.

The object model of ActiveX is Microsoft COM; Firefox's equivalent is XPCOM. Many other Windows features and applications that are nothing to do with web browsing use MS COM, and there have traditionally been endless problems where COM controls that were not written for secure web usage were nonetheless available to web pages. This caused many compromises. Firefox is better off here as XPCOM is not shared with the rest of the system. Newer versions of IE have better controls for mitigating what sites are allowed to use what controls.

(As a side-issue, because many add-ons for Firefox are themselves written in JavaScript, a high-level scripting language, they are often more secure from buffer overflow and string handling errors than extensions for IE which are commonly written in C[++].)

The control-downloader part of ActiveX has also been cleaned up a bit since the bad old days when anything in the My Computer zone could install any software it liked, and aggressive loader scripts could trap you in an alert loop until you agreed to approve the ActiveX prompt. Firefox's equivalent, XPInstall, behaves largely similarly, with the ‘information bar’ on all but Mozilla's sites by default and a suitable warning/prompt before installation.

There is another built-in way you can compromise yourself in Mozilla: signed scripts. I have never seen this actually used, and certainly there'll be another warning window appear before a script gains extra rights, but it kind of worries me that this is available to web pages at all.

for example an exploit through flash will gain access to my pc under my user rights

Yes, the majority of web exploits today occur in plugins. Adobe Reader, Java(*) and QuickTime are the most popular/vulnerable. IMO: get rid of those, and use FlashBlock to only show Flash when you want it.

(*: and Java's dialogues before it lets you give up all security to some untrusted applet is a bit bare too.)

Ubuntu gives you some questionable plugins by default, in particular a media player plugin that will make every vulnerability in any of your media codecs exploitable through the web (similar to the Windows Media Player plugin, only potentially with many more formats). Whilst I have yet to meet an exploit targeting Linux like this, that's really only security through obscurity.

Note that ActiveX itself is no different. A web browser compromise based on ActiveX still only gives user-level access; it's only because prior to Vista everyone habitually ran everything as Administrator that this escalated to a full-on rooting.

and then follow to exploit some known vulnerability in X to gain root rights. that is not "easy".

Maybe, maybe not. But I think you'll find the damage some malware can do from even a normal user account is quite bad enough. Copy all your personal data, observe your keypresses, delete all your documents...

Wayland is supposed to be a complete replacement for X, not an addon, and it addresses the problems which concern you. Note that at the moment Wayland is not production ready and there is limited driver and toolkit support for it.

It seems that Ubuntu does not have any plans to integrate Wayland at the moment, and wants to create its own display server called Mir, which may also address some of your security concerns. Mir is also not ready for general use.

DirectFB is a bare bones solution for embedded systems which gives programs direct access to the video card's framebuffer. It does not have any security mechanisms.

Another solution is not to run a GUI at all.