CVE-2026-90616

Flatpak versions before 1.18.1 fail to apply symlink protection when creating per-app data directories such as /var/cache, /var/data, /var/config, and /var/tmp inside each sandbox on every app launch. Because components of the host-side mount path (e.g., ~/.var/app/APP_ID/cache/tmp) are attacker-controlled, a malicious sandboxed app can plant symlinks that redirect where these directories are bind-mounted. The path is passed through bwrap --bind to the kernel, which follows symlinks, so the attacker-chosen target on the host becomes mounted inside the sandbox. This yields arbitrary file read and write access on the host, which can be escalated to arbitrary code execution outside the sandbox (CVSS 3.1: 7.4, CWE-61; a distinct issue from CVE-2026-76925). Any Linux system running Flatpak before 1.18.1 is affected, but exploitation requires the victim to have installed the attacker's malicious Flatpak app; no public PoC, KEV listing, or in-the-wild exploitation is known.

What to do: Upgrade to Flatpak 1.18.1 or later as soon as your distribution ships it, applying distro security backports where 1.18.1 is not yet packaged. Until patched, restrict installs to trusted remotes and verified Flathub publishers and remove any Flatpak apps from unverified sources. Inspect ~/.var/app/APP_ID/ directories for unexpected symlinks pointing outside the app's data tree, which would indicate a malicious app attempting this technique.