The Windows operating system will automatically, and without any way for applications to disable it, try to establish a connection to another host over the network and access it (over SMB or other protocols), if only the correct file path is accessed.
When first realizing this, the curl team tried to filter out such attempts in order to protect applications for inadvertent probes of for example internal networks etc. This resulted in CVE-2019-15601 and the associated security fix.
However, we’ve since been made aware of the fact that the previous fix was far from adequate as there are several other ways to accomplish more or less the same thing: accessing a remote host over the network instead of the local file system.
The conclusion we have come to is that this is a weakness or feature in the Windows operating system itself, that we as an application and library cannot protect users against. It would just be a whack-a-mole race we don’t want to participate in. There are too many ways to do it and there’s no knob we can use to turn off the practice.
We no longer consider this to be a curl security flaw!
If you use curl or libcurl on Windows (any version), disable the use of the FILE protocol in curl or be prepared that accesses to a range of “magic paths” will potentially make your system try to access other hosts on your network. curl cannot protect you against this.
We have updated relevant curl and libcurl documentation to make users on Windows aware of what using FILE:// URLs can trigger (this commit) and posted a warning notice on the curl-library mailing list.
Previous security advisory
This was previously considered a curl security problem, as reported in CVE-2019-15601. We no longer consider that a security flaw and have updated that web page with information matching our new findings. I don’t expect any other CVE database to update since there’s no established mechanism for updating CVEs!
Many thanks to Tim Sedlmeyer who highlighted the extent of this issue for us.