Analysis of TP-Link Firmware with Header Version 1

Introduction

Analysis of TP-Link firmware file wr841nv11_wr841ndv11_en_3_16_9_up_boot(150616).bin with header version 1.

Header Structure

Offset Size (Bytes) Item Value
0 4 Version 0x01000000
4 24 Vendor Name TP-Link Technologies
0x1C 36 FW Version Ver. 1.0
0x40 4 HW ID 0x08410011
0x44 4 HW Revision 0x00000001
0x48 4 Unused 0
0x4C 16 Image MD5SUM 0xBF...65
0x5C 4 Unused 0
0x60 16 MD5SUM2 0x23...F0
0x70 4 Unused 0
0x74 4 Kernel Load Address 0x80002000
0x78 4 Kernel Entry Point 0x801CDB90
0x7C 4 FW Length 0x003E0200
0x80 4 Kernel Data Offset 0x00000200
0x84 4 Kernel Data Length 0x000CF72B
0x88 4Rootfs Offset 0x00100000
0x8C 4 Rootfs Data Length 0x002C0000
0x90 4 Bootloader Offset 0x00000000
0x94 4 Bootloader Data Length 0x0000C3B2
0x98 2 FW Ver Hi 0x0003
0x9A 2 FW Ver Mid 0x0010 (16)
0x9C 2 FW Ver Lo 0x0009
0x9E 4Unused 0


Analysis

cp wr841nv11_wr841ndv11_en_3_16_9_up_boot\(150616\).bin image_with_boot.bin

1. Verify MD5SUM of file. MD5 salt for image containing bootloader is 0x8cef335bd5c5cefaa79c28dab2e90f42. Replace MD5SUM in header with salt and then calculate MD5SUM of whole file.

cp image_with_boot.bin tmp.bin

printf '\x8c\xef\x33\x5b\xd5\xc5\xce\xfa\xa7\x9c\x28\xda\xb2\xe9\x0f\x42' | dd of=tmp.bin bs=1 seek=$((0x4C)) conv=notrunc

md5sum tmp.bin

2. In firmware containing bootloader, bootloader starts immediately after the header.

dd if=image_with_boot.bin iflag=skip_bytes,count_bytes of=bootloader.bin skip=$((0x200)) count=$((0xC3B2))

3. Extract rest of the firmware which contain a header, kernel and rootfs. It starts at offset 0x20200.

dd if=image_with_boot.bin iflag=skip_bytes,count_bytes of=fw.bin skip=$((0x20200))

4. MD5 salt for firmware is 0xdcd73aa5c39598fbddf9e7f40eae4738. To verify MD5, replace MD5SUM value in header with salt and then calculate MD5SUM.

cp fw.bin tmp.bin

printf '\xdc\xd7\x3a\xa5\xc3\x95\x98\xfb\xdd\xf9\xe7\xf4\x0e\xae\x47\x38' | dd of=tmp.bin bs=1 seek=$((0x4C)) conv=notrunc

md5sum tmp.bin

5. Extract kernel:

dd iflag=skip_bytes,count_bytes if=fw.bin of=kernel.bin skip=$((0x200)) count=$((0xCF72B))

6. Extract rootfs:

dd iflag=skip_bytes,count_bytes if=fw.bin of=rootfs.bin skip=$((0x100000)) count=$((0x2C0000))

sudo unsquashfs rootfs.bin

Demo

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