repmgr: node names in event commands let database users run OS commands

repmgr 4.0.0 through 5.5.0 is affected by an OS command injection vulnerability (CWE-78) in event notifications. When event_notification_command is configured, repmgr substitutes the %a (node name), %c (connection string) and %d (details) placeholders into the command and runs it through the shell. %a and %c are inserted without escaping, and %d escapes only double quotes, so shell command substitution still works inside the quoting the documentation recommends. The node name and connection string come from node records in the repmgr.nodes table. An attacker with the credentials of a non-superuser repmgr database user can write a crafted node record and run OS commands, as the operating system user that runs repmgr or repmgrd (usually postgres), on other cluster nodes that process an event using these placeholders. The %a placeholder of failover_validation_command has the same flaw. Deployments whose repmgr database user is a superuser gain no new capability, because a superuser can already run OS commands, and deployments that do not use these placeholders are not affected. The issue is fixed in repmgr 5.5.1. After upgrading, operators must remove any quotes they placed around these placeholders, because a placeholder left inside double quotes is still open to command injection.