You may find that when doing decryption on a Palo Alto Networks Next Generation Firewall that images in Slack channels are not displayed or are only shown in a very low resolution – in addition images cannot be uploaded. When inspecting the HTTP error messages in your browser a 503 response may also be seen.
To top off the issue you may also see that User-ID isn’t mapping traffic from the Slack desktop application against the traffic which negates any User-ID based decryption exception you might have.
While helpful the Slack connection test tool at https://my.slack.com/help/test also doesn’t appear to throw any errors.
To fix this issue you need a decryption exception custom URL rule for the files.slack.com domain (which fixes viewing images) along with the base domain for your Slack tenancy (fixes uploads). This domain can be found by clicking the drop down in the top left corner of the Slack client.
If you have multiple Slack tenancies then you will need an exception for each one.
As always when making exceptions for your decryption policy please consider how it might degrade your ability to detect malicious usage of the network – in this case the sharing of files with unknown payloads.
For more information about Slack network usage visit: https://slack.com/intl/en-gb/help/articles/360001603387-Manage-Slack-connection-issues.