The commit diff rest endpoint in Bitbucket Server and Data Center before 5.16.10 (the fixed version for 5.16.x ), from 6.0.0 before 6.0.10 (the fixed version for 6.0.x), from 6.1.0 before 6.1.8 (the fixed version for 6.1.x), from 6.2.0 before 6.2.6 (the fixed version for 6.2.x), from 6.3.0 before 6.3.5 (the fixed version for 6.3.x), from 6.4.0 before 6.4.3 (the fixed version for 6.4.x), and from 6.5.0 before 6.5.2 (the fixed version for 6.5.x) allows remote attackers who have permission to access a repository, if public access is enabled for a project or repository then attackers are able to exploit this issue anonymously, to read the contents of arbitrary files on the system and execute commands via injecting additional arguments into git commands.
https://seclists.org/bugtraq/2019/Sep/43
https://jira.atlassian.com/browse/BSERV-11947
http://packetstormsecurity.com/files/154610/Bitbucket-Server-Data-Center-Argument-Injection.html