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

Summary

Multiple backends, when given a specially crafted object to copy, can escape the backend confinement.

Backend	Keep/Close	Per-backend severity
sftp	Medium	Real filesystem escape, fires under default encoding.
smb	Low-Medium	Escapes to a different SMB share the credential can reach.
ftp	Low	Real, leading-.. overshoot PoC is partly neutralized by encoding; escape bounded to at/below the login base.
webdav	Low	Server-side ACLs are the real boundary.
b2	Low	Same-account sibling bucket crossing on a flat keyspace.
swift	Low	Same, container.
qingstor	Low	Same.
oracleobjectstorage	Low	Same.
internetarchive	Low	IA items are owner-writable only; confined to user's own items.
storj	Low	Can retarget a different bucket in the same access grant.
filelu	Low	Confined to the user's own account.
shade	Low	Confined to the user's own drive.
sia	Low	siad API password already grants full-daemon access.

Root cause

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

# Step 1 - attacker, using NATIVE S3 tooling (NOT rclone) on a source the victim ingests from:
aws s3api put-object --bucket shared-drop --key '../../victim-backups/pwned.txt' --body evil.txt

# Step 2 - victim's ordinary ingest:
rclone copy s3-drop:shared-drop b2:victim-uploads/incoming
# path.Join("victim-uploads/incoming", "../../victim-backups/pwned.txt") = "victim-backups/pwned.txt"
# -> 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.

Precedent

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.