GHSA-8wc8-hf36-mjh9 File Browser: ScopedFs follows a dangling symlink on write, letting a scoped user create files outside their scope
Summary`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".Root cause`files/scoped.go` (commit `be23ab3`). The guard, including the maintainer comment that already flags this exact gap:When `p` is a symlink whose target does not exist, `EvalSymlinks(target)` returns `fs.ErrNotExist`. The loop takes the lexical parent of the link path (`filepath.Dir`), a real directory inside the scope, and `EvalSymlinks` of that resolves under the scope root. `within()` returns `true` and `guard()` permits the operation. The write then dereferences the link at the OS layer:The assumption that breaks: `within()` treats "target does not exist" as "brand-new in-scope file" and validates the containing directory. But the path component being created is itself a symlink pointing outside the scope. `O_CREATE` follows it and creates the file at the link target, not inside the validated directory. The existing-target case is correctly blocked, because the walk-up resolves the link itself to an out-of-scope path. Only the dangling case slips through.For the layout below:`EvalSymlinks(/tmp/root/scope/escape)` returns not-exist, and the fallback validates `/tmp/root/scope`. The final `OpenFile` still follows `/tmp/root/scope/escape` and creates `/tmp/root/outside/created-by-http.txt`.Reachability over HTTPEndpoint: `POST /api/resources/<linkname>?override=true` (also `PUT`, and `POST/PATCH /api/tus/...`). Verified trace against the audited source:`http/resource.go` `resourcePostHandler` requires `d.user.Perm.Create` (else 403).`files.NewFileInfo` is called. For a dangling symlink, `stat()` in `files/file.go` does `LstatIfPossible` (sees the symlink, `err == nil`, `IsSymlink = true`), then `Fs.Stat` follows the link and fails with ENOENT, so the code returns the symlink `FileInfo` with `err == nil`. The handler therefore enters the "file exists" branch.The branch requires `override == "true"` and `d.user.Perm.Modify`, then proceeds.`writeFile(d.user.Fs, r.URL.Path, r.Body, ...)` calls `afs.OpenFile(dst, os.O_RDWR|os.O_CREATE|os.O_TRUNC, fileMode)`.`ScopedFs.OpenFile` runs `guard()`, which passes for the dangling link, then `os.OpenFile` follows the link and creates the file outside the scope with the request body as content.The TUS path (`http/tus_handlers.go` `tusPostHandler` -> `OpenFile`, then `tusPatchHandler`) reaches the same sink.Why existing defenses do not apply`afero.BasePathFs` lexical confinement only neutralizes `..`. A plain link name passes it unchanged.`ScopedFs.within()` is the dedicated symlink defense, and it is the component that fails: the not-exist walk-up validates the link's parent directory instead of the link.The project's symlink tests (`http/tus_symlink_test.go` `TestTusHandlersRejectSymlinkScopeEscape`, `files/file_test.go`) only exercise escaping symlinks whose target exists. Those are blocked. The dangling variant is never tested, so the regression suite does not catch it.Proof of conceptThe PoC drives the real security boundary, `files.NewScopedFs`, with the same flags `http.writeFile` uses, and shows the write landing outside the scope. A control test shows the existing-target case is still blocked, proving the guard is real and the gap is specifically the dangling case.Observed result:The dangling test "fails" by design: the assertion fires because the file escaped. The control test passes: an `escape_link -> outside` (existing dir) write to `escape_link/injected.txt` is rejected by `OpenFile` with `permission denied` and nothing is created in `outside/`. That is exactly the scenario the project's own tests cover.End-to-end HTTP equivalent:The handler returns `200 OK`, and the outside file contains the uploaded body.Impact**Direct primitive (live-verified):** arbitrary file creation with attacker-controlled content at any non-existent path outside the user's scope that the File Browser process user can write to.**Cross-tenant integrity (source-reasoned):** in multi-user deployments the scopes are sibling directories under one server root. A low-priv user can plant files into another user's home (a script, an HTML page later served, a config the victim trusts).**Persistence / RCE on permissive deployments (source-reasoned):** creating a not-yet-existent `~/.ssh/authorized_keys` for the service account, a file under a web-served or later-executed directory, a cron or profile fragment. The official Docker image runs as non-root UID 1000, which bounds this to whatever that user owns. Bare-metal/systemd deployments that run File Browser as root raise this to host-level file write and RCE.**Scope limit (honest):** this is file creation, not overwrite. Overwriting an existing out-of-scope file is genuinely blocked, because an existing target makes the link "escaping" and `within()` rejects it. The negative control confirms this.Suggested remediationFor write/create/truncate operations, do not treat a dangling final symlink as safe merely because the nearest existing ancestor is inside the scope. The walk-up in `within()` should treat "the leaf is a symlink" as an escape candidate rather than validating its parent:In `guard()` / `within()`, `Lstat` the target. If it is a symlink, `Readlink` it, resolve the link target lexically (join with the link's directory, `Clean`), and require that target to be within the scope root, regardless of whether it currently exists.More robust: resolve path components one by one for the operation being attempted, or open the final component with `O_NOFOLLOW` (`unix.Openat(... O_NOFOLLOW)`) so creating through a symlink fails with `ELOOP`; or `fstat` the descriptor after opening and verify it resolves within the scope before writing.Add a regression test mirroring `TestTusHandlersRejectSymlinkScopeEscape` but with a dangling target (`os.Symlink(filepath.Join(outside, "newfile"), ...)`), asserting both a 4xx and that no file is created outside the scope.ReferencesIncomplete fix of CVE-2026-54094 / `GHSA-239w-m3h6-ch8v`.Affected code: `files/scoped.go` (`ScopedFs.within`, `ScopedFs.OpenFile`, `ScopedFs.Create`), reached from `http/resource.go` (`resourcePostHandler`, `writeFile`) and `http/tus_handlers.go`.Confirmed unpatched at `be23ab3a15bf957928ecfed88de5ab67850c1b9c` (`v2.63.15`).
CVSS v3.1 score: 6.3