Bleeping Computer explains.
Mozilla is warning website developers that the upcoming Firefox 100 and Chrome 100 versions may break websites when parsing user-agent strings containing three-digit version numbers.
A user-agent is a string used by a web browser that includes information about the software, such as the browser name, its version number, and the various technologies it uses.
When a person visits a website, the browser’s user-agent is sent along with the request for a web page. This allows the web page to check the visitor’s browser version and modify its response based on the features the browser supports.
The problem is that some websites – including some major ones – are hard-coded to expect a 2-digit browser version number, so will fall over in various ways when presented with a 3-digit one.
Mozilla and Google found a small number of websites that would not operate correctly when parsing a user-agent string that contained a three-digit version number.
Since then, Mozilla has been keeping track of web bugs caused by the version 100 change and has found problems on websites for HBO Go, Bethesda, Yahoo, Slack, and those created by the Duda website builder.
For the most part, these issues have ranged from the websites stating the browser is unsupported to user interface issues affecting portions of the site.
Chrome v100 is set for release on March 29, and Firefox v100 on May 3.
If you have a website and want to check whether yours will be affected, Mozilla has provided instructions for testing. However, both companies will also introduce mitigation measures designed to minimize issues, and have a backup plan to freeze version numbers at 99 if these precautions don’t prove effective.
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