Hugo produces static output, so once built, the runtime is the browser (assuming the output is HTML) and any server (API) that you integrate with.
But when developing and building your site, the runtime is the hugo executable. Securing a runtime can be a real challenge.
Hugo’s main approach is that of sandboxing:
Hugo has a virtual file system and only the main project (not third-party components) is allowed to mount directories or files outside the project root.
Only the main project can walk symbolic links.
User-defined components have only read-access to the filesystem.
We shell out to some external binaries to support Asciidoctor and similar, but those binaries and their flags are predefined. General functions to run arbitrary external OS commands have been discussed, but not implemented because of security concerns.
Hugo will soon introduce a concept of Content Source Plugins (AKA Pages from Data), but the above will still hold true.
Hugo builds as a static binary using Go Modules to manage its dependencies. Go Modules have several safeguards, one of them being the go.sum file. This is a database of the expected cryptographic checksums of all of your dependencies, including any transitive.
Hugo Modules is built on top of Go Modules functionality, and a Hugo project using Hugo Modules will have a go.sum file. We recommend that you commit this file to your version control system. The Hugo build will fail if there is a checksum mismatch, which would be an indication of dependency tampering.
Web Application Security
These are the security threats as defined by OWASP.
Templates authors (you) are trusted, but the data you send in is not.
This is why you sometimes need to use the safe functions, such as safeHTML, to avoid escaping of data you know is safe.
There is one exception to the above, as noted in the documentation: If you enable inline shortcodes, you also say that the shortcodes and data handling in content files are trusted, as those macros are treated as pure text.
It may be worth adding that Hugo is a static site generator with no concept of dynamic user input.
For content, the default Markdown renderer is configured to remove or escape potentially unsafe content. This behavior can be reconfigured if you trust your content.