The process of modifying the system image of a mobile operating system opens up endless possibilities for enthusiasts. Deep intervention in the structure firmware allows you to remove pre-installed software, introduce third-party drivers, change system fonts and optimize power consumption at the kernel level. However, working with low-level memory sections requires not only theoretical knowledge about the file system, but also extreme caution, since one error in the syntax of the configuration file can turn the device into a “brick.”
Before you begin direct editing, you need to clearly understand the architecture of modern Android images. Unlike earlier versions, where the data was stored openly, modern firmware uses complex encryption schemes and dynamic partitions. You will need a specialized set of utilities that run on Linux or via Windows emulation, as well as a full backup of the original memory dump. Without creating a backup, any experiments with system partition are an unjustified risk of data loss.
In this guide we will look at the full cycle of work: from obtaining a stock image to its final packaging and firmware back into the device. We will not use ready-made mods, but will analyze the process of creating your own custom assembly from scratch, which will give you full control over every byte in the firmware.
Preparing tools and obtaining source data
The foundation of a successful modification is the correct choice of software. Standard archivers like WinRAR or 7-Zip will not cope with the format img or specific containers dat. You will need specialized tools, which are often distributed as command line scripts. The most popular solution is a package for Windows or a set of utilities for a Linux environment. The source material for the work is a firmware dump. It can be obtained in several ways: download the stock firmware from the manufacturer's official website, extract it via recovery mode (if you have access to TWRP) or use dump tools via Android Image Kitchen for Windows or a set of utilities simghack And simg2img for Linux environment.
The source material for work is a firmware dump. You can get it in several ways: download the stock firmware from the manufacturer's official website, extract it via recovery mode (if you have access to TWRP) or use dump tools via fastboot. It is important to make sure that the downloaded file corresponds exactly to your device model, including the board revision, since differences in drivers can lead to inoperability of communication modules.
⚠️ Attention: Make sure that the version of the Bootloader on your device is unlocked. An attempt to write a modified image to a device with a locked bootloader will result in a signature verification error and a refusal to boot the system.
After receiving the firmware file, it must be unpacked. Manufacturers often package images in payload.bin or use compression new.dat.br (Brotli). To work with them, separate converter scripts are required. The conversion process may take considerable time depending on the size of the image and the power of your processor.
☑️ Preparing for modification
Unpacking the system image and working with the file system
The central element of any firmware is the partition system.img. This is where all applications, libraries, fonts and operating system configuration files are stored. Modern devices use a file system ext4 or erofs (Enhanced Read-Only File System), which imposes certain restrictions on mounting methods. Simply copying files from an image is impossible without a correct unpacking procedure.
The unpacking process is usually performed using a script unpackimg.sh or a similar utility included in Android Image Kitchen. The script automatically detects the compression type, extracts the header and deserializes the contents into a regular folder. As a result, you get a directory whose structure completely replicates the Android root file system. Here you will find the folders app, priv-app, framework and etc.
When working with the file system, it is important to respect access rights. Although you are running as an administrator on the computer, the original permissions and file owners (UID/GID) are preserved inside the image. When repackaging, the utility must correctly read these attributes. If the rights are reset, system processes such as SystemUI or android.process.systemwill not be able to start due to errors accessing their resources.
Features of the EROFS file system
EROFS is a modern file system developed by Huawei and introduced into the public domain. It provides better compression and read performance than ext4, but is read-only. Editing EROFS images requires conversion to ext4 before unpacking, which can increase the final firmware size.
After unpacking, you get direct access to the files. This allows you to remove unnecessary system applications (bloatware), replace standard icons or modify resources. However, remember that deleting critical libraries from a folder is guaranteed to result in a cyclic reboot (bootloop). file lib or framework is guaranteed to lead to a cyclic reboot (bootloop).
Modifying system properties and configuration
One of the most effective ways to change the behavior of the system without deep intervention in the code is to edit the file build.prop. This text file contains key parameters that are read by the bootloader and system services at startup. By changing the values in this file, you can trick applications into thinking that the device has a different model, change the screen density (DPI) or disable the operator logo when loading.
For editing, use advanced text editors such as Notepad++ or VS Code, with the obligatory saving of the encoding in UTF-8 without BOM. Having BOM characters at the beginning of a file can break the system's parsing of properties, causing all your changes to be ignored or the download to fail. The file syntax is simple: each line contains a key=value pair.
| Property (Key) | Description | Example value |
|---|---|---|
ro.build.fingerprint |
A unique build fingerprint used to check application compatibility (for example, Netflix HD). | google/pixel5/redfin:11/RQ3A.211001.001/... |
ro.sf.lcd_density |
Pixel density per inch, which affects the size of the interface. | 420 |
persist.sys.usb.config |
Configuration of the default USB operating mode when connected to a PC. | mtp,adb |
ro.adb.secure |
USB debugging security setting. A value of 0 disables signature verification. | 0 |
Changing the setting ro.build.fingerprint is often used to unblock Netflix in high definition on devices that are not certified by Google. Replacing your fingerprint with a certified device (for example, Pixel or Xiaomi Mi Mix) allows you to bypass DRM software restrictions. However, this requires an accurate selection of the Android version, since a mismatch of API versions can cause the application to crash.
Before changing build.prop, copy the original file to a safe place. If after editing the system does not boot, you can restore the file via Recovery by simply replacing the damaged file with the original.
In addition to build.prop, the folder /system/etc/ contains hardware configuration files. For example, the file gps.conf allows you to configure almanac data download servers to speed up GPS cold starts, and the files in the folder permissions define which Android functions are available to the system (camera, NFC, telephony).
