GHSA-r9w3-qx54-qvc8 (https://github.com/flatpak/flatpak/security/advisories/GHSA-r9w3-qx54-qvc8)

Description: The child temporary directories allocated under the user cache (/var/tmp/flatpak-cache-*) are created with mode 0777. On multi-user systems this could allow other local users to modify the app as it is being installed. This is related to GHSA-7rvf-rqr3-43j4 (CVE-2026-97025) but with a different impact. This is mitigated by a restrictive umask; it is believed to be a denial-of-service risk rather than an integrity threat, because a modified app/runtime would fail its signature or digest check. The flatpak(1) CLI, GNOME Software, and KDE Plasma Discover are believed unaffected since they set umask 022; other software using libflatpak with a permissive umask might be indirectly vulnerable.

Mitigation: Set a umask that does not allow other users to write to the cache directory (e.g. umask 022). Patched in 1.18.4 by commit 011dfae2 ("common: Restrict tmpdir permissions from 0777 to 0755"). Credit: Found by AISLE in cooperation with Red Hat.