When writing an object with metadata, the local backend applies the source-supplied mode, uid, and gid verbatim: it parses mode as an octal integer and passes it straight into os.Chmod(o.path, os.FileMode(umode)), and passes uid/gid straight into os.Chown. The value is never masked to permission bits, so any value with Go's ModeSetuid (1<<23) or ModeSetgid (1<<22) bit set causes the setuid/setgid bit to be applied. Because both the file content and its metadata come from the (attacker-controlled) source remote, an attacker stores a binary of their choosing with mode = 40000755 (and uid = 0); when the victim runs rclone copy -M <remote>: /dest, rclone writes the attacker's binary and makes it setuid. If the victim runs rclone as root (typical for system backup/restore), the uid=0 chown plus setuid produces a root-owned setuid binary with attacker content — any local user then escalates to root. When rclone runs as a non-root service user, the planted setuid binary is owned by that user, giving any local user that user's privileges (lateral escalation / persistent backdoor).

backend/local/metadata.go, writeMetadataToFile():

uid, hasUID := o.parseMetadataInt(m, "uid", 10)
gid, hasGID := o.parseMetadataInt(m, "gid", 10)
if hasUID {
  ...
  err = os.Chown(o.path, uid, gid)    // source-controlled owner, no same-uid guard
}
mode, hasMode := o.parseMetadataInt(m, "mode", 8)
if hasMode && mode >= 0 {
  umode := uint(mode)
  if umode <= math.MaxUint32 {
    err = os.Chmod(o.path, os.FileMode(umode))  // <-- raw value; ModeSetuid/ModeSetgid NOT masked off
  }
}

os.Chmod/os.FileMode honor ModeSetuid/ModeSetgid/ModeSticky. There is no &^ (os.ModeSetuid|os.ModeSetgid) mask and no check that the source is trusted, so an attacker-chosen mode string sets those bits on the freshly written, attacker-controlled file. (Note: a legitimate local source reports mode in unix st mode layout e.g. 0106755, whose bit 1<<23 is unset, so honest copies happen to drop setuid — but the attacker supplies the Go-FileMode layout 40000755 directly, which sets it.)

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

mkdir -p msrc mdst && cp /bin/true msrc/payload
./rclone copy -M --metadata-set mode=40000755 msrc mdst    # 40000755 = Go FileMode setuid|0755

stat -c '%A %a' mdst/payload
-rwsr-xr-x 4755                        # 's' = setuid bit SET on attacker binary

Variants: mode=20000755 → setgid (-rwxr-sr-x); mode=60000755 → both (-rwsr-sr-x). With rclone run as root and the source object also carrying uid=0/gid=0, the file is chown'd root:root, yielding a root-owned setuid binary executable by any local user.

A victim performing a metadata-preserving copy/restore (-M) from an untrusted or compromised remote installs an attacker-chosen executable with the setuid/setgid bit set. Run as root (system backup/restore, the common case for --metadata), this is a root-owned setuid root backdoor executable by any local user → local privilege escalation to root. Run as a non-root user, it is a setuid backdoor for that service account. The companion uid/gid application lets a root-run transfer also reassign ownership of written files arbitrarily.

Mask special bits before applying mode from metadata — os.Chmod(o.path, os.FileMode(umode).Perm()) (or umode & 0o777) — and do not honor setuid/setgid/sticky from source metadata; gate uid/gid/setuid application behind an explicit opt-in (e.g. --local-metadata-set-ownership) that is off by default, and document that -M from untrusted remotes must not restore privileged bits. Regression test: copying an object with mode=40000755/uid=0 must produce a non-setuid, caller-owned file unless the opt-in is set.