Failing to Install Lineage from Recovery: A Comprehensive Troubleshooting Guide

We understand the frustration that arises when attempting to install LineageOS, only to be halted by the notorious adb: sideload connection failed: device unauthorized error. This specific issue is a common roadblock for users navigating the custom ROM installation process, particularly on modern devices like the Google Pixel 8. While the process of unlocking the bootloader and flashing a custom recovery is often straightforward, the final step of sideloading the LineageOS zip file can introduce complexities that are not immediately apparent.

This guide is designed to provide an exhaustive, step-by-step resolution to the unauthorized ADB sideload error. We will delve into the root causes of this issue, moving beyond surface-level fixes to explore the intricate relationship between your computer’s ADB drivers, the device’s bootloader state, and the recovery environment. Our goal is to equip you with the knowledge to not only resolve the current error but also to understand the underlying mechanics, ensuring a smoother installation process for any future custom ROM endeavors. We will dissect the error message, validate the prerequisites, and walk through the precise sequence of actions required to successfully authorize the ADB connection and initiate the LineageOS installation.

Understanding the ADB Unauthorized Sideload Error

The error message adb: sideload connection failed: device unauthorized is not merely a generic connectivity issue; it is a specific security protocol failure. It indicates that your computer, via the Android Debug Bridge (ADB), is attempting to establish a privileged connection with your device, but the device has not granted permission for this connection. This handshake is a fundamental security feature of the Android Open Source Project (AOSP), designed to prevent unauthorized access to your device’s data and system partitions.

Deciphering the Error Message

When we break down the error, we find several key components:

• adb: sideload connection failed: This confirms that the ADB client on your computer has successfully initiated a connection attempt but was rejected by the target device.
• device unauthorized: This is the core of the problem. The device’s ADB daemon (adbd) running within the recovery environment has not received a cryptographic key from your computer that it recognizes as trusted.
• This adb server's $ADB_VENDOR_KEYS is not set: This line points to a potential configuration issue on your host computer, suggesting that the ADB server is not aware of any vendor-specific keys. However, in most standard user scenarios, this is a secondary issue to the primary “unauthorized” state.
• Try 'adb kill-server' if that seems wrong. Otherwise check for a confirmation dialog on your device.: This is the most crucial hint provided by the ADB client. It explicitly directs you to look for a prompt on your device screen that requires user interaction to authorize the connection.

Why Authorization is Crucial for Sideloading

Sideloading a zip file via ADB requires an elevated level of trust. Unlike a simple file transfer, which might occur within the standard Android operating system, the adb sideload command pushes a complete software package directly to the recovery partition. This action has the potential to modify or completely replace the entire operating system. Therefore, the recovery environment, whether it’s Lineage Recovery or a third-party option like TWRP, enforces a strict authorization check. It will not accept a software package from any source without explicit user confirmation on the device itself. This prevents malicious actors from pushing compromised firmware to your device if they gain physical or network access to your computer.

Prerequisites for a Successful LineageOS Installation

Before we address the specific sideload error, we must ensure that the entire environment is correctly configured. A failure at any of these prerequisite stages can manifest as an authorization error later in the process. We will cover the essential software, drivers, and device preparations.

Essential Software and Tools

To successfully install LineageOS, you will need the following components installed and configured on your host computer (Windows, macOS, or Linux):

1. ADB and Fastboot Platform Tools: These are the core command-line utilities that allow your computer to communicate with the device in either ADB (Android Debug Bridge) mode or Fastboot mode. You can download the latest version directly from the official Android developer website. Ensure the location of these tools is added to your system’s PATH environment variable for easy access from any terminal window.
2. Device-Specific USB Drivers (Windows Users): For Windows-based systems, generic USB drivers are often insufficient. You must install the specific USB drivers for your device model (e.g., Google Pixel 8 USB drivers). These drivers allow the operating system to correctly identify your device in different modes (Fastboot, ADB, and standard charging). Manufacturer websites or trusted developer communities are the best sources for these drivers.
3. LineageOS ROM Package: Download the correct LineageOS build for your specific device model. Using a build for a different model, even a similar one, can lead to severe malfunctions. Verify the file’s integrity using the provided checksum if available.
4. Custom Recovery Image: Depending on your device, you will need a compatible custom recovery image. For many modern devices, LineageOS provides its own recovery. For others, you may need to use TWRP (Team Win Recovery Project). This recovery image must be flashed to the device before you can initiate the sideload.
5. Official GApps Package (Optional): If you require Google Mobile Services (GMS), such as the Play Store, Google Maps, and Gmail, you will need to download a GApps package compatible with your Android version (e.g., ARM64, Android 14). These are not included with LineageOS due to licensing restrictions.

