My hugo.toml,
# [markup]
# defaultMarkdownHandler = "pandoc"
[markup]
defaultMarkdownHandler = "mmark"
[markup.goldmark]
[markup.goldmark.renderer]
unsafe = true # Critical for math support
[security]
enableInlineShortcodes = false # Keep this as-is unless needed
[security.exec]
allow = ['^pandoc$', '^mmark$'] # Whitelist pandoc (and mmark if used)
> which mmark
/opt/local/bin/mmark
> cd ~/tmp/quickstart
> hugo server -D
Error: markup: Configured defaultMarkdownHandler "mmark" not found.