rclone: source object names can escape the configured root on upload

rclone core does not sanitize .. in a source object's Remote() - verified: nothing in fs/march, fs/sync, fs/list, or fs/operations rejects .. segments before the name reaches the destination backend's Put/Update/Mkdir. Confinement is therefore each backend's responsibility, and these backends join root + remote without a check.

This divides into two classes:

• Bucket based backends - bucket.Split(path.Join(f.root, rootRelativePath)):

  • backend/b2/b2.go:404, backend/swift/swift.go:464, backend/qingstor/qingstor.go:198, backend/oracleobjectstorage/oracleobjectstorage.go:245, backend/internetarchive/internetarchive.go:1016, backend/smb/smb.go:885, backend/storj/fs.go:289.
  • path.Join collapses .. on the standard (ASCII) form before encoding is applied (e.g. FromStandardPath(path.Join(...)) at backend/b2/b2.go:1641), so EncodeDot never gets the chance to neutralize the ...
  • lib/bucket.Join does not clean paths (keeps .. as a literal key segment); path.Join does. backend/s3, backend/azureblob, backend/googlecloudstorage already use bucket.Join and are therefore not affected.

• Path based backends - path.Join(root, remote) onto a real path:

  • sftp: remotePath = path.Join(f.absRoot, f.opt.Enc.FromStandardPath(remote)) (backend/sftp/sftp.go:2497). Default encoding is encoder.Display (== Standard), and FromStandardPath short-circuits to a pass-through in that mode, so .. survives; f.absRoot is absolute, so path.Join("/home/user/root", "../../../../etc/passwd") -> /etc/passwd.
  • webdav: filePath at backend/webdav/webdav.go:426-432.
  • ftp: path.Join(f.root, remote) at ~14 sites (e.g. backend/ftp/ftp.go:1247).
  • filelu, shade, sia: analogous joins.

Precondition that limits reachability

For any of these to fire, a source must hand rclone a Remote() containing raw ... That is only possible when:

1. the source is a flat-keyspace object store (not a filesystem - a local/sftp/smb source cannot represent ../../x as one directory entry), and
2. the offending key was written with native, non-rclone tooling - rclone's own writer applies EncodeDot and rewrites a .. segment to fullwidth ．．, so you cannot create such a key through rclone.

rclone's source-side listing does pass a natively-planted raw .. key through unchanged (verified for b2: remote := file.Name[len(prefix):] after ToStandardPath, backend/b2/b2.go:858,867). The reports never establish this precondition; it is the same omission across every member of the class.

Example attack

 1# Step 1 - attacker, using NATIVE S3 tooling (NOT rclone) on a source the victim ingests from:
 2aws s3api put-object --bucket shared-drop --key '../../victim-backups/pwned.txt' --body evil.txt
 3 
 4# Step 2 - victim's ordinary ingest:
 5rclone copy s3-drop:shared-drop b2:victim-uploads/incoming
 6# path.Join("victim-uploads/incoming", "../../victim-backups/pwned.txt") = "victim-backups/pwned.txt"
 7# -> lands in the victim's victim-backups bucket instead of under incoming/

The blast radius is the victim's own account (a bucket/share/path the configured credential already reaches) - integrity misdirection, not a cross-tenant or confidentiality breach. sftp/smb are the exception in reach (server filesystem / other share), still bounded by the login's own permissions.

This is the same class as the already-fixed local backend advisory https://github.com/rclone/rclone/security/advisories/GHSA-7p4m-qxvv-g567, which added (*Fs).localPath returning errPathEscapes for names resolving outside the root (backend/local/local.go:819-826). That fix was justified because the destination was the operator's own OS filesystem; the same reasoning extends (at lower severity) to sftp/smb.