Today, the owner of the rather popular Web Developer toolbar noticed it had been hijacked and replaced with a malicious version. He disabled it, but the malicious version (diff) had been in the wild for at least an hour already.

  • What did it do while it was active (besides ad injection)?
  • What should I do to keep further ill-effects at bay?
    • Do I need to sign out/in of sessions for any webapps? Revoke keys? Change passwords?
  • Since the code loaded javascript from remote servers that may have changed over time, it's looking a little like the worst case may be helpful reading. Aug 2, 2017 at 16:58

4 Answers 4


They basically replaced advertisements and gathered Cloudflare credentials.

Proof Point has an article with a thorough analysis about what happened.


From the Twitter account you have linked, the author says:

I don’t know for sure yet, but it looks like it may have just been adware being injected. I’m still looking into the possible impact.

You have also asked whatever you should sign out of sessions, revoke keys and change passwords. While I do not have access to the malicious code, and am unable to tell what the code did, I feel pretty confident to suggest that you should end all sessions (to prevent session hijacks), and to change your passwords.

I would say that it is much easier to change passwords than later have to deal with your accounts hacked.


An extension such as Web Developer has access to pretty much everything that is happening in the browser. It can read all the site content, it can intercept traffic, sniff keystrokes, or whatever you can imagine. Here is a pretty good write-up.

What it actually did will need further analysis. The owner should be able to give some details, but it is really hard to completely assess the scope of this. Here is why: the extension had the ability to inject ads and according to a tweet it also injected some JS content that changed over time.

What should I do to keep further ill-effects at bay?

Well, this is hard to answer. Given the possible scope, the safest bet is to change any password, API keys you used when the rogue extension was active. Signing in and out is also a good idea as your session could have been compromised as well. Basically, anything that appeared in your browser while the extension was active could have been relayed somewhere else.

  • 1
    Thanks. You mention anything that "appeared in my browser." How about things that I did not navigate to or have loaded when the extension ran? Could it load a hidden iframe and attempt lots of popular websites to see if I had "remember me" sessions and click around in the ones that did? Aug 2, 2017 at 17:14
  • @Michael, I think non-visited sites are not in immediate danger. An extension has two ways of accessing such sites. It could, as you suggested, load a hidden iframe or open it from the extension. Looking at the diff, the latter does not seem to apply. The former should be pretty limited too, as most secure sites do not allow framing (X-Frame-Options header) and the Same Origin Policy would prevent tampering with the ones that do. The only thing left is CSRF attacks. Their effectiveness depends on the defenses deployed (CSRF tokens, referrer checking, etc). Aug 3, 2017 at 7:20

Without analysing the malicious version of the extension, it's impossible to say what it did. However, as an extension, it wouldn't have most of the limitations which apply to scripts on web pages: for example, the same origin policy wouldn't apply, meaning that any data you've entered on any page whilst the extension was active on your system could have been grabbed and sent to the attacker.

Chrome provides a bunch of other APIs to extensions which allow for access to other data within the browser: https://developer.chrome.com/extensions/declare_permissions These include access to cookies (both for sites which are visited while the extension is active, and those stored for other sites), access to the privacy controls for the browser, access to DNS settings for the browser, and access to the geolocation data, without requiring a prompt. I'm not sure which of these the web developer extension used, but given the target usage for it, it's likely that it could at the very least access cookie data.

Given that, any sites where you have an active session, even if you didn't visit them while the malicious extension was active, could have been compromised. Any passwords you entered while the extension was active could have been compromised. Any data you entered in other fields (chat windows, search boxes, file uploads) could have been compromised. Any data shown to you by a website could have been compromised. And that's just from the minimal set of permissions that the extension would need to work!

If more permissions were given (whether the legitimate extension required them, or if the malicious version prompted for and was given them), the possibilities get worse. It could turn off safe browsing mode, so you don't get warnings on malicious websites which Google has detected, for one.

It all depends on what permissions the extension had...

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