Mongo-Express before 1.0.0 is susceptible to remote code execution because it uses safer-eval to validate user supplied javascript. Unfortunately safer-eval sandboxing capabilities are easily bypassed leading to remote code execution in the context of the node server.
View the template here CVE-2020-24391.yaml
References:
https://securitylab.github.com/advisories/GHSL-2020-131-mongo-express/