ISSUE
Because of the age of the image, many packages are outdated and bring critical vulnerabilities into the image.

How to reproduce
Build any of the mono:6, 6.12, 6.12.0, 6.12.0.182, latest and run docker scan will find the following layer adds
apt-get update && apt-get install -y binutils curl mono-devel ca-certificates-mono fsharp mono-vbnc nuget referenceassemblies-pcl && rm -rf /var/lib/apt/lists/* /tmp/*

And these package versions are old and have known issues.

Also, if you find your way to update the Debian version to 11 or 12 as 10 is EOL.