CVE-2026-21548 Overview
CVE-2026-21548 is an improper input validation vulnerability [CWE-20] in the UniSoc New Radio (NR) modem component. An attacker can trigger a remote denial of service (DoS) condition by sending malformed input to the affected modem stack. Exploitation requires no user interaction and no privileges from the attacker, though the impact manifests at the modem's System execution context. The flaw affects device availability but does not compromise confidentiality or integrity. UniSoc published details in its Product Security Bulletin.
Critical Impact
Remote attackers can crash the baseband modem over the air, disrupting cellular connectivity on affected devices without authentication or user interaction.
Affected Products
- UniSoc NR (5G New Radio) modem firmware
- Mobile devices incorporating affected UniSoc baseband chipsets
- OEM platforms using vulnerable UniSoc modem software builds
Discovery Timeline
- 2026-08-03 - CVE-2026-21548 published to NVD
- 2026-08-03 - Last updated in NVD database
Technical Details for CVE-2026-21548
Vulnerability Analysis
The vulnerability resides in the NR modem's handling of untrusted protocol input. The modem accepts network-supplied data without adequately validating structural or length constraints. Processing of the malformed input drives the modem into an unrecoverable error state, causing a denial of service. Because the modem runs with System-level execution privileges on the baseband, the crash disrupts cellular radio functionality on the affected device.
The impact is limited to availability. There is no evidence that this flaw enables code execution, memory disclosure, or tampering with modem data. However, cellular connectivity loss can affect emergency calling, messaging, and data services until the modem or device is restarted.
EPSS data places the near-term exploitation probability at 0.403% (33rd percentile) as of 2026-08-06. No public proof-of-concept exploit is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.
Root Cause
The root cause is improper input validation [CWE-20] within the NR modem parser. Fields controlled by the serving network or an attacker-operated rogue base station are not bounded or type-checked before use. Downstream logic then operates on inconsistent state, producing a fatal fault in the modem process.
Attack Vector
The attack vector is the cellular air interface. An adversary within radio range, or one operating a rogue gNodeB, can transmit crafted NR signaling frames that the target modem parses. No pairing, authentication, or user interaction is required. Successful delivery of the malformed frame results in a modem crash and loss of cellular service until recovery.
Refer to the UniSoc Product Security Bulletin for vendor-supplied technical details.
Detection Methods for CVE-2026-21548
Indicators of Compromise
- Unexpected loss of cellular registration or repeated modem resets on affected devices
- Baseband crash logs referencing NR protocol parsing routines
- Concentrated modem failures across multiple devices in a single radio coverage area, suggesting a rogue base station
Detection Strategies
- Collect and review device bug reports and radio interface logs for abnormal NR signaling patterns and modem restart events
- Correlate device telemetry across fleets to identify geographic clusters of modem failures indicative of over-the-air exploitation
- Monitor mobile device management (MDM) inventories to confirm firmware versions align with UniSoc's patched builds
Monitoring Recommendations
- Track OEM and carrier security bulletins for firmware updates that reference the UniSoc advisory
- Enable crash reporting on managed mobile endpoints to surface baseband faults for triage
- Alert on anomalous rogue base station indicators, including unexpected PLMN identifiers or downgrade attempts
How to Mitigate CVE-2026-21548
Immediate Actions Required
- Identify devices in the fleet using UniSoc NR modem firmware through MDM inventory and vendor documentation
- Apply the OEM firmware update that incorporates UniSoc's patched modem image as soon as it is available
- Prioritize patching for high-risk users and devices operating in environments where rogue base stations are plausible
Patch Information
UniSoc has issued a fix in its coordinated security bulletin. Device manufacturers must integrate the updated modem image into their firmware releases and distribute it through carrier or OEM update channels. Consult the UniSoc Product Security Bulletin for the specific patch reference and affected build versions.
Workarounds
- Disable 5G NR on affected devices and restrict connectivity to LTE where operationally acceptable, reducing exposure to the vulnerable code path
- Restrict use of affected devices in untrusted radio environments until firmware updates are applied
- Enforce airplane mode in high-risk locations where rogue base station activity is suspected
# Example: disable 5G NR on Android to force LTE-only mode
# Dial code varies by OEM; consult device documentation
adb shell am start -a android.intent.action.MAIN -n com.android.settings/.Settings\$NetworkModeSettingsActivity
# Then select "LTE only" as preferred network type
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
