A critical security vulnerability (CVE-2026-34078) in Flatpak allows any Flatpak app to completely escape its sandbox, gaining read/write access to arbitrary host files and the ability to execute code in the host context. The flaw exists because the Flatpak portal accepts paths in the `sandbox-expose` options that can be app-controlled symlinks pointing to arbitrary host paths, which Flatpak then mounts inside the sandbox. The issue is patched in version 1.16.4 and will be addressed in the upcoming 1.18.0. A temporary mitigation is to disable the Flatpak Portal service, though this may cause some apps to misbehave. The vulnerability was reported by Codean Labs.
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