In Splunk Enterprise versions below 8.2.12, 9.0.6, and 9.1.1, an attacker can create an external lookup that calls a legacy internal function. The attacker can use this internal function to insert code into the Splunk platform installation directory. From there, a user can execute arbitrary code on the Splunk platform Instance.
https://research.splunk.com/application/ee69374a-d27e-4136-adac-956a96ff60fd/