Joomla! Core is prone to a vulnerability that lets remote attackers inject arbitrary formulas into CSV files. Attackers can possibly exploit this issue to execute arbitrary commands on the victim's system, by the use of Microsoft Excel DDE function, or to leak data via maliciously injected hyperlinks. Joomla! Core versions 3.9.x ranging from 3.9.0 and up to and including 3.9.6 are vulnerable.
Update to Joomla! Core version 3.9.7 or latest