Flatpak's process ID namespace separation does not prevent a sandboxed app's kill(0, signal) or killpg(0, signal) calls from reaching processes outside the sandbox that share the same process group. A malicious or compromised Flatpak app can use this to cause denial of service by terminating processes outside its sandbox, such as the desktop shell.

flatpak-1.18.4-1.1 on GA media

Affected: Ubuntu:Pro:18.04:LTS: flatpak, Ubuntu:Pro:20.04:LTS: flatpak, Ubuntu:Pro:22.04:LTS: flatpak, Ubuntu:Pro:24.04:LTS: flatpak, Ubuntu:26.04:LTS: flatpak

Flatpak's process ID namespace separation does not prevent a sandboxed app's kill(0, signal) or killpg(0, signal) calls from reaching processes outside the sandbox that share the same process group. A malicious or compromised Flatpak app can use this to cause denial of service by terminating processes outside its sandbox, such as the desktop shell.