Rendering policy bypass via substring allow-list checks

Type confusion in DigitalOcean do-markdownit 1.16.1 and earlier allows remote attackers to bypass configured rendering allow-lists. When allowedClasses or allowedEnvironments is a string rather than an array, callout and fence_environment use String.includes(), accepting partial substrings instead of exact list members. Exploitation requires a downstream service to expose attacker-controlled Markdown while configuring either option as a string, enabling unauthorized classes or environments in rendered output.