It was possible for double OGNL evaluation in FreeMarker templates through Struts FreeMarker tags to occur. An attacker who has restricted administration rights to Bamboo or who hosts a website that a Bamboo administrator visits, is able to exploit this vulnerability to execute Java code of their choice on systems that run a vulnerable version of Bamboo. All versions of Bamboo before 6.1.6 (the fixed version for 6.1.x) and from 6.2.0 before 6.2.5 (the fixed version for 6.2.x) are affected by this vulnerability.
https://jira.atlassian.com/browse/BAM-18842
https://confluence.atlassian.com/bamboo/bamboo-security-advisory-2017-12-13-939939816.html