Hi everyone,
I've been working on Hikvision K41 NVR firmware modification for a while, and I wanted to share my findings and give back to this community. This post covers unpacking, modifying, repacking, and verifying four K41 firmware versions. I'm also responding to a few existing threads that were very helpful to me.
Academic Research Statement: The unpacking, modification, and repacking of Hikvision K41 NVR firmware described in this post are performed solely for academic research and educational purposes. The goal is to understand embedded firmware structure, boot flow, and static integrity verification methods. This work does not involve, support, or encourage any other security threat, including unauthorized access, attacks, bypassing security mechanisms, planting backdoors, or any malicious use. No physical flashing was performed, and no attempt was made to bypass digital signature verification. The test target is K41 NVR only; DVR devices and other platforms were not tested.
What I worked with
Firmware: hikvision_3.4.87.dav, hikvision_3.5.30.dav, hikvision_3.5.35.dav, hikvision_4.1.50.dav
Platform: K41 series NVR
Tools: hikpack 2.5 (with -t k41), mkcramfs, mount -t cramfs -o loop, Python 3.10
Environment: Ubuntu 22.04 LTS on WSL2
What I found: two different firmware structures
K41 firmware comes in two layouts depending on version:
Structure A — Multi-file (4.1.50): After unpacking, you get separate files: uImage, start.sh, sys_app.tar.lzma, gui_res.tar.gz, webs.tar.lzma, TX1_Version.bin, etc. start.sh is plaintext and directly editable.
Structure B — cramfs image (3.4.87, 3.5.30, 3.5.35): After unpacking, you get cramfs.img, new_10.bin, new_20.bin. You need to mount the cramfs image to reach start.sh inside the root filesystem.
cramfs extraction — mount vs. binwalk
I tried binwalk first and ran into permission errors under WSL2. The reliable method was native Linux mount:
bash
mkdir -p /mnt/cramfs_tmp rootfs
sudo mount -t cramfs -o loop cramfs.img /mnt/cramfs_tmp
sudo cp -a /mnt/cramfs_tmp/* rootfs/
sudo umount /mnt/cramfs_tmp
After modifying files inside rootfs/, repack with:
bash
mkcramfs rootfs new_cramfs.img
Key finding: hikpack handles checksums automatically
This was the most important discovery for me. I compared new_10.bin before and after repacking, and confirmed that hikpack regenerates all checksum data during packing — file CRCs, new_10.bin, new_20.bin, and the header CRC. You do not need to manually update any CRC or MD5 values.
The pack command is simply:
bash
./hikpack -t k41 -p modified_firmware.dav -o firmware_extracted
And verify with:
bash
./hikpack -t k41 -i modified_firmware.dav
devclass must be preserved
The devclass field in the .dav header is the hardware platform identifier. I extracted it automatically from the original firmware and kept it unchanged during repacking. If you change it, the device may reject the firmware.
What I injected
I appended a simple command to the end of start.sh:
bash
echo "Custom script running" > /tmp/custom.log
This does not alter any existing logic — it just adds a line at the end of the boot script.
Results
Firmware devclass Structure Status
3.4.87 0x44 cramfs Repacked + verified
3.5.30 0x5A cramfs Repacked + verified
3.5.35 0x5A cramfs Repacked + verified
4.1.50 0x3D multi-file Repacked + verified
All four modified firmware files passed hikpack -i header CRC validation. I re-unpacked each one and confirmed the injected command appeared at the end of start.sh.
Structural verification (no physical flashing)
I did not flash these to a device. Instead, I built a static verification method I call SIS (Structural Integrity Signature) , which compares the boot-critical metadata before and after modification:
cramfs: superblock magic + size + version
uImage: magic + load address + entry point + image CRC
start.sh: sh -n syntax check
For all four firmware versions, the SIS was identical before and after modification.
Responses to existing threads
@montecrypto's hikpack thread (2016) — Your tool worked perfectly on my K41 targets. The automatic checksum regeneration you built in saved me a lot of manual CRC work. I'm not sure about the newest firmware versions, but on 3.4.87 through 4.1.50, hikpack 2.5 -t k41 works.
“How to extract .dav firmware files” (2023) — The warning about newer firmware possibly blocking hikpack is noted. I did not test the latest versions, so I can't confirm or deny it. For cramfs extraction, I'd add that mount -o loop is more reliable than binwalk on WSL2.
“Reverse engineering Hikvision DVR firmware” (2017) — Your workflow still holds up. My test target was K41 NVR, not DVR, so the device type is different, but the method has reference value. I automated the structure detection (multi-file vs. cramfs) and the repacking pipeline into a Python script, which handles both layouts transparently.
“Hikvision FIRMWARE TOOLS” (2015) — The early tools laid the groundwork. On my K41 units, the 3.x versions use cramfs, while 4.1.50 switched to a multi-file layout. hikpack -t k41 handles both.
What I have NOT verified
To be clear and avoid misleading anyone:
I have not physically flashed any of these modified firmware files to a device.
I have not tested whether the latest Hikvision firmware blocks hikpack.
I have not attempted to bypass digital signature verification.
I have not verified system functionality (hard drive, network, web UI, etc.) after modification.
This was tested only on K41 NVR — not on DVR, K51, R0, R1, or other platforms.
I hope this is useful to someone. If you've tried similar modifications on other K41 versions or other platforms, I'd like to hear your results.
Again: The unpacking, modification, and repacking in this experiment are for purely academic research only, with no other security threat or malicious purpose. They are not intended for unauthorized access, attacks, bypassing security mechanisms, planting backdoors, or any illegal use.
