More malicious extensions in Chrome Web Store

Two weeks ago I wrote about the PDF Toolbox extension containing obfuscated malicious code. Despite reporting the issue to Google via two different channels, the extension remains online. It even gained a considerable number of users after I published my article.

A reader tipped me off however that the Zoom Plus extension also makes a request to serasearchtop[.]com. I checked it out and found two other versions of the same malicious code. And I found more extensions in Chrome Web Store which are using it.

So now we are at 18 malicious extensions with a combined user count of 55 million. The most popular of these extensions are Autoskip for Youtube, Crystal Ad block and Brisk VPN: nine, six and five million users respectively.

Update (2023-06-01): With an increased sample I was able to find some more extensions. Also, Lukas Andersson did some research into manipulated extension ratings in Chrome Web Store and pointed out that other extensions exhibited similar patterns in their review. With his help I was able to identify yet another variant of this malicious code and a bunch more malicious extensions. So now we are at 34 malicious extensions and 87 million users.

Update (2023-06-02): All but eight of these extensions have been removed from Chrome Web Store. These eight extensions are considerably different from the rest, so I published a follow-up blog post discussing the technical aspects here.

The extensions

So far I could identify the following 34 malicious extensions. Most of them are listed as “Featured” in Chrome Web Store. User counts reflect the state for 2023-05-30.

Update (2023-06-12): The complete list of extension IDs from this article series can be found here. This repository also contains the check-extensions command-line utility which will search local browser profiles for these extensions.

Name Weekly active users Extension ID
Autoskip for Youtube 9,008,298 lgjdgmdbfhobkdbcjnpnlmhnplnidkkp
Soundboost 6,925,522 chmfnmjfghjpdamlofhlonnnnokkpbao
Crystal Ad block 6,869,278 lklmhefoneonjalpjcnhaidnodopinib
Brisk VPN 5,595,420 ciifcakemmcbbdpmljdohdmbodagmela
Clipboard Helper 3,499,233 meljmedplehjlnnaempfdoecookjenph
Maxi Refresher 3,483,639 lipmdblppejomolopniipdjlpfjcojob
Quick Translation 2,797,773 lmcboojgmmaafdmgacncdpjnpnnhpmei
Easyview Reader view 2,786,137 icnekagcncdgpdnpoecofjinkplbnocm
PDF toolbox 2,782,790 bahogceckgcanpcoabcdgmoidngedmfo
Epsilon Ad blocker 2,571,050 bkpdalonclochcahhipekbnedhklcdnp
Craft Cursors 2,437,224 magnkhldhhgdlhikeighmhlhonpmlolk
Alfablocker ad blocker 2,430,636 edadmcnnkkkgmofibeehgaffppadbnbi
Zoom Plus 2,370,645 ajneghihjbebmnljfhlpdmjjpifeaokc
Base Image Downloader 2,366,136 nadenkhojomjfdcppbhhncbfakfjiabp
Clickish fun cursors 2,353,436 pbdpfhmbdldfoioggnphkiocpidecmbp
Cursor-A custom cursor 2,237,147 hdgdghnfcappcodemanhafioghjhlbpb
Amazing Dark Mode 2,228,049 fbjfihoienmhbjflbobnmimfijpngkpa
Maximum Color Changer for Youtube 2,226,293 kjeffohcijbnlkgoaibmdcfconakaajm
Awesome Auto Refresh 2,222,284 djmpbcihmblfdlkcfncodakgopmpgpgh
Venus Adblock 1,973,783 obeokabcpoilgegepbhlcleanmpgkhcp
Adblock Dragon 1,967,202 mcmdolplhpeopapnlpbjceoofpgmkahc
Readl Reader mode 1,852,707 dppnhoaonckcimpejpjodcdoenfjleme
Volume Frenzy 1,626,760 idgncaddojiejegdmkofblgplkgmeipk
Image download center 1,493,741 deebfeldnfhemlnidojiiidadkgnglpi
Font Customizer 1,471,726 gfbgiekofllpkpaoadjhbbfnljbcimoh
Easy Undo Closed Tabs 1,460,691 pbebadpeajadcmaoofljnnfgofehnpeo
Screence screen recorder 1,459,488 flmihfcdcgigpfcfjpdcniidbfnffdcf
OneCleaner 1,457,548 pinnfpbpjancnbidnnhpemakncopaega
Repeat button 1,456,013 iicpikopjmmincpjkckdngpkmlcchold
Leap Video Downloader 1,454,917 bjlcpoknpgaoaollojjdnbdojdclidkh
Tap Image Downloader 1,451,822 okclicinnbnfkgchommiamjnkjcibfid
Qspeed Video Speed Controller 732,250 pcjmcnhpobkjnhajhhleejfmpeoahclc
HyperVolume 592,479 hinhmojdkodmficpockledafoeodokmc
Light picture-in-picture 172,931 gcnceeflimggoamelclcbhcdggcmnglm

