CVE-2026-21553 Overview
CVE-2026-21553 is an improper input validation vulnerability [CWE-20] affecting Unisoc modem components. A remote attacker can exploit this flaw to trigger a denial of service condition without requiring authentication or user interaction. Exploitation does not grant additional execution privileges but disrupts modem availability on affected devices.
The vulnerability was disclosed through the Unisoc Product Security Bulletin. The EPSS probability of exploitation stands at 0.403% with a percentile of 33.161, indicating limited near-term exploitation likelihood. No public proof-of-concept code is available at this time.
Critical Impact
Remote attackers can trigger denial of service against Unisoc modem-equipped devices over the network without authentication, disrupting cellular connectivity.
Affected Products
- Unisoc modem components (specific chipset models listed in the Unisoc Security Bulletin)
- Mobile devices integrating affected Unisoc baseband processors
- Consult the Unisoc Security Bulletin for the authoritative product list
Discovery Timeline
- 2026-08-03 - CVE-2026-21553 published to NVD
- 2026-08-03 - Last updated in NVD database
Technical Details for CVE-2026-21553
Vulnerability Analysis
The vulnerability resides in modem firmware code responsible for processing network-delivered input. The affected component fails to properly validate incoming data before further processing. An attacker can send crafted network traffic to the modem to trigger the fault condition.
Successful exploitation causes the modem to enter a denial of service state. The Unisoc advisory classifies the impact as availability loss only, with no confidentiality or integrity compromise. The attack does not require privileges or user interaction, making it reachable from an adjacent or remote network position.
Baseband processor vulnerabilities are particularly notable because the modem operates as a privileged subsystem separate from the application processor. Disruption at this layer affects all cellular services on the device.
Root Cause
The root cause is improper input validation [CWE-20] within the modem firmware. The vulnerable code path accepts input that violates expected structural or semantic constraints, leading to a fault that terminates or hangs modem processing. Public technical details describing the exact affected function or message type have not been released.
Attack Vector
The attack vector is network-based. An attacker within radio range or capable of injecting traffic into the cellular signaling path can deliver malformed input to the modem. The low attack complexity and absence of authentication requirements make exploitation straightforward once a working payload is developed. Refer to the Unisoc Security Bulletin for vendor-supplied specifics.
No verified exploit code is publicly available. The vulnerability description does not disclose the specific protocol layer or message format that triggers the fault.
Detection Methods for CVE-2026-21553
Indicators of Compromise
- Repeated unexplained cellular modem crashes, resets, or loss of registration on affected devices
- Baseband firmware watchdog events logged by the platform
- Abnormal patterns of malformed signaling traffic observed by mobile network operators
Detection Strategies
- Monitor mobile device management (MDM) telemetry for elevated modem reset or radio subsystem failure events across managed fleets
- Correlate cellular connectivity loss events with device logs to identify systemic patterns rather than isolated incidents
- Track vendor advisories from Unisoc and downstream OEMs for updated affected-model lists and firmware build identifiers
Monitoring Recommendations
- Ingest device health and radio telemetry into a centralized analytics platform for fleet-wide visibility
- Alert on repeated modem crash signatures on the same device or across devices with matching baseband firmware versions
- Review carrier-side signaling anomaly reports where available to identify targeted denial of service attempts
How to Mitigate CVE-2026-21553
Immediate Actions Required
- Inventory devices containing Unisoc baseband processors and cross-reference against the affected models listed in the Unisoc bulletin
- Apply firmware updates from device OEMs as soon as they incorporate the Unisoc patch
- Prioritize patching for devices operating in high-risk environments where cellular availability is critical
Patch Information
Unisoc has published remediation guidance in its product security bulletin. Downstream mobile device OEMs must integrate the fixed modem firmware into device software updates. Consult the Unisoc Security Bulletin for the authoritative patch reference and affected component list. Contact the device manufacturer for OEM-specific update availability.
Workarounds
- No vendor-published workaround eliminates the vulnerability without applying the firmware update
- Restrict use of affected devices in environments where cellular denial of service would materially impact operations until patches are available
- Maintain alternative connectivity paths (Wi-Fi, wired) for critical device functions where feasible
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
