If an attacker comes into the possession of a victim's OctoPrint session cookie through whatever means, the attacker can use this cookie to authenticate as long as the victim's account exists.
An attacker can freely brute force username and password and can takeover any account. An attacker could easily guess user passwords and gain access to user and administrative accounts.
OctoPrint through 1.3.9 allows remote attackers to obtain sensitive information or cause a denial of service via HTTP requests on port 8081. NOTE: the vendor disputes the significance of this report because their documentation states that with "blind port forwarding ... Putting OctoPrint onto the public internet is a terrible idea, and I really can't emphasize that enough.