CVE-2026-79783

rclone before 1.74.4 fails to mask special permission bits when applying source-supplied mode metadata in the local backend, allowing attackers to set setuid/setgid bits on attacker-controlled files. When copying with metadata preservation from an untrusted remote, attackers can plant a setuid binary that escalates privileges to root if rclone runs as root, or to the service account user otherwise.

rclone before 1.74.4 fails to mask special permission bits when applying source-supplied mode metadata in the local backend, allowing attackers to set setuid/setgid bits on attacker-controlled files. When copying with metadata preservation from an untrusted remote, attackers can plant a setuid binary that escalates privileges to root if rclone runs as root, or to the service account user otherwise.

Summary

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).

Details

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.)

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 a payload (the attacker-controlled binary content) and copy it with the malicious mode metadata:

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

3. Observe — the destination file is now setuid:

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.

Impact

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.

Remediation

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…

rclone before 1.74.4 fails to mask special permission bits when applying source-supplied mode metadata in the local backend, allowing attackers to set setuid/setgid bits on attacker-controlled files. When copying with metadata preservation from an untrusted remote, attackers can plant a setuid binary that escalates privileges to root if rclone runs as root, or to the service account user otherwise.

rclone before 1.74.4 fails to mask special permission bits when applying source-supplied mode metadata in the local backend, allowing attackers to set setuid/setgid bits on attacker-controlled files. When copying with metadata preservation from an untrusted remote, attackers can plant a setuid binary that escalates privileges to root if rclone runs as root, or to the service account user otherwise.

rclone before 1.74.4 fails to mask special permission bits when applying source-supplied mode metadata in the local backend, allowing attackers to set setuid/setgid bits on attacker-controlled files. When copying with metadata preservation from an untrusted remote, attackers can plant a setuid binary that escalates privileges to root if rclone runs as root, or to the service account user otherwise.