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The following steps are recommended for modifying device startup configurations and bootloaders like U-boot:

  1. Access Bootloader's Interpreter Shell:

    • During boot, press "0", space, or other identified "magic codes" to access the bootloader's interpreter shell.

  2. Modify Boot Arguments:

    • Execute the following commands to append 'init=/bin/sh' to the boot arguments, allowing execution of a shell command: %%% #printenv #setenv bootargs=console=ttyS0,115200 mem=63M root=/dev/mtdblock3 mtdparts=sflash:<partitiionInfo> rootfstype=<fstype> hasEeprom=0 5srst=0 init=/bin/sh #saveenv #boot %%%

  3. Setup TFTP Server:

    • Configure a TFTP server to load images over a local network: %%% #setenv ipaddr 192.168.2.2 #local IP of the device #setenv serverip 192.168.2.1 #TFTP server IP #saveenv #reset #ping 192.168.2.1 #check network access #tftp ${loadaddr} uImage-3.6.35 #loadaddr takes the address to load the file into and the filename of the image on the TFTP server %%%

  4. Utilize ubootwrite.py:

    • Use ubootwrite.py to write the U-boot image and push a modified firmware to gain root access.

  5. Check Debug Features:

    • Verify if debug features like verbose logging, loading arbitrary kernels, or booting from untrusted sources are enabled.

  6. Cautionary Hardware Interference:

    • Be cautious when connecting one pin to ground and interacting with SPI or NAND flash chips during the device boot-up sequence, particularly before the kernel decompresses. Consult the NAND flash chip's datasheet before shorting pins.

  7. Configure Rogue DHCP Server:

    • Set up a rogue DHCP server with malicious parameters for a device to ingest during a PXE boot. Utilize tools like Metasploit's (MSF) DHCP auxiliary server. Modify the 'FILENAME' parameter with command injection commands such as 'a";/bin/sh;#' to test input validation for device startup procedures.

Note: The steps involving physical interaction with device pins (*marked with asterisks) should be approached with extreme caution to avoid damaging the device.

References ​

Learn AWS hacking from zero to hero with htARTE (HackTricks AWS Red Team Expert)!

Other ways to support HackTricks: