CVE-2026-20526

Executive Summary

CVE-2026-20526 is an out-of-bounds write vulnerability in cellular modem firmware, caused by a missing bounds check according to the NVD description. The flaw affects mobile devices (User Equipment, UE) that connect to an attacker-controlled rogue base station, enabling remote escalation of privilege. Exploitation requires user interaction and no additional execution privileges, and the patch reference MOLY01898195 / MSV-8906 indicates MediaTek modem firmware. No public exploit, PoC, or CISA KEV entry exists as of this writing.

Technical Analysis

The vulnerability resides in modem firmware code that processes data received over the cellular radio interface. The NVD description states: "In Modem, there is a possible out of bounds write due to a missing bounds check." This places the flaw in the CWE-787 category (Out-of-bounds Write), meaning the modem processes attacker-influenced input without validating that a write operation stays within allocated buffer boundaries. A write past the end of a buffer can corrupt adjacent memory, potentially overwriting control structures or function pointers, which in the baseband context may allow the attacker to redirect execution within the modem's processing environment.

The attack surface is the radio interface between the UE and the base station. Because the flaw is triggered by data the modem accepts during connection to a base station, the attacker must control the base station or impersonate one. The description explicitly notes that exploitation requires the UE to be connected to a rogue base station controlled by the attacker, and that user interaction is needed. The "no additional execution privileges needed" wording indicates the attacker does not require prior code execution on the device or elevated access on the modem; the malformed network-side input alone reaches the vulnerable write path.

The modem runs a separate real-time operating system with its own memory space and privileges, distinct from the application processor. An out-of-bounds write in this context can allow escalation from the modem's processing domain toward more privileged modem functions, potentially crossing into the application processor environment depending on the device's inter-processor communication design. The patch is identified by Patch ID MOLY01898195 and Issue ID MSV-8906, a naming convention associated with MediaTek's modem firmware maintenance. The specific buffer, message type, or parsing function involved is not disclosed in the NVD description, so the exact code-level write site cannot be named without fabrication. What is clear from the description is the class of defect: an unchecked length or index used in a memory write during processing of cellular signaling or data received from the serving base station.

Because the vulnerability is exploited remotely over the air, it affects any UE that can be induced to camp on or connect to the attacker's base station. Rogue base station attacks are practical with software-defined radio hardware, and the requirement for user interaction — for example, the victim accepting a connection, operating in a location where the rogue station is the strongest cell, or performing an action that triggers the vulnerable code path — is the primary friction point. HSG Note: The media coverage and vendor-provided identifiers suggest the affected component is a widely deployed modem IP (MOLY-series naming), which historically ships across many OEM handsets and IoT modules, giving the flaw a broad potential device footprint even though specific model lists are absent here. The absence of CVSS scoring from NVD and absence of EPSS data means severity must be assessed from the CNA description alone; the described impact (remote escalation of privilege from the modem domain) is high, while exploitability is moderated by the rogue base station and user interaction preconditions.

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred from the vulnerability class (CWE-787) and the CNA description, and does not name any function, endpoint, or file path because none are provided.

1. Reconnaissance and target identification: The attacker sets up or rents a rogue base station using software-defined radio equipment. The attacker identifies a target area and configures the station to broadcast a cell that the victim UE will prefer — for example, by using a stronger signal, a matching or spoofed network identity, or by operating in a location where legitimate coverage is weak. The objective is to get the target UE to select and connect to the rogue cell.

2. Triggering the flaw: Once the UE connects, the attacker sends crafted cellular signaling or data over the air. The malformed input reaches the modem code path that lacks a bounds check. Because the write is not validated against the destination buffer length, the modem writes attacker-controlled data past the end of the intended buffer. This is the direct manifestation of CWE-787: a missing bounds check permits an out-of-bounds write during processing of the attacker's input.

3. Payload and execution method (vulnerability-class level): The precise message format that triggers the unchecked write is not described in the available data. At the vulnerability-class level, the attacker would craft the field or length value that the modem uses without validation, causing the out-of-bounds write to overwrite adjacent memory. Depending on how the overwritten memory is used by the modem firmware, the attacker may gain control over data structures or execution flow within the modem domain, achieving the described escalation of privilege. No specific offsets, structures, or function names can be provided without fabrication.