Cheers
I've been working on Hikvision K41 NVR firmware modification for a while, and I wanted to share my findings and give back to this community. This post covers unpacking, modifying, repacking, and verifying four K41 firmware versions. I'm also responding to a few existing threads that were very helpful to me.
Academic Research Statement: The unpacking, modification, and repacking of Hikvision K41 NVR firmware described in this post are performed solely for academic research and educational purposes. The goal is to understand embedded firmware structure, boot flow, and static integrity verification methods. This work does not involve, support, or encourage any other security threat, including unauthorized access, attacks, bypassing security mechanisms, planting backdoors, or any malicious use. No physical flashing was performed, and no attempt was made to bypass digital signature verification. The test target is K41 NVR only; DVR devices and other platforms were not tested.
What I worked with
Firmware: hikvision_3.4.87.dav, hikvision_3.5.30.dav, hikvision_3.5.35.dav, hikvision_4.1.50.dav
Platform: K41 series NVR
Tools: hikpack 2.5 (with -t k41), mkcramfs, mount -t cramfs -o loop, Python 3.10
Environment: Ubuntu 22.04 LTS on WSL2
What I found: two different firmware structures
K41 firmware comes in two layouts depending on version:
Structure A — Multi-file (4.1.50): After unpacking, you get separate files: uImage, start.sh, sys_app.tar.lzma, gui_res.tar.gz, webs.tar.lzma, TX1_Version.bin, etc. start.sh is plaintext and directly editable.
Structure B — cramfs image (3.4.87, 3.5.30, 3.5.35): After unpacking, you get cramfs.img, new_10.bin, new_20.bin. You need to mount the cramfs image to reach start.sh inside the root filesystem.
cramfs extraction — mount vs. binwalk
I tried binwalk first and ran into permission errors under WSL2. The reliable method was native Linux mount:
bash
mkdir -p /mnt/cramfs_tmp rootfs
sudo mount -t cramfs -o loop cramfs.img /mnt/cramfs_tmp
sudo cp -a /mnt/cramfs_tmp/* rootfs/
sudo umount /mnt/cramfs_tmp
After modifying files inside rootfs/, repack with:
bash
mkcramfs rootfs new_cramfs.img
Key finding: hikpack handles checksums automatically
This was the most important discovery for me. I compared new_10.bin before and after repacking, and confirmed that hikpack regenerates all checksum data during packing — file CRCs, new_10.bin, new_20.bin, and the header CRC. You do not need to manually update any CRC or MD5 values.
The pack command is simply:
bash
./hikpack -t k41 -p modified_firmware.dav -o firmware_extracted
And verify with:
bash
./hikpack -t k41 -i modified_firmware.dav
devclass must be preserved
The devclass field in the .dav header is the hardware platform identifier. I extracted it automatically from the original firmware and kept it unchanged during repacking. If you change it, the device may reject the firmware.
What I injected
I appended a simple command to the end of start.sh:
bash
echo "Custom script running" > /tmp/custom.log
This does not alter any existing logic — it just adds a line at the end of the boot script.
Results
Firmware devclass Structure Status
3.4.87 0x44 cramfs Repacked + verified
3.5.30 0x5A cramfs Repacked + verified
3.5.35 0x5A cramfs Repacked + verified
4.1.50 0x3D multi-file Repacked + verified
All four modified firmware files passed hikpack -i header CRC validation. I re-unpacked each one and confirmed the injected command appeared at the end of start.sh.
Structural verification (no physical flashing)
I did not flash these to a device. Instead, I built a static verification method I call SIS (Structural Integrity Signature) , which compares the boot-critical metadata before and after modification:
cramfs: superblock magic + size + version
uImage: magic + load address + entry point + image CRC
start.sh: sh -n syntax check
For all four firmware versions, the SIS was identical before and after modification.
Responses to existing threads
@montecrypto's hikpack thread (2016) — Your tool worked perfectly on my K41 targets. The automatic checksum regeneration you built in saved me a lot of manual CRC work. I'm not sure about the newest firmware versions, but on 3.4.87 through 4.1.50, hikpack 2.5 -t k41 works.
“How to extract .dav firmware files” (2023) — The warning about newer firmware possibly blocking hikpack is noted. I did not test the latest versions, so I can't confirm or deny it. For cramfs extraction, I'd add that mount -o loop is more reliable than binwalk on WSL2.
“Reverse engineering Hikvision DVR firmware” (2017) — Your workflow still holds up. My test target was K41 NVR, not DVR, so the device type is different, but the method has reference value. I automated the structure detection (multi-file vs. cramfs) and the repacking pipeline into a Python script, which handles both layouts transparently.
“Hikvision FIRMWARE TOOLS” (2015) — The early tools laid the groundwork. On my K41 units, the 3.x versions use cramfs, while 4.1.50 switched to a multi-file layout. hikpack -t k41 handles both.
What I have NOT verified
To be clear and avoid misleading anyone:
I have not physically flashed any of these modified firmware files to a device.
I have not tested whether the latest Hikvision firmware blocks hikpack.
I have not attempted to bypass digital signature verification.
I have not verified system functionality (hard drive, network, web UI, etc.) after modification.
This was tested only on K41 NVR — not on DVR, K51, R0, R1, or other platforms.
I hope this is useful to someone. If you've tried similar modifications on other K41 versions or other platforms, I'd like to hear your results.
Again: The unpacking, modification, and repacking in this experiment are for purely academic research only, with no other security threat or malicious purpose. They are not intended for unauthorized access, attacks, bypassing security mechanisms, planting backdoors, or any illegal use.
Cheers