Permission denied

Your OS is blocking access to the USB device.

PERMISSION_DENIED

Symptoms

  • The probe appears in lsusb (Linux) but MCUHex/pyOCD cannot open it.
  • Errors mention LIBUSB_ERROR_ACCESS, "Access denied (insufficient permissions)", or the probe list stays empty unless the SDK runs with sudo.
  • On Windows, the probe shows a warning icon in Device Manager, or only works in one specific tool.

Likely causes

  • Linux: missing udev rules — USB devices are owned by root by default, so your user account cannot open the probe.
  • Windows: wrong or missing driver — the ST-Link needs ST’s driver; a vendor-specific driver bound by another tool can also block generic access.
  • The SDK runs as a different user than the one the udev rule grants access to.

How to fix it

  1. Linux: install the probe udev rules

    Install ST’s rules, or pyOCD’s ruleset covering ST-Link, J-Link and CMSIS-DAP, then reload udev. A minimal ST-Link V2/V3 rule is shown below.

    sudo cp 49-stlinkv2.rules 49-stlinkv3.rules /etc/udev/rules.d/
sudo udevadm control --reload-rules
sudo udevadm trigger

# minimal rule contents (V2 / V2-1 / V3):
SUBSYSTEM=="usb", ATTR{idVendor}=="0483", ATTR{idProduct}=="3748", MODE="0666", TAG+="uaccess"
SUBSYSTEM=="usb", ATTR{idVendor}=="0483", ATTR{idProduct}=="374b", MODE="0666", TAG+="uaccess"
SUBSYSTEM=="usb", ATTR{idVendor}=="0483", ATTR{idProduct}=="374e", MODE="0666", TAG+="uaccess"

  2. Linux: replug the probe

    udev rules apply on device insertion — unplug and replug the USB cable after reloading rules. Do not keep running the SDK with sudo as a workaround.

  3. Windows: install the ST-Link driver

    Install ST’s official ST-Link USB driver, then check Device Manager: the probe should appear without a warning icon.

  4. macOS: allow USB access if prompted

    Recent macOS versions may prompt when a new USB device is accessed; approve it in System Settings → Privacy & Security.

  5. Retry in MCUHex

    Refresh the device list. The probe should now open without elevated privileges.

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