Language: 日本語

This is a PoC exploit for a vulnerability in the Nothing Phone (2a) / CMF Phone 1 secure boot chain (and possibly other MediaTek devices).

It abuses a logic flaw where one component isn't properly verified if the seccfg unlock state is set to unlocked.

The exploit achieves code execution at EL3 and breaks the secure boot chain after Preloader execution.

This exploit abuses a flaw in the MediaTek boot chain. When the bootloader is unlocked (seccfg), the Preloader skips verification of the bl2_ext partition, even though bl2_ext is responsible for verifying everything that comes after it.

The issue is critical because:

• Preloader jumps directly into bl2_ext while still running at EL3 (highest privilege level)
• bl2_ext controls the transition to EL1 and the non-secure world
• An unverified bl2_ext can load any subsequent images without checks

By patching bl2_ext to skip verification, the entire chain of trust collapses.

graph LR
    A["BootROM (SoC)"] --> B[Preloader]
    B --> C["bl2_ext (EL3, verified)"]
    C --> D[TEE]
    D --> E["GenieZone (GZ)"]
    E --> F[LK/AEE]
    F --> G["Linux kernel (EL1)"]
    
    classDef secure fill:#2ecc71,stroke:#27ae60,stroke-width:2px,color:#fff
    classDef trusted fill:#3498db,stroke:#2980b9,stroke-width:2px,color:#fff
    classDef kernel fill:#9b59b6,stroke:#8e44ad,stroke-width:2px,color:#fff
    classDef bootloader fill:#34495e,stroke:#2c3e50,stroke-width:2px,color:#fff
    
    class A,B bootloader
    class C,D,E secure
    class F trusted
    class G kernel

graph LR
    A["BootROM (SoC)"] --> B[Preloader]
    B -. skips verification .-> C["bl2_ext (patched, EL3)"]
    C --> D[TEE]
    C --> E["GenieZone (GZ)"]
    C --> F[LK/AEE]
    C --> G["Linux kernel (EL1)"]
    
    classDef normal fill:#95a5a6,stroke:#7f8c8d,stroke-width:2px,color:#fff
    classDef compromised fill:#e74c3c,stroke:#c0392b,stroke-width:3px,color:#fff
    classDef affected fill:#f39c12,stroke:#d68910,stroke-width:2px,color:#fff
    
    class A,B normal
    class C compromised
    class D,E,F,G affected

The actual exploit just patches sec_get_vfy_policy() to always return 0, so an unverified bl2_ext running at EL3 now happily loads unverified images for the rest of the boot chain.

Place your bootloader image in the bin/ directory with your device codename (e.g., bin/pacman.bin)

Once you have your bootloader image ready, you can build the exploit using the provided build.sh script:

# Using default bootloader location (bin/[device].bin)
./build.sh pacman

# Using custom bootloader path
./build.sh pacman /path/to/your/bootloader.bin

After building, you should see a new file named lk.patched in the root directory. You can now flash this patched bootloader image to your device:

./flash.sh

Currently, only the Nothing Phone (2a) (Pacman) is supported by the exploit. The exploit is also known to work on the CMF Phone 1 (Tetris), but support for that device is still incomplete.

Adding support for a new device isn’t straightforward, but it is possible with some effort and reverse engineering. A good starting point is to check whether your phone’s bl2_ext partition is verified.

If it isn’t, you may be able to use the exploit. You can verify this by examining an expdb dump and looking for the img_auth_required flag when the partition is being loaded:

[PART] img_auth_required = 0
[PART] Image with header, name: bl2_ext, addr: FFFFFFFFh, mode: FFFFFFFFh, size:654944, magic:58881688h
[PART] part: lk_a img: bl2_ext cert vfy(0 ms)

The payload isn’t strictly necessary, it’s more of a neat addition to the exploit. At the moment, it can’t modify memory at runtime, as doing so triggers an MMU fault that I haven’t resolved yet.

What it can do is register custom fastboot commands, control the boot mode, and dynamically call any built-in bootloader functions on the fly (you can probably do more, just use your imagination).

In addition to patching sec_get_vfy_policy(), the included PoC also spoofs the device’s lock state as locked so you can pass strong integrity checks anywhere while being unlocked. In theory, it should work with custom ROMs as well, though I haven’t personally tested this yet (it may require some additional vbmeta adjustments).

Another device known to be affected by this vulnerability is the Vivo X80 Pro. From my previous testing, it didn’t even verify bl2_ext despite having a locked bootloader (which makes it even worse).

• Add proper porting guide for new devices
• Fix MMU crashes when modifying memory at runtime
• Figure out proper payload appending method

This project is licensed under the GNU Affero General Public License v3.0 (AGPL-3.0).

Key points to be aware of:

• You are free to use, modify, and distribute the software.
• If you modify and use the software publicly, you must release your source code.
• You must retain the same license (AGPL-3.0) when redistributing modified versions.
• You cannot keep modifications private if the software is used to provide a networked service.

For full details, please refer to the LICENSE file.