The MKdocs 1.2.2 built-in dev-server allows directory traversal using the port 8000, enabling remote exploitation to obtain sensitive information. Note the vendor has disputed the vulnerability (see references) because the dev server must be used in an unsafe way (namely public) to have this vulnerability exploited.
View the template here CVE-2021-40978.yaml
References:
https://github.com/mkdocs/mkdocs/pull/2604