CVE-2025-20696: Yocto Privilege Escalation Vulnerability

CVE-2025-20696 Overview

CVE-2025-20696 is an out-of-bounds write vulnerability in the MediaTek Download Agent (DA) component. The flaw results from a missing bounds check [CWE-787] and affects a wide range of MediaTek chipsets used across Android smartphones, IoT devices, and embedded Linux platforms. An attacker with physical access to the device can leverage this issue to achieve local escalation of privilege. Exploitation requires user interaction but no additional execution privileges. MediaTek assigned the fix tracking identifier Patch ID ALPS09915215 and Issue ID MSV-3801, and published the advisory in the MediaTek Product Security Bulletin August 2025.

Critical Impact
Physical attackers can corrupt memory in the MediaTek Download Agent to escalate privileges on dozens of chipset models shipped in Android devices, OpenWrt routers, Yocto builds, and Zephyr-based products.

Affected Products

MediaTek chipsets including MT6739, MT6761, MT6765, MT6768, MT6781, MT6789, MT6833, MT6853, MT6877, MT6893, MT6983, MT6989, MT6991, MT8188, MT8196, MT8370, MT8390, and MT8676
Google Android 13.0, 14.0, and 15.0 builds on affected MediaTek silicon
Linux Foundation Yocto 4.0, RDK-B 2024q1, OpenWrt 21.02.0 and 23.05, Zephyr 3.7.0

Discovery Timeline

2025-08-04 - CVE-2025-20696 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-20696

Vulnerability Analysis

The vulnerability resides in the MediaTek Download Agent (DA), a low-level component used during device bring-up and firmware flashing over the preloader interface. The DA accepts command and data buffers from a connected host and processes them in SRAM before handing control to higher-level loaders. Because the DA runs with elevated privileges below the operating system, a memory corruption flaw here translates directly to control over the boot chain.

The root issue is a missing bounds check preceding a write operation [CWE-787]. When the DA parses an attacker-supplied field, it writes beyond the end of the destination buffer, overwriting adjacent memory used by the agent itself. This enables an attacker to influence control-flow structures or configuration state inside the DA runtime.

Root Cause

The DA does not validate that the size or offset derived from an incoming request stays within the bounds of the target buffer. The code path trusts length information supplied by the host without comparing it to the allocated region, which is the classic pattern described by CWE-787 (Out-of-bounds Write).

Attack Vector

Exploitation requires physical access to the target device, typically by connecting it to a host over USB in a mode that exposes the preloader or Download Agent interface. User interaction is also required, consistent with workflows that require the device to be powered into a flashing or recovery state. Once the DA session is established, the attacker submits a crafted payload that triggers the out-of-bounds write and achieves local privilege escalation below the operating system.

No verified public exploit code is available. See the MediaTek Product Security Bulletin August 2025 for vendor-supplied technical details.

Detection Methods for CVE-2025-20696

Indicators of Compromise

Unexpected entries into preloader, BROM, or Download Agent mode on provisioned production devices
Firmware or bootloader hashes that do not match vendor-signed baselines after a device has been in physical custody of a third party
Device management logs showing attestation failures or unexpected serial-over-USB connections

Detection Strategies

Enforce measured boot and remote attestation so that tampered DA sessions produce verifiable attestation mismatches
Monitor MDM or UEM telemetry for devices entering flashing modes outside approved service workflows
Audit factory and repair workflows that expose the preloader interface, and alert on out-of-policy host connections

Monitoring Recommendations

Track Android ro.boot.verifiedbootstate and bootloader-locked status across the MediaTek fleet
Review MediaTek Product Security Bulletin updates monthly and correlate patch IDs such as ALPS09915215 against installed firmware
For OpenWrt, Yocto, RDK-B, and Zephyr builds on affected chipsets, monitor vendor firmware release notes and rebuild images against patched BSPs

How to Mitigate CVE-2025-20696

Immediate Actions Required

Apply the MediaTek firmware update containing Patch ID ALPS09915215 from the device OEM as soon as it is published for the affected chipset
Keep bootloaders locked and verified boot enabled on all production Android devices
Restrict physical access to deployed devices, especially those used in field, retail, or kiosk deployments

Patch Information

MediaTek addressed the vulnerability under Patch ID ALPS09915215 / Issue ID MSV-3801, listed in the MediaTek Product Security Bulletin August 2025. Downstream OEMs integrate the fix into Android security patch levels and into Yocto, RDK-B, OpenWrt, and Zephyr board support packages built on the affected chipsets. Validate the installed firmware revision against the OEM advisory before considering a device remediated.

Workarounds

Maintain a locked bootloader and reject OEM unlock requests on managed devices through MDM policy
Disable or restrict USB data access on managed devices to limit exposure of the Download Agent interface
Enforce chain-of-custody controls for devices sent to repair or returned from the field before redeployment

bash

# Verify Android bootloader and verified boot state on affected MediaTek devices
adb shell getprop ro.boot.verifiedbootstate
adb shell getprop ro.boot.flash.locked
adb shell getprop ro.build.version.security_patch