Replacing system applications and resources
The most visual part of the modification — replacement of system applications and graphic resources. The folder /system/priv-app/ contains critical applications such as SystemUI, Settings and Framework-res. Replacing these components allows you to completely change the appearance of the status bar, settings menu and system animations.
When replacing applications, you must take the signature into account. System applications are signed with the firmware developer key. If you simply copy a modified APK file from a third-party developer, the system will refuse to run it due to a signature mismatch. In such cases, you need to either re-sign the application itself with the platform key (which is difficult without access to the original manufacturer keys), or use overlay mechanisms (RRO - Runtime Resource Overlay), if they are supported by your version of Android.
To replace resources (pictures, sounds, fonts) inside system APKs, the tool APK Editor or MT Manager (on the device itself), or Apktool on the computer. The process involves decoding the APK, replacing the files in the folder res and reverse compiling. It is important to maintain the structure of the manifest and not violate the integrity of resources, otherwise the application will crash when trying to access a missing resource ID.
Particular attention should be paid to fonts. Replacing a file Roboto-Regular.ttf in a folder /system/fonts/ changes the font throughout the system. This is a simple and safe way of customization, which rarely leads to critical errors if the font file format is correct.
Building the image and checking integrity
After making all the necessary changes, the stage of repacking begins. This is the critical point where most newbies make mistakes. A packaging utility (for example packimg.sh) must correctly calculate the block size, create section headers, and apply compression if necessary. An incorrectly calculated image size will result in the partition not fitting into its allocated space in the partition table.
An important aspect is maintaining the security context. Modern versions of Android use Verified Boot (AVB). This is a mechanism that verifies the cryptographic signature of each partition upon boot. If you change at least one bit in the image system.img, the hash sum will change and the bootloader will block the system from starting, displaying an integrity violation warning.
⚠️ Attention: Bypassing the integrity check (AVB) is only possible on devices with an unlocked bootloader by disabling verification through kernel parameters or file patching
vbmeta.img. An attempt to disable verification on a locked bootloader is impossible.
To disable verification, you often need to create an empty or modified file vbmeta.img with flags that prohibit verification. The command for creating such a file through the utility avbtool looks like this:
./avbtool make_vbmeta_image --output vbmeta.img --flags 3This file is then flashed to the appropriate partition via fastboot. Without this procedure, the modified firmware will not load on modern devices running Android 9 and higher. It is also necessary to make sure that file attributes were not lost during packaging, especially the executability bit (chmod +x) for scripts and binaries.
Successful image assembly depends not only on the contents of the files, but also on the correct calculation of section headers and disabling signature verification mechanisms (AVB).
Flashing a modified image into the device
The final stage is writing the finished image into the device’s memory. To do this, the phone is switched to fastboot mode (usually by pressing the volume down button when turning it on). The connection to the computer must be stable, preferably through the original cable and USB 2.0 port on the motherboard, to avoid data transfer errors.
The command for flashing the firmware depends on the section name. For a system image, the command will look like fastboot flash system system.img. If you have modified the boot image (to introduce root access or a custom kernel), the command fastboot flash boot boot.imgis used. After flashing all the necessary partitions, it is necessary to clear the cache and data, since old settings may conflict with the new system.
The command fastboot erase userdata completely deletes all user data. This is a mandatory procedure when loading custom or heavily modified firmware for the first time. Ignoring this step often results in a hang on the splash screen logo due to a settings database conflict.
After cleaning, the command fastboot rebootis executed. The first launch may take up to 10-15 minutes, as the system optimizes applications (ART compilation). Do not interrupt this process, even if the screen goes blank or the device seems frozen.
Frequently asked questions (FAQ)
Is it possible to edit the firmware without unlocking the bootloader?
No, it is not possible. Android's security mechanism prevents writing to system partitions if the bootloader is locked. Any attempt to flash a modified image will be rejected with a signature error. Unlocking the bootloader usually requires obtaining a code from the manufacturer or using exploits, and it inevitably leads to a complete reset of the device.
What is a bootloop and how to get out of it after modification?
Bootloop is a cyclic reboot of the device that occurs when a critical system error occurs. If you get into bootloop after editing the firmware, you need to boot into Recovery mode (TWRP or stock). From there, you can try to wipe the data (Wipe Data/Factory Reset) or, if you have access to the file system, replace the modified file (for example, build.prop) with a backup copy through the terminal or file manager.
Is it safe to delete applications from the priv-app folder?
Removing applications from priv-app risky. Unlike a regular folder app, components that have extended access rights to the system are stored here. Removing applications such as Phone, Contacts, SystemUI or Google Play services will result in basic phone functions not working or loading completely. Delete only those applications whose purpose you are 100% sure of.
Why does the banking application not work after updating the firmware?
Banking applications use the Google SafetyNet service (or Play Integrity API) to check the integrity of the system. The presence of modified firmware, an unlocked bootloader or root access changes the device status to “unsafe”. To bypass this limitation, it is often necessary to use special modules (for example, Magisk Hide or Zygisk), which hide the fact of modification from checking applications.
Is it necessary to rebuild the recovery image when modifying the system?
Usually no. A Recovery image (such as TWRP) is an independent system maintenance tool. However, if you modify the boot image itself (boot.img) to implement superuser rights, then it is this that needs to be flashed. Recovery only needs to be rebuilt if you change the partition structure or implement specific recovery scripts into it.