> viewing data is a fundamental failure of the principle of least privilege.
I read the cwe not cve, was wrong. It's still early in the morning...
> If I read it correctly (but could be mistaken), it runs with setuid root
I am certain you are mistaken. I couldn't find anything that hints at notepad running with elevated privileges.
You are mistaken:
> The malicious code would execute in the security context of the user who opened the Markdown file, giving the attacker the same permissions as that user.