CVE-2025-20656: Yocto Out-of-Bounds Write Privilege Escalation

CVE-2025-20656 Overview

CVE-2025-20656 is an out-of-bounds write vulnerability in the MediaTek Download Agent (DA) component. The flaw stems from a missing bounds check that allows an attacker with physical access to the device to escalate privileges locally. Exploitation requires no additional execution privileges and no user interaction. The issue affects a broad range of MediaTek chipsets used in Android smartphones, IoT devices, and network equipment based on Yocto, RDK-B, and OpenWrt. MediaTek addressed the flaw with Patch ID ALPS09625423 (Issue ID MSV-3033) in the April 2025 Product Security Bulletin.

Critical Impact
A physical attacker can trigger a memory corruption condition in the MediaTek Download Agent to gain elevated privileges on affected devices without user interaction.

Affected Products

MediaTek chipsets: MT6781, MT6789, MT6835, MT6855, MT6878, MT6879, MT6886, MT6895, MT6897, MT6983, MT6985, MT6989, MT6990, MT8196, MT8370, MT8390
Google Android 12.0, 13.0, 14.0, 15.0 running on affected MediaTek SoCs
OpenWrt 21.02.0 and 23.05, RDK-B 2024q1, and Yocto 4.0 builds bundling the affected DA firmware

Discovery Timeline

2025-04-07 - CVE-2025-20656 published to NVD following MediaTek's April 2025 Product Security Bulletin
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-20656

Vulnerability Analysis

The vulnerability resides in the MediaTek Download Agent (DA), a low-level firmware component responsible for handling flashing and provisioning operations on MediaTek platforms. The DA runs early in the boot chain and interacts with tools such as SP Flash Tool over USB. A missing bounds check on attacker-controlled input allows a write past the end of an allocated buffer, corrupting adjacent memory in the DA's execution context. Because the DA operates outside the Android security model, memory corruption at this layer bypasses standard OS-level mitigations. The classification aligns with [CWE-787] Out-of-Bounds Write.

Root Cause

The root cause is the absence of input length validation before a memory write operation in the DA component. When the DA processes a crafted request or payload, it copies data into a fixed-size buffer without verifying that the source length fits the destination. The resulting overflow can overwrite control data, function pointers, or return addresses within the DA's memory space.

Attack Vector

Exploitation requires physical access to the target device, typically through a USB connection while the SoC is in a mode that exposes the Download Agent. An attacker connects the device to a controlled host and sends a malformed request that triggers the out-of-bounds write. Successful exploitation yields code execution at the DA's privilege level, which can be leveraged to install unauthorized firmware, bypass verified boot, or extract secrets prior to Android initialization. No user interaction on the target device is required.

Refer to the MediaTek Security Bulletin April 2025 for vendor technical details.

Detection Methods for CVE-2025-20656

Indicators of Compromise

Unexpected device reboots into flashing or download mode without administrator initiation
Firmware version or bootloader hash mismatches against known-good vendor baselines
Anomalous USB connection events on managed mobile devices logged by MDM or endpoint telemetry

Detection Strategies

Monitor mobile device management (MDM) attestation results and flag devices reporting altered boot components or failed Verified Boot checks
Compare firmware and bootloader fingerprints collected from fleet devices against MediaTek reference values published in the April 2025 bulletin
Investigate devices that spend unexplained time disconnected from managed networks, which may indicate offline physical tampering

Monitoring Recommendations

Enable and centrally collect Android SafetyNet or Play Integrity attestation results across the managed device fleet
Track chain-of-custody for devices used by executives, developers, and other high-value targets
Alert on repeated boot loops or DA-mode entries reported through OEM device management channels

How to Mitigate CVE-2025-20656

Immediate Actions Required

Inventory devices using the affected MediaTek chipsets and prioritize patch deployment for models based on MT6781 through MT8390
Apply OEM firmware updates that incorporate MediaTek Patch ID ALPS09625423 as soon as they are released for each device model
Restrict physical access to sensitive devices and enforce tamper-evident handling procedures for lost or unattended hardware

Patch Information

MediaTek released the fix under Patch ID ALPS09625423 (Issue ID MSV-3033) in the April 2025 Product Security Bulletin. Downstream vendors including Google, OpenWrt, RDK-B, and Yocto integrators must incorporate the corrected DA firmware into device-specific images. Consult the MediaTek Security Bulletin April 2025 for the authoritative patch reference and confirm the fixed version with the device OEM.

Workarounds

Disable or lock down USB debugging and download modes on production devices through OEM or MDM policy
Enforce bootloader locking and Verified Boot on all deployed Android devices to raise the cost of post-exploitation persistence
Physically secure devices in transit and storage; use tamper-evident seals for high-risk assets pending patch rollout

bash

# Example: verify Android Verified Boot state and patch level via adb
adb shell getprop ro.boot.verifiedbootstate
adb shell getprop ro.build.version.security_patch
adb shell getprop ro.boot.flash.locked