Summary

CSV injection is a vulnerability where untrusted user input in CSV files can lead to unauthorized access or data manipulation.
In my subsequent testing of the application.

Details

I discovered that there is an option to "Export Data" from the web app to your personal computer, which exports a "csv" file that can be opened with Excel software that supports macros.

P.S
I discovered that the web application's is offering a demo-site that anyone may access to play with the web application. So, there's a chance that someone will export the data (CVS) from the demo site and execute it on their PC, giving the malicious actor a complete control over their machine. (if a user enters a malicious payload to the website).

PoC

You can check out my vulnerability report if you need more details/PoC with screenshots: (removed by JC5)

Impact

An attacker can exploit this by entering a specially crafted payload to one of the fields, and when a user export the csv file using the "Export Data" function, the attacker can potentiality can RCE.

Addendum by JC5, the developer of Firefly III

There is zero impact on normal users, even on vulnerable versions.