GHSA-mf74-qq7w-6j7v: Zmarkdown Server-Side Request Forgery (SSRF) in remark-download-images

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:

![](http://192.168.1.2/private-img.png)

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.