Flameshot: OCTOU symlink attack via predictable /tmp path in Flameshot "Open With"

Summary

Flameshot is powerful yet simple to use screenshot software. Prior to 14.0.0, the Open With feature wrote screenshots to a predictable temporary path and followed symlinks, creating a time-of-check to time-of-use race that allowed a local unprivileged attacker on the same machine to pre-plant a symlink and cause Flameshot to write PNG data through it, overwriting any file the victim user could write. This issue is fixed in version 14.0.0.