4. Post-exploitation impact: Successful exploitation yields privilege escalation in the modem, as explicitly stated in the description. The modem's privileged position — with access to the baseband, SIM, and inter-processor channels — makes this a significant foothold. The description does not state whether the escalation crosses into the application processor or enables persistent compromise, so the ultimate blast radius beyond the modem domain is not established by the available data. What is established is remote code-level impact without prior execution privileges, mediated by the requirement that the UE be connected to the attacker's rogue base station and that user interaction occur.

The practical barrier is the user-interaction requirement and the need to control the radio environment. An attacker capable of operating a rogue base station in proximity to the target can attempt this repeatedly across many passing UEs; the flaw is therefore attractive to well-resourced adversaries targeting specific individuals or mass-casualty device populations, less so to opportunistic remote attackers.

Impact Assessment

The described impact is remote escalation of privilege within the modem. Because NVD provides no CVSS score for this CVE, the vector in this report is an analyst-derived CVSS:3.1 estimate: AV:N (network-adjacent over the air), AC:L (no special conditions beyond the rogue base station and user interaction), PR:N (no additional execution privileges), UI:R (user interaction required), S:U, and high confidentiality, integrity, and availability impact reflecting code execution in a privileged component. The score is not vendor- or NVD-issued and should be treated as provisional.

Business impact centers on mobile endpoint compromise, potential exfiltration of cellular and device data accessible to the modem domain, and the risk of persistent or difficult-to-detect compromise at the baseband layer, which is often invisible to host-based endpoint security. Affected systems are any device using the affected modem firmware; the specific model list is not available. Blast radius: individual targeted users are at direct risk; if the flaw ships across a broad handset and IoT module base, the population at risk could be large, but the rogue base station and user-interaction preconditions constrain real-world exploitation to capable, proximity-based adversaries.

Detection Guidance

Detection is constrained by the baseband attack surface: much of the activity occurs over the air and beneath the OS visibility layer. Detection strategy therefore combines host telemetry for modem and baseband anomalies with network-side monitoring where modem traffic is visible (for example, in IoT gateways or test benches with baseband logging).

Sigma Rule

This rule monitors endpoint telemetry for the modem/baseband crash, reload, or kernel log signatures that may accompany a successful out-of-bounds write in modem firmware. Adapt the log source and event identifiers to your platform.

title: Suspected Cellular Modem Baseband Memory Corruption Indicator
id: 8e2b7c14-6f0a-4d3a-9d5e-2c1b0a7f4e33
status: experimental
description: Detects host-side indicators that may accompany exploitation of CVE-2026-20526 (modem out-of-bounds write) such as baseband crashes, modem resets, or memory fault reports.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-20526
author: valtersit.com
date: 2026/07/13
tags:
  - attack.t1190
logsource:
  product: linux
  service: kernel
detection:
  keywords_modem:
    - 'modem'
    - 'baseband'
    - 'moly'
  keywords_fault:
    - 'out of bounds'
    - 'memory corruption'
    - 'watchdog reset'
    - 'modem crash'
  condition: keywords_modem and keywords_fault
falsepositives:
  - Benign firmware update cycles
  - Legitimate modem resets during poor coverage
level: medium

YARA Rule

This rule targets files or firmware images for the vendor patch and issue identifiers and modem out-of-bounds write artifacts. It is intended for firmware inventory and forensic triage rather than runtime detection.

rule CVE_2026_20526_Modem_OOB_Write
{
    meta:
        description = "Detects modem firmware artifacts referencing CVE-2026-20526 patch and issue identifiers"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-20526"
        author = "valtersit.com"
        date = "2026-07-13"
        hash = "unknown"
    strings:
        $patch = "MOLY01898195" ascii wide
        $issue = "MSV-8906" ascii wide
        $nvd    = "CVE-2026-20526" ascii wide
        $cwe    = "out of bounds write" ascii wide nocase
    condition:
        uint32be(0) == 0x7F454C46 or
        uint32be(0) == 0x0A0D0A0D or
        filename matches /\\.(bin|img|mbn|elf)$/i and
        any of them
}

Suricata IDS Rule

Cellular signaling is generally not visible on IP networks, so this rule covers the gateway/modem-management IP path and flags transport to or from baseband management interfaces in combination with oversized or anomalous payloads. Adjust the IP variables to your baseband management subnet.

