CVE-2026-21552: Modem Denial of Service Vulnerability

CVE-2026-21552 Overview

CVE-2026-21552 is an improper input validation vulnerability [CWE-20] affecting Unisoc modem components. A remote attacker can trigger a denial-of-service condition on the affected modem without requiring authentication or user interaction. The flaw does not expose data confidentiality or integrity, but successful exploitation causes a high-impact availability loss on the targeted device.

The issue is documented in the Unisoc Product Security Bulletin. No exploit code is publicly available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
Remote attackers can crash or disrupt modem functionality on affected Unisoc-based devices without privileges or user interaction, resulting in loss of cellular connectivity.

Affected Products

Unisoc modem components (specific chipset models listed in the vendor bulletin)
Mobile devices integrating affected Unisoc baseband firmware
IoT and embedded systems using vulnerable Unisoc modem stacks

Discovery Timeline

2026-08-03 - CVE-2026-21552 published to NVD
2026-08-03 - Last updated in NVD database

Technical Details for CVE-2026-21552

Vulnerability Analysis

The vulnerability resides in the modem firmware component of Unisoc chipsets. It stems from improper validation of input received over the network-facing modem interface. When the modem processes a malformed or unexpected input structure, the validation logic fails to enforce expected constraints on fields such as length, type, or value ranges.

Exploitation results in a denial-of-service condition affecting the availability of the modem subsystem. Because the attack targets baseband processing, a successful trigger can disrupt cellular connectivity on the affected device. No code execution or data disclosure occurs, per the vendor advisory.

The attack vector is network-based with low complexity and requires no privileges or user interaction. This exposure profile is characteristic of baseband vulnerabilities, where over-the-air signaling can reach the modem stack before higher-layer authentication is enforced.

Root Cause

The root cause is improper input validation [CWE-20] within the modem's message-parsing routines. The affected code path accepts attacker-controlled input without sufficient bounds or structural checks, leading to a fault that terminates modem operation.

Attack Vector

An attacker sends crafted signaling or protocol messages to the affected modem over the network. The malformed input reaches the vulnerable parsing routine, which fails to reject the invalid data and instead triggers a fault condition. The result is disruption of modem services on the target device.

Technical specifics of the malformed input format are not disclosed in the public advisory. Refer to the Unisoc Product Security Bulletin for vendor-provided details.

Detection Methods for CVE-2026-21552

Indicators of Compromise

Unexpected loss of cellular connectivity on Unisoc-based devices without corresponding network outages
Repeated modem crashes, restarts, or radio interface resets logged by the device
Abnormal baseband log entries referencing input validation failures or parser exceptions

Detection Strategies

Monitor device telemetry for modem subsystem crashes and reboot loops across fleets of Unisoc-based hardware
Correlate cellular disconnection events with geographic clustering that may indicate localized radio-based attacks
Review mobile device management (MDM) health data for elevated modem fault counts on affected chipsets

Monitoring Recommendations

Enable baseband and radio interface logging on managed devices where supported
Track vendor security bulletins for Unisoc firmware updates and correlate patch status against device inventory
Establish baselines for normal modem restart frequency to detect anomalous spikes

How to Mitigate CVE-2026-21552

Immediate Actions Required

Inventory all devices using Unisoc modem chipsets and identify those running unpatched firmware
Apply the firmware update referenced in the Unisoc Product Security Bulletin as soon as vendor and OEM releases become available
Coordinate with device OEMs and mobile carriers to accelerate distribution of patched baseband images

Patch Information

Unisoc has published remediation guidance in the Unisoc Product Security Bulletin. Device OEMs must integrate the corrected modem firmware into their platform updates and deliver it to end users through standard update channels.

Workarounds

Restrict device use in untrusted radio environments where feasible until patched firmware is deployed
Disable cellular connectivity on non-essential devices that cannot receive timely firmware updates
Rely on Wi-Fi connectivity for critical operations on affected devices until remediation is complete

bash

# Verify installed baseband/modem firmware version on Android devices
adb shell getprop gsm.version.baseband

# Compare the reported version against the fixed version listed in the
# Unisoc Product Security Bulletin before deploying to production fleets.