Firefox 100 and Chrome 100 may have user-agent issues

Mozilla developers have warned that sites may experience problems with the upcoming versions of Firefox 100 and Chrome 100 (released May 3 and March 29, 2022). The fact is that the release of new versions will mean that the user-agent values will become three-digit.

The user-agent string contains information such as the name of the browser, its version number, and information about the various technologies it uses. So, when a person visits a website, the browser’s user-agent is sent along with the web page request. This allows the resource to check the visitor’s software version and change their response depending on the features supported by the browser.

For example, the current user-agent of Mozilla Firefox version 97 looks like this: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:97.0) Gecko/20100101 Firefox/97.0. And the user-agent for Google Chrome 98 is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/98.0.4758.82 Safari/537.36.

Bleeping Computer says that back in August 2021, Mozilla engineers began an experiment to find out if the three-digit user-agent “Firefox/100” would cause problems when working with sites. Google developers soon conducted a similar experiment for Chrome 100. As a result, the experts found a small number of sites that did not work correctly with the new user-agent.

Since then, Mozilla has been monitoring version 100 bugs and has already found issues on HBO Go, Bethesda, Yahoo, Slack, and resources created with the Duda builder. Most of these bugs are limited to “browser not supported” messages, as well as user interface issues that can affect different parts of sites.

Without a single specification to follow, different browsers have different formats for the user-agent string, and user-agent parsing is site-specific. Perhaps some parsing libraries contain hard-coded assumptions or bugs that do not take into account three-digit version numbers. Many libraries have improved parsing performance when browsers have moved to two-digit version numbers, so it is expected that the transition to three-digit versions will cause less problems.Mozilla engineers explain.

If problems with sites do arise and are too numerous, and Mozilla or Google cannot fix it before the release of new versions, developers have backup plans. In particular, Firefox has a mechanism that allows to “freeze” the user-agent to “Firefox/99” or inject CSS into a problem site. Similarly, Chrome can “freeze” the version displayed by the user-agent at 99 and list the actual browser version on a different part of the string.

Mozilla asks site administrators to check in advance whether their resources accept the user-agent Firefox 100 and Chrome 100 normally. To do this, in Firefox admin needs to open the Firefox Nightly settings menu, find “Firefox 100”, and then activate the “Firefox 100 User-Agent String” flag . This will change the Firefox user-agent string to Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:100.0) Gecko/20100101 Firefox/100.0.

In Chrome, go to chrome://flags/#force-major-version-to-100, enable the setting, and the user-agent string will change to Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4758.102 Safari/537.36.

If problems are found, then bug reports should be submitted to so that the developers have time to fix them.

Let me remind you that we also reported that Chrome 90 gets a new security feature to protect against attacks on Windows 10.

And, you may be interested in the information that Mozilla Downsizing Affects Security Professionals.

By Vladimir Krasnogolovy

Vladimir is a technical specialist who loves giving qualified advices and tips on GridinSoft's products. He's available 24/7 to assist you in any question regarding internet security.

Leave a comment

Your email address will not be published. Required fields are marked *