Note that this list is unlikely to be complete. It’s based on a sample of roughly 1,600 extensions that I have locally, not all the Chrome Web Store contents.

The malicious code

There is a detailed discussion of the malicious code in my previous article. I couldn’t find any other extension using the same code as PDF Toolbox, but the two variants I discovered now are very similar. There are minor differences:

  • First variant masquerades as Mozilla’s WebExtension browser API Polyfill. The “config” download address is<Extension_ID>/polyfill.json, and the mangled timestamp preventing downloads within the first 24 hours is localStorage.polyfill.
  • The second variant masquerades as Day.js library. It downloads data from<Extension_ID>/locale.json and stores the mangled timestamp in localStorage.locale.

Both variants keep the code of the original module, the malicious code has been added on top. The WebExtension Polyfill variant appears to be older: the extensions using it usually had their latest release end of 2021 or early in 2022. The extensions using the Day.js variant are newer, and the code has been obfuscated more thoroughly here.

The extension logic remains exactly the same however. Its purpose is making two very specific function calls, from the look of it: chrome.tabs.onUpdated.addListener and chrome.tabs.executeScript. So these extensions are meant to inject some arbitrary JavaScript code into every website you visit.

What does it actually do?

As with PDF Toolbox, I cannot observe the malicious code in action. The configuration data produced by serasearchtop[.]com is always empty for me. Maybe it’s not currently active, maybe it only activates some time after installation, or maybe I have to be in a specific geographic region. Impossible to tell.

So I went checking out what other people say. Many reviews for these extensions appear to be fake. There are also just as many reviews complaining about functional issues: people notice that these extensions aren’t really being developed. Finally, a bunch of Brisk VPN reviews mention the extension being malicious, sadly without explaining how they noticed.

But I found my answer in the reviews for the Image Download Center extension:

Review by Sastharam Ravendran in July 2021: SPAM. Please avoid. Few days after install, my search results in google were randomly being re-directed elsewhere. I was lost and clueless. I disabled all extensions and enabled them one by one to catch this culprit. Hate it when extension developers, use us as baits for such things. google should check and take action ! A reply by Mike Pemberton in January 2022: had the same happen to me with this extension from the Micrsoft edge store. Another reply by Ande Walsh in September 2021: This guy is right. This is a dirty extension that installs malware. AVOID.

So it would seem that at least back in 2021 (yes, almost two years ago) the monetization approach of this extension was redirecting search pages. I’m pretty certain that these users reported the extension back then, yet here we still are. Yes, I’ve never heard about the “Report abuse” link in Chrome Web Store producing any result. Maybe it is a fake form only meant to increase customer satisfaction?

There is a similar two years old review on the OneCleaner extension:

Review by Vincent Descamps: Re-adding it to alert people: had to remove it, contains a malware redirecting to bing search engine when searching something on google using bullcrap

Small correction: the website in question was actually called CharmSearching[.]com. If you search for it, you’ll find plenty discussions on how to remove malware from your computer. The domain is no longer active, but this likely merely means that they switched to a less known name. Like… well, maybe serasearchtop[.]com. No proof, but serasearchtop[.]com/search/?q=test redirects to Google.

Mind you: just because these extensions monetized by redirecting search pages two years ago, it doesn’t mean that they still limit themselves to it now. There are way more dangerous things one can do with the power to inject arbitrary JavaScript code into each and every website.


