Ending mixed scripting vulnerabilities

Ending mixed scripting vulnerabilities: Last year, we posted on the Google Online Security Blog about our desire to end mixed scripting vulnerabilities. A “mixed scripting” vulnerability affects HTTPS websites that are improperly implemented; these vulnerabilities are serious because they eliminate most of the security protections afforded by HTTPS. All web browsers have historically taken it upon themselves to try and work around these bugs by informing or protecting users in some way.

With the recent release of Chrome 21, we’ve taken several steps forward:

• We continue to protect end users by blocking mixed scripting conditions by default, but we now do it in a way that is less intrusive. This change minimizes “security dialog fatigue” and reduces the likelihood that users will expose themselves to risk by clicking through the warning. 


• We’ve improved resistance to so-called “clickjacking” attacks. Electing to run any mixed script is now a two-click process. 


• We now silently block mixed scripting conditions for websites that opt in to the HSTS security standard. This is the strongest default protection available. 

If you visit a non-HSTS web site with a mixed scripting condition, a new shield icon in the omnibox (to the right, next to the star) indicates that Chrome’s protection has kicked in:



You can click on the shield to see the option to run the mixed script, but we don’t recommend it. Instead, if you see the shield icon, we recommend contacting the website owners to make sure they know they may have a security vulnerability.

It has been an interesting journey to get to this point. For about a year, we blocked mixed scripting by default on Chrome’s Dev and Beta channel releases. Rolling out the block to Stable was more challenging because of widespread mixed scripting across the web. To move forward, we turned blocking on for certain web sites, starting with google.com. Later, we reached out to and then collaborated with twitter.com and facebook.com to opt them into blocking, too. All these websites hold themselves to a high standard of security, so this approach worked well. We later took the additional step of opting in sites to mixed script blocking for any site using the HSTS standard.

We bit the bullet and let full mixed script blocking for all sites hit Stable back in Chrome 19. Predictably, we uncovered a range of buggy web sites, and some users were confused about the “infobar” warning displayed by the older versions of Chrome:



Fortunately—and no doubt driven by the high visibility of this warning—some prominently affected websites were able to deploy quick fixes to resolve their mixed scripting vulnerabilities. This work aligns with one of our Core Security Principles: Make the web safer for everyone. Unfortunately, the warning confused some users, which conflicts with another principle: Don’t get in the way. (We’re sorry for any temporary disruption.)

With Chrome 21, we believe we’ve achieved a good balance between top-flight protection for end users, a pleasant UI experience, and notifications that help buggy websites improve their security.

Posted by Chris Evans and Tom Sepez, Software Engineers

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