External risk intelligence

Modem OTA Stack Buffer Overflow Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-66086

The vulnerability exists in a modem OTA (Over-the-Air) stack, which handles low-level wireless signaling protocols between a device and a cellular base station. This interface is not a general-purpose public-facing network service like a web or API endpoint and is not exposed to the public internet in the manner intended by this scoring rubric.

Remote Code Execution

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability in the modem's Over-the-Air (OTA) update system could allow an attacker to execute remote code or cause a denial of service by sending specially crafted scheduling information. This issue affects the system's ability to securely manage software updates over cellular networks.

Flaw in cellular update system.
Potential for remote code execution.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending specially crafted scheduling information within SIB1 messages to a device's modem. Because the modem's Over-the-Air stack does not properly validate this information, a buffer overflow can occur, potentially allowing the attacker to execute arbitrary code or cause a denial of service. This attack requires no special privileges and can be initiated remotely.

No entry preconditions.
Triggered by malformed SIB1 messages.
Leads to code execution or denial of service.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability in the modem's Over-the-Air update stack could allow an attacker to remotely execute code or cause a denial of service when processing specially crafted scheduling information within SIB1 messages. This could affect the device's normal operation and potentially lead to unauthorized code execution without needing any special privileges.

Modem OTA stack is at risk.
Processed malformed SIB1 messages.
Remote code execution or DoS.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability resides within the modem's Over-the-Air (OTA) stack, specifically impacting how it handles scheduling information from SIB1 messages. Given this low-level, device-centric nature, ownership likely falls to teams responsible for device firmware, platform integration, or embedded systems, potentially in coordination with vendor management if the modem component is third-party. The immediate priority is to determine the scope of affected devices, confirm any potential exposure, and identify the accountable internal owner before planning remediation during a suitable maintenance window.

Device firmware or platform teams should own.
Confirm device reachability and business criticality.
Plan remediation based on assessed risk.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is the modem OTA stack in CVE-2026-66086?

The modem Over-the-Air (OTA) stack is a specialized component within cellular-enabled devices. It handles the low-level wireless signaling protocols that allow a device to communicate with cellular base stations. This software manages essential connectivity tasks, including the reception of system information broadcasted by networks, ensuring the device stays synchronized and functional within the mobile infrastructure.

How does this stack-based buffer overflow occur?

This vulnerability, classified as CWE-121, happens when the modem software writes more data to a memory buffer than it can hold. In this specific case, the stack-based overflow is triggered by the modem's failure to properly validate the size and structure of scheduling information contained within incoming SIB1 messages. By exceeding these memory boundaries, the malformed input can potentially overwrite adjacent memory, leading to unauthorized code execution or system instability.

Do I need to be authenticated to trigger this vulnerability?

No, this vulnerability does not require the attacker to have any special execution privileges or user authentication. The flaw is triggered automatically when the modem processes a malformed SIB1 message received over the air. It is important to note that this trigger relies specifically on the reception of these cellular signaling messages; it is not triggered by standard user-initiated network traffic like web browsing or encrypted application data.

Is this CVE a risk to my public-facing web servers?

No. According to Halo Surface Signal, this vulnerability exists in a low-level wireless signaling interface rather than a general-purpose public-facing network service. Because it operates at the cellular radio protocol layer—not on an internet-facing API or web endpoint—it is generally unlikely to be accessible to standard internet-based remote attackers in the same way typical software vulnerabilities are.

How should I respond if my devices are affected?

Your first step is to identify the specific devices in your environment that utilize the affected modem hardware. Since this involves low-level firmware, you should coordinate with your embedded systems or platform integration teams to track vendor-provided security updates. Review your device inventory to assess business criticality and plan for the deployment of patches during a scheduled maintenance window.

References

www.unisoc.com/en/support/product-security-bulletin/2109454821805424…

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.