Medium [CVE-2026-96276] Flatpak: flatpak: arbitrary write in host context via flatpak build-init

If a malicious SDK container declares an extension point with a crafted `directory` path, and a developer runs `flatpak build-init --writable-sdk --sdk-extension` with that SDK, attacker-chosen files could be written outside the working directory, since the target path is resolved via a function that allows `..` traversal.

Red Hat estimates the CVSSv3.1 vector of CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N. The SDK content is retrieved from a remote registry, so no local access to the victim's system is required by the attacker (AV:N), and the attacker needs no privileges of their own (PR:N).

Once the developer builds against the malicious SDK, exploitation is deterministic: the extension point's directory field is resolved via g_file_resolve_relative_path, which does not reject.. components, allowing a crafted value such as directory=../../ to redirect the copy operation outside the build tree (AC:L).

Meaningful user interaction is required, as the developer must actively choose to build against the untrusted SDK (UI:R). flatpak build-init runs entirely with the invoking developer's own privileges, so the impact is bounded by that user's existing filesystem access and no privilege boundary is crossed (S:U).

Affected products named by the advisory: Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 7; Red Hat Enterprise Linux 8; Red Hat Enterprise Linux 9; and 1 more.