Device Preparation and Configuration

Your device must be in a specific state to be recognized by ADB and Fastboot. Follow these steps meticulously:

1. Enable Developer Options: Navigate to Settings > About Phone and tap on the Build Number seven times. This will unlock the hidden Developer Options menu.
2. Enable USB Debugging: Go to Settings > System > Developer Options and enable USB Debugging. This is the master switch that allows ADB communication.
3. Unlock the Bootloader: This is a mandatory step for installing any custom ROM. The process varies by manufacturer. For Pixel devices, it involves using the fastboot flashing unlock command. Warning: Unlocking the bootloader will wipe all data from your device. Ensure you have backed up everything important.
4. Charge Your Device: Ensure your device has at least a 60% charge to prevent it from shutting down during the flashing process, which could result in a bricked device.

Step-by-Step Troubleshooting: Resolving the Unauthorized Error

This section provides a detailed, actionable guide to solving the device unauthorized error. We will proceed from the most common and simple solutions to more advanced troubleshooting steps.

The Critical Missing Step: Authorizing the ADB Sideload Request

The single most common reason for this error, as hinted in the original user’s post, is a lack of user interaction on the device screen. The authorization prompt does not always appear as a prominent pop-up in the center of the screen, especially within the minimalist interface of a custom recovery.

1. Initiate Sideload on Your Computer: Open a terminal or command prompt, navigate to your ADB and Fastboot platform tools directory, and run the sideload command:

   adb sideload lineage-21.0-xxx-xxx.zip
   

   (Replace the filename with the actual name of your LineageOS zip file).
2. Observe Your Device Screen Immediately: As soon as you execute the command, turn your full attention to your device’s screen, which should be displaying the Lineage Recovery interface.
3. Locate the Authorization Prompt: Look for any new text, box, or checkbox that has appeared. It might be located at the bottom or top of the screen. The prompt is often a simple checkbox or a “Start” button that you must interact with. It may say something like “Allow ADB Sideload?” or simply show a progress bar with an unchecked box next to it.
4. Authorize the Connection: Use the volume keys to navigate and the power button to select. Check the box or press “Start” or “Yes” to grant permission. Once authorized, the sideload process will begin, and you will see a progress indicator on both your computer and your device. This step is non-negotiable and must be performed every time you initiate an ADB sideload from recovery.

Resolving Computer-Side ADB and Driver Issues

If the authorization prompt never appears, the problem likely lies with your computer’s connection to the device.

1. Restart the ADB Server: The ADB server on your computer may be in a stuck state. In your terminal, run the following commands in order:

   adb kill-server
   adb start-server
   

   After restarting the server, disconnect and reconnect your device’s USB cable, then attempt the sideload again.
2. Check Device Detection: Before trying to sideload, verify that your computer can see the device in both Fastboot and ADB modes.
   • Fastboot Mode: Reboot your device into Fastboot mode (often done by holding Power + Volume Down). Run fastboot devices. You should see your device’s serial number. If not, your Fastboot drivers are incorrect or not installed.
   • ADB Mode: Reboot your device to recovery. Run adb devices. Your device should appear, but with the status unauthorized. If it appears with a serial number, it means the connection is being recognized, and the issue is purely the authorization step on the device. If it does not appear at all, your ADB drivers are faulty.
3. Update or Reinstall USB Drivers (Windows): Open Device Manager on your Windows PC. Look for your device under “Portable Devices,” “Other Devices,” or “Android Phone.” If you see a device with a yellow exclamation mark, the driver is not working correctly. Right-click on it, select “Update driver,” and choose the option to manually install the drivers you downloaded earlier. Alternatively, use the Google USB Driver package available through the Android SDK Manager.
4. Use a Different USB Port and Cable: This may seem simple, but it is a very common cause of connectivity issues. Some USB cables are power-only and lack the data lines required for ADB. Try a high-quality, data-enabled USB cable. Also, switch from a USB 3.0 port (usually blue) to a USB 2.0 port, as some devices have compatibility issues with USB 3.0 in recovery or fastboot modes.