  • Jeroen

    Thanks for your interesting article (and the other ones too!). I noticed the same things in extensions. I also discovered two plugins which inserted malicious code on facebook, pinterest and other social media websites, that sends stuff back to 'home'. Another plugin had become a giant affiliate-tracker, which rewrote urls of a bunch a of ecommerce sites to include their own affiliatecode to make money. I believe this last one was a screenshot extension. I also contacted Google multiple times through multiple channels, but there was no reply whatsoever. Only when I replied to a warning-email about one of my own extensions (I had to update my privacy policy) I finally got a human response. Then I addressed the issue that I found on the screenshot extension. Finally, after emailing a bit, they even took the extension offline. So there actually are some employees who care about the users, but it seems that the formal channels to report something are not (very actively) monitored or responded too.

  • Lukas

    Very interesting find! I'm currently working on a project where I try to detect reputation manipulation in the Chrome store and construct clusters of extensions with similar patterns of seemingly faked or incentivized reviews. 16 out of 18 of the extensions you mentioned are flagged in my project as suspected targets of reputation manipulation; when tracing back through the clusters created of the said 16 extensions, another 40 extensions are flagged for having similar suspected reputation manipulation, indicating the same group or individual is behind the action.

  • RatoGBM

    I actually had Font Customizer installed, thanks.

    I myself actually found an interesting extension called NoteTab, it changes your default search engine to their own that basically sends your query to the server just to be redirected to

    Wladimir Palant

    Yes, NoteTab changes the default search engine by “official” means. This is clearly something that can and should be indicated during installation, but for some reason I can see no such warning.

  • Anonysafe

    Thanks for keeping us safe, Google

  • Dominic Hébert

    Is it the same problem with Microsoft Edge Chromium based?

    Wladimir Palant

    Yes, at least some of these extensions are available via Microsoft’s Edge Add-ons site – then with a different extension ID. Also, from what I know Edge users can install extensions from Chrome Web Store.

  • Denis

    Maybe you can write a small extension for Chrome that will look at the names of installed extensions in the browser and, if it is on your list of malicious ones, warn the user about it with a brief description (for example, tracks user actions, opens hidden tabs to cheat visits, and so on)? And then the user himself decides to delete the extension or not.

    Wladimir Palant

    Actually, this already exists. It’s even built into the browser: Google Safe Browsing. It only needs to be used with these extensions…

  • Martin Valjavec

    Is this similar to (or in any way related to) the problems mentioned here:

    I guess, this means at least that Google had info earlier on that "something strange" is happening.

    Wladimir Palant

    No. I checked out that Downloader for Instagram extension (no longer on Chrome Web Store, downloaded it elsewhere), it’s different code and not related.

  • Denis

    Can I have a link to your extension in the store?

    Wladimir Palant

    I’m not talking about an extension, it’s a browser feature. Google merely has to activate it for these extensions.

  • Geoff

    But wait! There's more! These extension seem to have the same functionality :

    Blaze VPN - anenfchlanlnhmjibebhkgbnelojooic Snap VPN - nkklhdhlfknnhmmldffbofbbomlicpig Safum VPN - kbdlpfmnciffgllhfijijnakeipkngbe

    I'm not sure why these guys are getting approved, and a "Featured" badge, with obfuscated code when it is clearly against the guidelines.

    Wladimir Palant

    I’ve only looked into the first of these extensions. Yes, it looks similar, and there are indicators that it’s authored by the same people. It does not seem to contain any malicious functionality at this point however.

  • Geoff

    What is it you're looking at? I'm looking at extensions like fcfhplploccackoneaefokcmbjfbkenj and see obfuscated code and things like :

    function w(e) { if ("function" == typeof e) return null != /^function\s+\w\s(\s)\s{\s+[native code]\s+}$/i.exec( }


    var o, s =, a = /[object (.*)]/i.exec(s);

    and think to myself that's super shady.

    Wladimir Palant

    This code isn’t obfuscated, merely minified. The former detects whether a function is native – similar code can be found in jQuery for example, it’s used for browser feature detection. jQuery also has code similar to the latter, used to recognize plain objects.