If you're using Mist browser version Beta v0.9.3 or below, you should avoid browsing to untrusted sites.
According to a blog post published a few hours ago on Ethereum blog:
Due to a Chromium vulnerability affecting all released versions of the Mist Browser Beta v0.9.3 and below, we are issuing this alert warning users not to browse untrusted websites with Mist Browser Beta at this time. Users of “Ethereum Wallet” desktop app are not affected.
The problem relies not on the Mist code itself, but somewhere in the middle of the stack development, namely at the Electron level. Citing more form the blog post above, we learned that:
The layer between Mist and Chromium, Electron, is a project led by GitHub that aims to ease the creation of cross-platform applications using JavaScript. Recently, Electron hasn’t kept up to date with Chromium, leading to an increasing potential attack surface as time passes.
A core problem with the current architecture is that any 0-day Chromium vulnerability is several patch-steps away from Mist: first Chromium needs to be patched, then Electron needs to update the Chromium version, and finally, Mist needs to update to the new Electron version.
So, if you do you use Mist, better stay safe than be sorry.
I'm a serial entrepreneur, blogger and ultrarunner. You can find me mainly on my blog at Dragos Roua where I write about productivity, business, relationships and running. Here on Steemit you may stay updated by following me 
You can also vote for me as witness here:
https://steemit.com/~witnesses
If you're new to Steemit, you may find these articles relevant (that's also part of my witness activity to support new members of the platform):
