CVE-2026-79783

### 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()`: ```go 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 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.

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.

Unsafe file permission restoration from metadata in github.com/rclone/rclone