The
Sec-CH-UA
user agent client hint
request header provides the user-agent's branding and significant version information.
The
Sec-CH-UA
header provides the brand and significant version for each brand associated with the browser in a comma-separated list.
A brand is a commercial name for the user agent like: Chromium, Opera, Google Chrome, Microsoft Edge, Firefox, and Safari.
A user agent might have several associated brands.
For example, Opera, Chrome, and Edge are all based on Chromium, and will provide both brands in the
Sec-CH-UA
header.
The
significant version
is the "marketing" version identifier that is used to distinguish between major releases of the brand.
For example a Chromium build with
full version number
"96.0.4664.45" has a significant version number of "96".
The header therefore allows the server to customize its response based on both shared brands and on particular customizations in their respective versions.
Sec-CH-UA
is a
low entropy hint
.
Unless blocked by a user agent permission policy, it is sent by default, without the server opting in by sending
Accept-CH
.
The header may include "fake" brands in any position and with any name.
This is a feature designed to prevent servers from rejecting unknown user agents outright, forcing user agents to lie about their brand identity.
Note:
Sec-CH-UA-Full-Version-List
is the same as
Sec-CH-UA
, but includes the full version number rather than the significant version number for each brand.