repmgr versions 4.0.0 through 5.5.0 contain an OS command injection vulnerability in event notification commands when certain placeholders (%a, %c, %d) are used without proper escaping. This allows a non-superuser database user with write access to node records to execute arbitrary OS commands as the operating system user running repmgr on other cluster nodes. The vulnerability is fixed in repmgr 5.5.1, and operators must adjust their configuration to remove quotes around placeholders after upgrading.