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Mozilla: Screenshots, Phabricator Etiquette, and Examining JavaScript Inter-Process Communication in Firefox

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  • The screenshot option in Firefox has moved. Here’s how to find it.

    If you have updated Firefox recently, you may have noticed that Take a Screenshot is not in the page actions menu. Don’t fret. The feature is still in Firefox; it has just been moved.

  • Phabricator Etiquette Part 2: The Author

    Just as with reviewers, the author is also responsible for making sure the revision is in the appropriate state. As a reminder, ambiguous states happen when the revision needs action from someone, but doesn’t show up in their queue.

    As the author, a common way to get into this predicament is failing to re-request review when no changes are submitted. A typical review might start with the revision in Needs Review (in the reviewer’s queue). The reviewer may request some changes and set it to Needs Revision, which is now back in the author’s queue. The author will then fix the problems and re-submit their patch. Importantly, moz-phab (or maybe Phabricator itself?) will automatically set the state back to Needs Review and all is right with the world.

  • Examining JavaScript Inter-Process Communication in Firefox – Attack & Defense

    Firefox uses Inter-Process Communication (IPC) to implement privilege separation, which makes it an important cornerstone in our security architecture. A previous blog post focused on fuzzing the C++ side of IPC. This blog post will look at IPC in JavaScript, which is used in various parts of the user interface. First, we will briefly revisit the multi-process architecture and upcoming changes for Project Fission, Firefox’ implementation for Site Isolation. We will then move on to examine two different JavaScript patterns for IPC and explain how to invoke them. Using Firefox’s Developer Tools (DevTools), we will be able to debug the browser itself.

    Once equipped with this knowledge, we will revisit a sandbox escape bug that was used in a 0day attack against Coinbase in 2019 and reported as CVE-2019-11708. This 0day-bug has found extensive coverage in blog posts and publicly available exploits. We believe the bug provides a great case study and the underlying techniques will help identify similar issues. Eventually, by finding more sandbox escapes you can help secure hundreds of millions of Firefox users as part of the Firefox Bug Bounty Program.


    In this blog post, we have given an introduction to Firefox IPC using JavaScript and how to debug the child and the parent process using the Content Toolbox and the Browser Toolbox, respectively. Using this setup, you are now able to simulate a fully compromised child process, audit the message passing in source code and analyze the runtime behavior across multiple processes.

    If you are already experienced with Fuzzing and want to analyze how high-level concepts from JavaScript get serialized and deserialized to pass the process boundary, please check our previous blog post on Fuzzing the IPC layer of Firefox.

    If you are interested in testing and analyzing the source code at scale, you might also want to look into the CodeQL databases that we publish for all Firefox releases.

    If you want to know more about how our developers port legacy MessageManager interfaces to JSActors, you can take another look at our JSActors documentation and at how Mike Conley ported the popup blocker in his Joy of Coding live stream Episode 204.

    Finally, we at Mozilla are really interested in the bugs you might find with these techniques – bugs like confused-deputy attacks, where the parent process can be tricked into using its privileges in a way the content process should not be able to (e.g. reading/writing arbitrary files on the filesystem) or UXSS-type attacks, as well as bypasses of exploit mitigations. Note that as of April 2021, we are not enforcing full site-isolation. Bugs that allow one to impersonate another site will not yet be eligible for a bounty. Submit your findings through our bug bounty program and follow us at the @attackndefense Twitter account for more updates.

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today's howtos

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