alert ip $HOME_NET any -> $HOME_NET any (msg:"CVE-2026-20526 Possible Modem Baseband Memory Corruption Attempt"; flow:to_server; dsize:>1024; content:"MOLY"; nocase; rawbytes; threshold: type both, track by_src, count 5, seconds 60; classtype:attempted-admin; sid:2026052601; rev:1;)

Elastic Detection

This EQL rule looks for baseband/modem fault events preceded by connection state changes, which may indicate a rogue base station exploitation attempt against CVE-2026-20526.

sequence by host.id with maxspan=5m
  [ process where process.name == "modemd" and event.action in ("start", "connect") ]
  [ file where file.path : "*baseband*" and event.action in ("write", "creation") ]
  [ any where event.category == "firmware" or event.action like "*modem*reset*" or event.action like "*baseband*crash*" ]

Splunk SPL Query

This search correlates modem and baseband log messages with fault or reset indicators and ranks by device to surface clusters that could indicate repeated exploitation attempts.

index=main (sourcetype=modem OR source=*baseband* OR host=*modem*)
| eval suspect = if(match(_raw, "(?i)(out of bounds|memory corruption|modem crash|watchdog reset|moly01898195|msv-8906)"), 1, 0)
| where suspect=1
| stats count min(_time) as first_seen max(_time) as last_seen values(host) as devices by host
| where count > 2
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Wazuh OSSEC Rule

This rule fires when modem or baseband logs contain crash, reset, or memory-corruption keywords associated with CVE-2026-20526.

<group name="modem,cve_2026_20526,">
  <rule id="100526" level="10">
    <decoded_as>syslog</decoded_as>
    <match>MOLY01898195|MSV-8906</match>
    <description>CVE-2026-20526: Modem firmware patch/issue identifier observed</description>
  </rule>
  <rule id="100527" level="12">
    <decoded_as>syslog</decoded_as>
    <regex>(?i)(modem|baseband).{0,40}(out of bounds|memory corruption|watchdog reset|crash)</regex>
    <description>CVE-2026-20526: Possible modem out-of-bounds write corruption event</description>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

Immediate mitigations:

• Treat untrusted or unknown cellular networks as hostile. Disable automatic connection to arbitrary or unknown base stations where the platform permits it, and for high-value devices consider locking to known network identities.
• Reduce exposure to rogue base stations by disabling 2G fallback where feasible (rogue base station attacks historically rely on downgrade to legacy, less-authenticated generations); this is a device/OEM-dependent setting.
• Maintain strict physical and RF hygiene for IoT and industrial devices with cellular modems: restrict the RF environment and monitor for unexpected cell changes.
• Apply defense in depth at the host: limit access to modem/baseband management interfaces, restrict inter-processor communication, and harden the application processor so that escalation out of the modem is contained.
• Monitor for and alert on modem crashes, resets, and memory fault reports as potential exploitation indicators.

Patch/upgrade steps:

• Contact the device OEM or module vendor and reference Patch ID MOLY01898195 and Issue ID MSV-8906 to confirm availability and obtain the fixed modem firmware. Because the patch confidence is zero, verify the fix is actually present rather than assuming it is shipped.
• Where the device OEM has released a firmware or OTA update that includes the modem patch, deploy it on an accelerated schedule for high-value or high-exposure devices.
• For devices that cannot be updated, consider compensating controls (network restrictions, removing the device from sensitive environments, or replacing fielded units with updated hardware).

Long-term hardening:

• Require modem and baseband vendors to publish clear advisories with affected version identifiers; the absence of an affected-version range for this CVE impedes asset management.
• Build inventory of devices by modem chipset/module, not just OS version, so baseband vulnerabilities can be tracked separately from platform updates.
• Adopt cellular equipment and network testing practices that detect anomalous base stations on site, and integrate baseband crash telemetry into the SOC even though it is often invisible to standard EDR.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-20526
• CWE-787 Out-of-bounds Write: https://cwe.mitre.org/data/definitions/787.html
• MediaTek product security / vulnerability disclosure: vendor to be confirmed via MOLY01898195 / MSV-8906 identifiers
• CISA Known Exploited Vulnerabilities catalog: not listed (checked; no CISA KEV evidence)
• FIRST EPSS: not yet scored (no EPSS value available)

No public PoC, exploit code, or Nuclei template is available for CVE-2026-20526, and there is currently no evidence of exploitation in the wild or association with a named threat actor campaign in the provided data.