Versions of extend prior to 3.0.2 (for 3.x) and 2.0.2 (for 2.x) are vulnerable to Prototype Pollution. The extend() function allows attackers to modify the prototype of Object causing the addition or modification of an existing property that will exist on all objects.

Recommendation

If you're using extend 3.x upgrade to 3.0.2 or later.
If you're using extend 2.x upgrade to 2.0.2 or later.

References

• https://nvd.nist.gov/vuln/detail/CVE-2018-16492
• https://hackerone.com/reports/381185
• GHSA-qrmc-fj45-qfc2
• https://www.npmjs.com/advisories/996