CVE-2026-21555 Overview
CVE-2026-21555 is an improper input validation vulnerability [CWE-20] in Unisoc modem components. Attackers can trigger a remote denial of service condition without requiring authentication or user interaction. The flaw stems from the modem's handling of malformed input over the network interface. Successful exploitation causes service disruption on affected devices but does not grant additional execution privileges to the attacker.
Unisoc disclosed this issue through its Product Security Bulletin. The vulnerability affects modem baseband processing, a component widely deployed in mobile and embedded devices using Unisoc chipsets.
Critical Impact
Remote attackers can cause denial of service on devices with vulnerable Unisoc modem firmware without authentication or user interaction.
Affected Products
- Unisoc modem firmware (specific versions listed in the vendor bulletin)
- Mobile devices and embedded systems using affected Unisoc baseband chipsets
- Downstream OEM devices integrating vulnerable Unisoc modem components
Discovery Timeline
- 2026-08-03 - CVE-2026-21555 published to NVD
- 2026-08-03 - Last updated in NVD database
Technical Details for CVE-2026-21555
Vulnerability Analysis
The vulnerability resides in the Unisoc modem's input validation logic. The modem fails to properly validate structure or content of incoming data received over the radio interface. When the modem processes a crafted input, it enters an error state that halts normal operation.
The issue maps to CWE-20 (Improper Input Validation). Baseband processors handle protocol messages at a low level. Any parsing flaw in this layer can crash the modem stack and interrupt cellular connectivity for the device.
Exploitation requires network adjacency to reach the target radio interface. No credentials are needed. The attack does not compromise confidentiality or integrity, but availability is fully impacted on the modem subsystem.
Root Cause
The root cause is missing or insufficient validation of a field in a protocol message processed by the modem firmware. The parser accepts inputs outside expected ranges or structural constraints and reaches an unsafe code path. This triggers a fault that terminates the modem process or forces a reset.
Attack Vector
An attacker within radio range of the target, or operating a rogue base station, transmits a crafted message to the victim modem. The malformed payload passes reachability checks but fails deeper processing logic. The modem crashes, dropping calls, data sessions, and any dependent services until the baseband recovers or the device reboots. See the Unisoc Product Security Bulletin for details on affected message handling.
No verified proof-of-concept code has been published for CVE-2026-21555. Refer to the vendor advisory for technical specifics on the affected protocol handler.
Detection Methods for CVE-2026-21555
Indicators of Compromise
- Unexpected modem resets, baseband crashes, or radio interface restarts across a fleet of devices
- Loss of cellular connectivity clustered by geography, suggesting a nearby rogue transmitter
- Kernel or vendor log entries referencing modem exception dumps and ramdumps
- Repeated RIL (Radio Interface Layer) reconnection events in device telemetry
Detection Strategies
- Correlate device-side crash telemetry with cellular network events to identify malformed message triggers
- Monitor mobile device management (MDM) alerts for elevated modem restart counts on affected chipsets
- Analyze baseband ramdumps for signatures matching the affected parser function once identified in the vendor advisory
Monitoring Recommendations
- Ingest device crash and modem event logs into a centralized SIEM for anomaly detection
- Track modem firmware versions across the device inventory to identify unpatched systems
- Alert on clusters of simultaneous connectivity loss that may indicate active exploitation attempts
How to Mitigate CVE-2026-21555
Immediate Actions Required
- Identify all devices in the environment running affected Unisoc modem firmware
- Apply vendor patches from device OEMs as soon as they are distributed for the affected chipsets
- Prioritize remediation on devices operating in high-risk radio environments or public spaces
Patch Information
Unisoc has published fixes through its Product Security Bulletin. OEMs must integrate the patched modem firmware into their device updates. Consult device vendors for availability of firmware updates addressing CVE-2026-21555.
Workarounds
- Restrict use of affected devices in untrusted radio environments until firmware updates are applied
- Where supported, disable cellular radios or switch to Wi-Fi-only mode on high-value devices pending patch deployment
- Deploy rogue base station detection where feasible to identify hostile transmitters attempting exploitation
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
