Zmarkdown Server-Side Request Forgery (SSRF) in remark-download-images

Summary

### Impact

A major blind SSRF has been found in `remark-images-download`, which allowed for requests to be made to neighboring servers on local IP ranges. The issue came from a loose filtering of URLs inside the module.

Imagine a server running on a private network `192.168.1.0/24`. A private service serving images is running on `192.168.1.2`, and is not expected to be accessed by users. A machine is running `remark-images-download` on the neighboring `192.168.1.3` host. An user enters the following Markdown:

```markdown  ```

The image is downloaded by the server and included inside the resulting document. Hence, the user has access to the private image.

It has been corrected by preventing images downloads from local IP ranges, both in IPv4 and IPv6. To avoid malicious domain names, resolved local IPs from are also forbidden inside the module. This vulnerability impact is moderate, as it is can allow access to unexposed documents on the local network, and is very easy to exploit.

### Patches

The vulnerability has been patched in version 3.1.0. If impacted, you should update to this version as soon as possible.

Please note that a minor version has been released instead of a bugfix. This is due to a new option included to prevent another vulnerability, upgrading to the new version will not break compatibility.

### Workarounds

No workaround is known, the package should be upgraded.

### For more information

If you have any questions or comments about this advisory, open an issue in [ZMarkdown](https://github.com/zestedesavoir/zmarkdown/issues).