Advanced Troubleshooting: SSH Keys and Vendor Keys

The error message mentions $ADB_VENDOR_KEYS. This is relevant for developers who have specific vendor keys for their devices, but for the average user, the solution lies in the adbkeys folder within the user’s home directory. Sometimes, these keys can become corrupted.

1. Delete Corrupted ADB Keys:
   • Linux/macOS: Navigate to ~/.android/ and delete the adbkey and adbkey.pub files.
   • Windows: Navigate to C:\Users\[YourUsername]\.android\ and delete the same files.
2. Regenerate Keys: After deleting the old keys, run adb kill-server followed by adb start-server. This will force the ADB server to generate new keys. The next time you connect your device, you should see a new authorization prompt on the device screen (if it appears in the standard Android OS). While this prompt may not show in recovery, ensuring your host keys are clean can resolve underlying connection issues.
3. Verify Fastboot and ADB Versions: Ensure you are using a recent version of the platform tools. Older versions may lack the necessary protocols to communicate with modern devices like the Pixel 8.

A Corrected, Chronological Installation Walkthrough

To prevent this error from occurring in the first place, we provide a corrected and verified chronological guide for installing LineageOS on a Pixel 8 or similar device.

1. Backup All Data: As stated, the bootloader unlock process will wipe your device.
2. Unlock the Bootloader:
   • Connect your powered-on device to your computer.
   • Open a terminal and run: adb reboot bootloader
   • Once in Fastboot mode, run: fastboot flashing unlock
   • On your device, use the volume keys to select “Unlock the bootloader” and confirm with the power button. The device will wipe its data and reboot.
3. Flash the Custom Recovery:
   • Download the correct Lineage Recovery or TWRP image for your device.
   • Reboot to Fastboot mode again: adb reboot bootloader
   • Flash the recovery image: fastboot flash recovery recovery.img (or fastboot flash boot recovery.img for some modern devices that use a boot.img-based recovery).
   • Do not reboot into the system yet. Immediately boot into recovery to prevent the stock recovery from overwriting your custom one.
4. Sideload LineageOS:
   • In the recovery menu, select “Apply update” or “Install.”
   • Choose “Apply from ADB” or “Sideload.”
   • This is the critical moment. On your computer, run: adb sideload [LineageOS_Zip_File].zip
   • IMMEDIATELY look at your device screen. Find the authorization prompt, check the box, or press “Start” to begin the transfer.
   • Wait for the process to complete. It can take several minutes.
5. Sideload GApps (Optional):
   • If you are installing GApps, you must sideload them in the same session. Do not reboot between sideloading LineageOS and GApps.
   • After the LineageOS sideload finishes, the recovery menu will reappear. Select “Apply update from ADB” again.
   • On your computer, run: adb sideload [GApps_Zip_File].zip
   • Again, ensure you authorize the connection on your device screen.
6. Wipe Data/Cache (If Necessary): Some installations require a final wipe of the Dalvik/ART cache and cache partition. Follow the specific instructions provided with your LineageOS build.
7. Reboot to System: Select “Reboot system now.” The first boot may take longer than usual as the system sets up for the first time.

Conclusion

The adb: sideload connection failed: device unauthorized error is a frustrating but entirely solvable obstacle. By understanding the security protocols behind ADB and following a methodical troubleshooting process, we can successfully overcome this hurdle. The key is to meticulously check every link in the chain: from ensuring the correct drivers are installed on your computer to recognizing and interacting with the authorization prompt on your device’s screen. By adhering to the detailed steps and corrected workflow outlined in this guide, you can confidently proceed with your LineageOS installation, transforming your device and unlocking its full potential. Should you encounter further complexities, the vast community of custom Android enthusiasts, including forums like XDA-Developers, remains an invaluable resource for sharing knowledge and finding device-specific solutions.