Adblock Plus filter exploit to run arbitrary code discovered

Martin Brinkmann
Apr 16, 2019
Updated • Apr 16, 2019

Most content blockers use and load filter lists that include instructions to block or change certain content on visited sites in the web browser by default; this is done to ensure that default configurations do block a good chunk of unwanted content right away.

Most extensions support custom lists and individual filters. Users may load custom lists in most extensions and add their own filters to the list as well.

Update: Eyeo GMHB announced today that it will remove the $rewrite function going forward. Expect a new release soon that removes if from the extension. End

Security researcher Armin Sebastian discovered an exploit in certain adblockers such as Adblock Plus that could be used to run malicious code on sites visited in the browser.

adblock plus exploit

The exploit uses a filter option called $rewrite that Adblock Plus supports to inject arbitrary code in web pages. The $rewrite filter is used to replace code on sites by rewriting it. The filter option restricts the operation; it is designed to load content only from the first-party source and not third-party sites or servers, and some requests, e.g. script or object, are not permitted either.

Sebastian discovered a vulnerability in $rewrite that attackers may exploit to load content from remote locations. The conditions that need to be met are:

  1. A JavaScript string needs to be loaded using XMLHttpRequest or Fetch, and the return code must be executed.
  2. Origins cannot be restricted on the page, e.g. by using Content Security Policy directives, and the final request URL cannot be validated before execution.
  3. The origin of the code must have a server-side open redirect, or must host arbitrary user content.

Properties that match all three requirements include Google Maps, Gmail, or Google Images among others. A proof of concept was published on the author's website and you may try it on Google Maps to verify that it works.

I tried the exploit in Chrome and Firefox, and could not get it to work. Lawrence Abrams over on Bleeping Computer managed to get it to work though.

Closing Words

The attack has another requirement, as it relies on filters. A manipulated filter needs to be added to the list of filters used by the content blocker. The two most common options include users adding filters manually to their content blockers, or that a manipulated filter is on a filter list that gets loaded.

The second option seems more likely, especially in cases were users load other lists in the extensions. It is not the first time that lists get manipulated but it does not happen very often.

The extension uBlock Origin is not affected by the issue as it does not support $rewrite.

Adblock Plus filter exploit to run arbitrary code discovered
Article Name
Adblock Plus filter exploit to run arbitrary code discovered
Security researcher Armin Sebastian discovered an exploit in certain adblockers such as Adblock Plus that could be used to run malicious code on sites visited in the browser.
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  1. ilev said on August 4, 2012 at 7:53 pm

    Doesn’t Windows 8 know that www. or http:// are passe ?

    1. Martin Brinkmann said on August 4, 2012 at 7:57 pm

      Well it is a bit difficulty to distinguish between domains and files for instance.

    2. Leonidas Burton said on September 4, 2023 at 4:51 am

      I know a service made by google that is similar to Google bookmarks.

  2. VioletMoon said on August 16, 2023 at 5:26 pm

    @Ashwin–Thankful you delighted my comment; who knows how many “gamers” would have disagreed!

  3. Karl said on August 17, 2023 at 10:36 pm


    The comments section under this very article (3 comments) is identical to the comments section found under the following article:

    Not sure what the issue is, but have seen this issue under some other articles recently but did not report it back then.

  4. Anonymous said on August 25, 2023 at 11:44 am

    Omg a badge!!!
    Some tangible reward lmao.

    It sucks that redditors are going to love the fuck out of it too.

  5. Scroogled said on August 25, 2023 at 10:57 pm

    With the cloud, there is no such thing as unlimited storage or privacy. Stop relying on these tech scums. Purchase your own hardware and develop your own solutions.

    1. lollmaoeven said on August 27, 2023 at 6:24 am

      This is a certified reddit cringe moment. Hilarious how the article’s author tries to dress it up like it’s anything more than a png for doing the reddit corporation’s moderation work for free (or for bribes from companies and political groups)

  6. El Duderino said on August 25, 2023 at 11:14 pm

    Almost al unlmited services have a real limit.

    And this comment is written on the dropbox article from August 25, 2023.

  7. John G. said on August 26, 2023 at 1:29 am

    First comment > @ilev said on August 4, 2012 at 7:53 pm

    For the God’s sake, fix the comments soon please! :[

  8. Kalmly said on August 26, 2023 at 4:42 pm

    Yes. Please. Fix the comments.

  9. Kim Schmidt said on September 3, 2023 at 3:42 pm

    With Google Chrome, it’s only been 1,500 for some time now.

    Anyone who wants to force me in such a way into buying something that I can get elsewhere for free will certainly never see a single dime from my side. I don’t even know how stupid their marketing department is to impose these limits on users instead of offering a valuable product to the paying faction. But they don’t. Even if you pay, you get something that is also available for free elsewhere.

    The algorithm has also become less and less savvy in terms of e.g. English/German translations. It used to be that the bot could sort of sense what you were trying to say and put it into different colloquialisms, which was even fun because it was like, “I know what you’re trying to say here, how about…” Now it’s in parts too stupid to translate the simplest sentences correctly, and the suggestions it makes are at times as moronic as those made by Google Translations.

    If this is a deep-learning AI that learns from users’ translations and the phrases they choose most often – which, by the way, is a valuable, moneys worthwhile contribution of every free user to this project: They invest their time and texts, thereby providing the necessary data for the AI to do the thing as nicely as they brag about it in the first place – alas, the more unprofessional users discovered the translator, the worse the language of this deep-learning bot has become, the greater the aggregate of linguistically illiterate users has become, and the worse the language of this deep-learning bot has become, as it now learns the drivel of every Tom, Dick and Harry out there, which is why I now get their Mickey Mouse language as suggestions: the inane language of people who can barely spell the alphabet, it seems.

    And as a thank you for our time and effort in helping them and their AI learn, they’ve lowered the limit from what was once 5,000 to now 1,500…? A big “fuck off” from here for that! Not a brass farthing from me for this attitude and behaviour, not in a hundred years.

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