Atlassian Connect Express is a Node.js package for building Atlassian Connect apps. Authentication between Atlassian products and the Atlassian Connect Express app occurs with a server-to-server JWT or a context JWT. Atlassian Connect Express versions between 3.0.2 - 6.5.0 erroneously accept context JWTs in lifecycle endpoints (such as installation) where only server-to-server JWTs should be accepted, permitting an attacker to send authenticated re-installation events to an app. This is fixed in version 6.6.0.
3.0.2 - 6.5.0
6.6.0 and later
This is an independent assessment and you should evaluate its applicability to your own IT environment.
CVSS v3 score: 9.1 => Critical severity
What you need to do
Atlassian recommends that you upgrade to the latest version. Upgrade to atlassian-connect-express to 6.6.0 or higher.