GHSA-45pq-889g-fcgh rclone: Incomplete path validation allows backend root escape in serve restic
Summary`rclone serve restic` does not correctly reject URL paths beginning with `../`. On affected backends, an attacker who can access the REST endpoint can read, create, overwrite, or delete objects outside the path configured by the operator.The issue affects `rclone v1.40` through `rclone v1.74.4`. The proof of concept and backend matrix were validated with the official Linux AMD64 binary for `v1.74.4`, and the latest `master` commit reviewed at the time (`2217d38`) contained the same vulnerable validation. The main proof of concept uses WsgiDAV as an independent storage server and one rclone process.Affected versionsAll releases from `v1.40` through `v1.74.4` are affected.Affected components and backend propagationThe primary vulnerable component is the backend-independent `WithRemote` middleware in `cmd/serve/restic/restic.go`, lines 235-264. It accepts a leading parent component and stores that unsafe relative path in the request context. The REST handlers then pass the same value to whichever rclone backend the operator configured. Therefore, the flaw is not specific to WebDAV.The backend determines whether the accepted `../` path escapes, is preserved, or is encoded as safe filename characters. The source locations and line numbers below correspond to the release used for dynamic testing:| Layer or backend | File and function | Relevant lines | Path propagation | Dynamic evidence |
|---|---|---:|---|---|
| REST server, primary cause | `cmd/serve/restic/restic.go`, `WithRemote` | 235-264 | Accepts a leading `../` remote and shares it with GET, HEAD, POST, and DELETE handlers | Confirmed through WebDAV |
| WebDAV | `backend/webdav/webdav.go`, `(*Fs).filePath` | 421-427 | `path.Join(f.root, file)` removes the configured root when resolving `../` | read, write, delete |
| FTP | `backend/ftp/ftp.go`, `(*Fs).NewObject`, `(*Object).Open`, `Update`, and `Remove` | 844-848, 1308-1311, 1349-1356, 1411-1415 | Each operation joins the backend root and remote with `path.Join` before the FTP request | read, write, delete |
| HTTP | `backend/http/http.go`, `(*Fs).url` | 386-395 | Appends the escaped remote containing `../` to the configured endpoint URL | read |
| Memory | `backend/memory/memory.go`, `(*Fs).split` | 227-231 | Joins `f.root` and the relative path before splitting the in-memory bucket and key | read, write, delete |
| SFTP | `backend/sftp/sftp.go`, `(*Fs).remotePath` | 2086-2089 | Joins `f.absRoot` and the remote, allowing the parent component to remove the published subdirectory | read, write, delete |These are backend-specific manifestations of the same `WithRemote` validation flaw, not separate vulnerabilities.Technical Details`WithRemote` obtains the decoded URL path, removes external slashes, and tries to reject traversal by comparing the path with `path.Clean`:The comment describes the intended behavior, but the condition does not reject every parent component. `path.Clean` preserves leading parent components in a relative path:Because both strings are equal, the middleware accepts the path. Internal traversal behaves differently:These strings differ, so that request returns HTTP 400. This explains why the existing check appears to work while the leading variant bypasses it.After validation, `WithRemote` stores the accepted value in the request context:GET, POST, and DELETE handlers retrieve this same value. GET passes it to `s.f.NewObject`, POST passes it to `operations.RcatSize`, and DELETE resolves the object and calls `Remove`. There is no second containment check.WebDAV is used below as the concrete end-to-end example because it was the backend used for the main proof of concept. WebDAV is not the source of the validation flaw. The example demonstrates one way in which an unsafe remote accepted by `WithRemote` is propagated by a backend.The WebDAV backend joins its configured root with the attacker-controlled remote:For the proof of concept:The configured root is removed before encoding. WsgiDAV receives a normal operation for `/outside-secret.txt`, which is outside the root published by `rclone serve restic`.The same accepted leading parent path propagates through the other affected backends tested. FTP joins its root and remote with `path.Join` before object operations; HTTP preserves `served-root/../outside-secret.txt` when constructing the endpoint request; Memory joins the root and relative path before splitting the bucket and key; and SFTP joins `f.absRoot` and the remote in `remotePath`. In each case, the backend receives the leading parent component already accepted by `WithRemote`. The exact escape mechanism and available operations vary by backend. Conversely, S3-compatible and local backends did not escape in the tested configuration because they encoded `..` as filename characters.Expected behavior is HTTP 400 before any backend operation. Actual behavior is HTTP 200 followed by an operation outside `served-root`.Preconditions and impactThe operator must publish a backend subdirectory, the endpoint must be reachable, and the backend credential must have access to a parent or sibling object. Exploitability also depends on backend path semantics.An attacker may:read files and objects outside the published backup root;create or overwrite sibling objects;delete objects when deletion is permitted;cross isolation boundaries between users, repositories, or automation jobs;indirectly compromise another system if it later trusts an overwritten configuration, script, or artifact.`--append-only` reduces overwrite and delete impact but does not prevent traversal reads or creation of new objects.Proof of conceptThe following procedure was executed on Linux Mint 22.3 with the official `rclone v1.74.4` Linux AMD64 binary, WsgiDAV 4.3.5, and Cheroot 10.0.1. The rclone binary reports that it was built with Go 1.26.5.1. Create the storage layout2. Start the independent WebDAV server3. Download, verify, and start rclone4. Confirm normal access5. Read outside the published root6. Write outside the published root7. Delete outside the published root8. Compare with internal traversalThis demonstrates why the existing check appears to work for interior traversal while the leading variant bypasses it.Tested backends| Backend | Local implementation | Result | Operations tested |
|---|---|---|---|
| WebDAV | WsgiDAV 4.3.5 | Affected | read, write, delete |
| FTP | pyftpdlib 2.2.0 | Affected | read, write, delete |
| HTTP | Python `http.server` 3.12.3 | Affected | read |
| Memory | rclone memory backend | Affected | read, write, delete |
| SFTP | `atmoz/sftp` OpenSSH server | Affected | read, write, delete |
| S3 compatible | MinIO | No root escape observed | read, write, delete |
| Local filesystem | default local encoding | No root escape observed | read, write, delete |Only the backends listed in this table were tested or classified. Every row was dynamically repeated with the same official `v1.74.4` Linux AMD64 binary identified in the proof of concept.Suggested remediationReject `.` and `..` components in `WithRemote` before storing the remote in the context. Validating the decoded relative path with `io/fs.ValidPath`, with explicit handling for the empty API root, is one possible approach. Authorization and backend lookup should use the same validated representation.Regression tests should cover GET, HEAD, POST, and DELETE with `..`, `../x`, `../../x`, `%2e%2e/x`, `a/../x`, and `a/../../x`, both with and without `--private-repos`.Additional impact scenarios identified by the maintainer`GET /../` could reach the list handler and enumerate the parent directory, allowing an attacker to discover object names before accessing them.With `--append-only`, a request such as `DELETE /../locks/<name>` could satisfy the existing delete guard and delete an object outside the served root.A bare `.` path was also accepted. On bucket-based backends, `POST /.` could write an object outside the intended served path.Credit: Caubi Loureiro of Vorpcel Research
CVSS v4.0 score: 8.6