From the alert:

The unset-value package is vulnerable to Prototype Pollution. The unset function in index.js files allows for access to object prototype properties. An attacker can exploit this to override the behavior of object prototypes, resulting in a possible Denial of Service (DoS), Remote Code Execution (RCE), or other unexpected behavior.

More explanation here:
https://cwe.mitre.org/data/definitions/1321.html