CVE-2026-20434: Mediatek Lr12a Privilege Escalation Flaw

CVE-2026-20434 Overview

CVE-2026-20434 is an out-of-bounds write vulnerability in MediaTek modem firmware caused by a missing bounds check. This memory corruption flaw affects a wide range of MediaTek chipsets used in smartphones, tablets, and other mobile devices. An attacker controlling a rogue cellular base station can exploit this vulnerability to achieve remote escalation of privilege on affected devices without requiring any execution privileges on the target.

The vulnerability is particularly concerning as it targets the modem component, which operates at a privileged level within the device's architecture. Successful exploitation requires that the victim's User Equipment (UE) connects to a malicious base station controlled by the attacker, and some user interaction is needed for exploitation.

Critical Impact

Remote privilege escalation via rogue base station attack affecting 90+ MediaTek chipsets used in millions of mobile devices worldwide.

Affected Products

• MediaTek MT67xx Series (MT6739, MT6761-MT6769, MT6771-MT6899)
• MediaTek MT68xx Series (MT6833-MT6991)
• MediaTek MT87xx Series (MT8666-MT8893)
• MediaTek MT27xx Series (MT2735, MT2737)
• MediaTek LR Series (LR12A, LR13)
• MediaTek NR Series (NR15, NR16, NR17)

Discovery Timeline

• March 2, 2026 - CVE-2026-20434 published to NVD
• March 2, 2026 - Last updated in NVD database

Technical Details for CVE-2026-20434

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write), indicating that the modem firmware writes data beyond the boundaries of an allocated memory buffer. The flaw stems from insufficient validation of input data received during cellular communication, allowing an attacker to corrupt adjacent memory regions.

The modem subsystem processes signaling messages from base stations as part of normal cellular communication. When a device connects to a malicious base station, the attacker can craft specially formatted messages that exploit the missing bounds check. This causes the modem firmware to write attacker-controlled data outside the intended buffer, potentially overwriting critical control structures or code pointers.

Since the modem operates with elevated privileges and handles sensitive operations like cellular authentication and encryption, compromising it can have severe security implications. An attacker could potentially intercept communications, inject malicious data, or use the compromised modem as a stepping stone to attack the application processor.

Root Cause

The root cause is a missing bounds check in the MediaTek modem firmware when processing certain input data. The firmware fails to validate that incoming data fits within the allocated buffer before performing write operations. This allows oversized or malformed input to overflow the buffer boundaries.

The vulnerability is tracked internally by MediaTek as Patch ID: MOLY00782946 and Issue ID: MSV-4135, indicating it was identified through their internal security processes.

Attack Vector

The attack requires the adversary to operate a rogue cellular base station within radio range of the target device. The attack flow involves:

1. Base Station Setup: The attacker deploys a rogue base station (IMSI catcher or fake cell tower) broadcasting a stronger signal than legitimate towers
2. Device Connection: The victim's device connects to the rogue base station, either automatically or through user interaction
3. Malicious Payload Delivery: The attacker sends specially crafted signaling messages containing oversized data
4. Memory Corruption: The modem processes the malicious input without proper bounds checking, causing out-of-bounds writes
5. Privilege Escalation: The corrupted memory state allows the attacker to escalate privileges within the modem subsystem

This attack is classified as Adjacent Network (AV:A) because it requires proximity to the target device via the cellular radio interface.

Detection Methods for CVE-2026-20434

Indicators of Compromise

• Unexpected cellular network disconnections followed by connections to unknown cell towers
• Unusual modem crash logs or firmware exceptions in device diagnostics
• Abnormal cellular signaling patterns detected by network monitoring tools
• Device connecting to base stations with suspicious parameters (unusual Cell IDs, weak authentication)

Detection Strategies

• Monitor device logs for modem subsystem crashes or watchdog resets
• Implement network-level detection for rogue base stations using specialized RF scanning equipment
• Deploy cellular intrusion detection systems (IDS) to identify anomalous signaling
• Use baseband security monitoring tools where available to detect exploitation attempts

Monitoring Recommendations

• Enable detailed modem logging on managed mobile devices for forensic analysis
• Coordinate with mobile carriers to implement rogue base station detection
• Monitor firmware integrity through device management solutions
• Track MediaTek security bulletins for additional IOCs and detection signatures

How to Mitigate CVE-2026-20434

Immediate Actions Required

• Apply firmware updates from device manufacturers that incorporate MediaTek patch MOLY00782946
• Check device manufacturer websites for security updates addressing the March 2026 MediaTek bulletin
• Prioritize updates for devices in high-security environments or handling sensitive data
• Consider restricting device usage in areas where rogue base station attacks are feasible

Patch Information

MediaTek has released a security patch addressing this vulnerability as documented in the MediaTek Security Bulletin March 2026. The patch is identified as MOLY00782946. Device manufacturers (OEMs) must integrate this patch into their firmware updates and distribute them to end users.

Organizations should check with their device manufacturers for the availability of updated firmware that includes this fix. The broad range of affected chipsets means updates may be released on varying schedules depending on the device manufacturer and carrier.

Workarounds

• Avoid connecting devices to unfamiliar or untrusted cellular networks when possible
• Disable automatic network selection in high-risk environments to prevent connection to rogue base stations
• Use WiFi calling where available as an alternative to cellular connectivity in sensitive locations
• Implement mobile device management (MDM) policies to enforce firmware update compliance

# Check device firmware version on Android
# Settings > About Phone > Build Number
# Compare against manufacturer's security patch level

# Verify MediaTek chipset model
adb shell getprop ro.hardware
adb shell getprop ro.board.platform

# Check current security patch level
adb shell getprop ro.build.version.security_patch