CVE-2026-55668: File Browser: ScopedFs follows a dangling symlink on write, letting a scoped user create files outside their scope

ScopedFs confines every File Browser user to a scope directory. Its within() guard is meant to reject any operation that follows a symbolic link out of that scope. When the link target does not exist yet, the guard walks up to the nearest existing ancestor and validates that instead. For a dangling symlink (target does not exist), the nearest existing ancestor is the in-scope directory containing the link, so the guard returns “in scope” and the subsequent os.OpenFile(O_CREATE) follows the link and creates the file at its out-of-scope target.

A post-auth user with Create and Modify permission can write attacker-controlled content to any non-existent path outside their scope that the File Browser process can write to. The precondition is a dangling symlink present inside the user’s scope, which is the same out-of-band precondition the rest of ScopedFs is built to defend against.

This is a patch-gap variant of the GHSA-239w-m3h6-ch8v symlink confinement issue, not a resubmission of the already-published vulnerable-version behavior: GHSA-239w-m3h6-ch8v marks <= 2.63.13 vulnerable and 2.63.14 patched, while this proof reproduces on current master / v2.63.15 (be23ab3a15bf957928ecfed88de5ab67850c1b9c). The escaping-symlink-to-an-existing-target case is defended and tested. The dangling case is neither, and the gap is acknowledged in a code comment as “best-effort”.