In the @digitalocean/do-markdownit package through 1.16.1 (in npm), the callout and fence_environment plugins perform .includes substring matching if allowedClasses or allowedEnvironments is a string (instead of an array).

Affected: npm: @digitalocean/do-markdownit

### Overview A type confusion issue exists in the `@digitalocean/do-markdownit` package. In the `callout` and `fence_environment` plugins, the `allowedClasses` and `allowedEnvironments` options are expected to be arrays of strings. If these options are provided as a single string, the code applies `.includes` directly on the string, resulting in substring matching instead of membership checks against an array.

### Affected Versions All versions up to and including 1.16.1 (npm).

### Impact Supplying crafted input can bypass intended allow-lists (e.g., class/environment constraints) due to substring checks, which may enable rendering of unintended classes or environments and lead to policy bypass in downstream consumers.

### Mitigation Until an upstream fix is released, ensure configuration normalization before invoking the plugins: - Validate that `allowedClasses` and `allowedEnvironments` are arrays (`Array.isArray(...)`), converting single strings into one-element arrays when necessary. - Consider sanitizing or strictly validating user-controlled values that influence Markdown rendering.