CVE-2026-20503 Overview
CVE-2026-20503 is a denial-of-service vulnerability in the MediaTek Modem component. The flaw stems from a missing bounds check that can trigger a system crash when a User Equipment (UE) device connects to a rogue base station controlled by an attacker. Exploitation requires no user interaction and no additional execution privileges. MediaTek addressed the issue under Patch ID MOLY01371002 and Issue ID MSV-9020. The vulnerability is cataloged under [CWE-617] (Reachable Assertion) and affects a broad range of MediaTek chipsets used in smartphones, tablets, and connected devices.
Critical Impact
A remote attacker operating a rogue cellular base station can crash the modem of any nearby vulnerable MediaTek-powered device, disrupting cellular connectivity without user interaction.
Affected Products
- MediaTek modem firmware across the MT68xx smartphone series (including MT6833, MT6853, MT6873, MT6877, MT6885, MT6893, MT6983, MT6989, MT6991)
- MediaTek MT27xx series firmware (MT2716, MT2735, MT2737)
- MediaTek MT87xx series firmware used in tablets and automotive/IoT platforms (MT8675, MT8755, MT8791, MT8798, MT8893)
Discovery Timeline
- 2026-09-07 - CVE-2026-20503 published to the National Vulnerability Database (NVD)
- September 2026 - MediaTek publishes the MediaTek Security Bulletin - September 2026 with patch MOLY01371002
- 2026-09-09 - Last updated in NVD database
Technical Details for CVE-2026-20503
Vulnerability Analysis
The vulnerability resides in the baseband modem software that processes signaling messages received over the cellular air interface. A missing bounds check in the parser allows a malformed or oversized field from the network side to reach a code path that terminates the modem process. The condition maps to [CWE-617], a reachable assertion or unchecked condition that leads to abrupt termination.
Because the flaw is inside the modem firmware, a successful trigger halts cellular communication for the affected device. Voice, SMS, and mobile data become unavailable until the modem restarts. The attack vector is adjacent (radio proximity), and no authentication or user interaction is required.
Root Cause
The root cause is the absence of length or range validation on a protocol field parsed by the modem prior to use. When a rogue base station transmits a value outside the expected range, the modem reaches an unhandled condition and crashes. MediaTek's fix, tracked as Patch ID MOLY01371002 and Issue ID MSV-9020, introduces the missing input validation.
Attack Vector
An attacker sets up a rogue base station (for example, using a software-defined radio and open-source cellular stack) that advertises itself with parameters attractive to nearby UEs. Once a vulnerable MediaTek-powered device attaches or attempts attach procedures, the attacker sends a specifically crafted signaling message. The malformed message reaches the vulnerable parser, the missing bounds check fails to reject it, and the modem crashes. The result is a loss of cellular service for every affected device in radio range.
No verified public exploit code is available for CVE-2026-20503.
The vulnerability is described in prose only. Refer to the
MediaTek Security Bulletin for authoritative technical details.
Detection Methods for CVE-2026-20503
Indicators of Compromise
- Unexpected modem restarts, radio subsystem resets, or repeated cellular reattach events on MediaTek-based devices
- Simultaneous loss of cellular service across multiple devices in the same physical area
- Presence of unknown cell towers in device diagnostics with unusual Mobile Country Code (MCC) or Mobile Network Code (MNC) values
Detection Strategies
- Monitor mobile device management (MDM) telemetry for spikes in modem crash events, radio resets, or baseband exceptions on MediaTek chipsets
- Correlate cellular disconnection events across geographically clustered users to detect potential rogue base station activity
- Use RF survey tools in sensitive locations to identify unauthorized 4G/5G cells operating on licensed spectrum
Monitoring Recommendations
- Track vendor security bulletins from MediaTek and downstream device OEMs for the September 2026 patch level
- Ingest MDM and endpoint mobile telemetry into a centralized analytics platform to identify anomalous modem behavior at scale
- Establish a baseline of expected cellular attach patterns for corporate fleets and alert on deviations
How to Mitigate CVE-2026-20503
Immediate Actions Required
- Inventory all mobile devices, tablets, and IoT products built on the affected MediaTek chipsets listed in the advisory
- Coordinate with device OEMs to obtain firmware updates that incorporate MediaTek Patch ID MOLY01371002
- Prioritize patching for devices operated in high-risk environments where rogue base station attacks are plausible (executive travel, sensitive facilities)
Patch Information
MediaTek released the fix in the MediaTek Security Bulletin - September 2026. The correction is delivered as Patch ID MOLY01371002 addressing Issue ID MSV-9020. Because modem firmware ships through the device OEM, end users must install the vendor-provided system update that incorporates the September 2026 MediaTek patch level.
Workarounds
- Where feasible, restrict devices to 5G Standalone (SA) or trusted operator networks to reduce exposure to rogue 2G/4G cells
- Disable automatic network selection on high-risk devices and manually select trusted operators
- Use hardware or software tools that detect and alert on suspicious base stations (IMSI catchers) in sensitive locations
# No configuration-based mitigation is provided by the vendor.
# The authoritative remediation is to install the OEM firmware
# update containing MediaTek patch MOLY01371002 (September 2026).
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
