Kibana 7.10.0 user management reports "Error loading user" when clicking a user with an @ in their username

[chrisxaustin]

Kibana version: 7.10.0

Elasticsearch version: 7.10.0

Server OS version: CenOS 8, using ECK 1.2 default image

Browser version: Chrome 86.0.4240.75 and Firefox 81.0.2

Browser OS version: Windows 10

Original install method (e.g. download page, yum, from source, etc.): ECK 1.2

Describe the bug:

Users with an @ in their username cannot be edited in Kibana 7.10.0.
This is a regression, as it worked in 7.9.0 and all previous versions I've used in this environment.
It appears to be caused by double encoding, as the link to internal/security/users replaces the @ with %2540
First level of escaping becomes %40, then next level escapes the % as %25.

If I navigate to that path it fails, but if I replace the %2540 with @ it works.

Steps to reproduce:

1. Create a user with an @ in their username
2. Stack Management -> Users
3. Click the user that has an @ in their username

Expected behavior:
Edit user page displayed

Screenshots (if relevant):

Errors in browser console (if relevant):
GET request to internal/security/users fails with 404.

Provide logs and/or server output (if relevant):

Any additional context:
Anyone affected by this can right-click the username, copy the url, replace the %40 with @, and open the fixed url in a new tab.
This opens the edit user page properly

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[legrego]

Hey @chrisxaustin, thanks for the bug report. I'm marking this as a duplicate of #83541, since that issue has more traction (even though yours was opened first).