This advisory discloses a critical severity security vulnerability which was introduced in version 2.3.1 of Bamboo. Versions of Bamboo starting with 2.3.1 before 5.11.4.1 (the fixed version for 5.11.x) and from 5.12.0 before 5.12.3.1 (the fixed version for 5.12.x) are affected by this vulnerability.