CVE-2026-20519

Executive Summary

CVE-2026-20519 is an out-of-bounds write (CWE-787) in the MediaTek Modem component affecting the MT6833 (Dimensity 700) chipset firmware. The flaw stems from a missing bounds check in modem processing logic; an attacker operating a rogue base station can exploit it against a connected user equipment (UE) to achieve remote privilege escalation without user interaction. With no public proof-of-concept, no exploit code, and no confirmed patch evidence, the risk is concentrated in mobile-network interception scenarios affecting devices built on the MT6833 platform.

Technical Analysis

The vulnerability resides in the modem subsystem — the baseband processing environment that handles radio-layer communications between the UE and the cellular network. At its core is a missing bounds check preceding a write operation, producing a CWE-787 out-of-bounds write. Because bounds validation is absent at the point of the write, attacker-influenced input can cause data to be written beyond the intended buffer's boundaries.

The attack surface is the air interface. The NVD description states the attack requires the UE to be "connected to a rogue base station controlled by the attacker." This places the vulnerability in the class of baseband vulnerabilities exploitable via over-the-air attack infrastructure (e.g., IMSI catchers / false base stations). The vulnerability mechanism is therefore:

1. The UE associates with a base station whose signaling the attacker controls.
2. The attacker supplies crafted signaling or data to the modem's processing path.
3. A bounds check that should validate the size/offset of the data is missing.
4. A write operation occurs past the end of the target buffer, corrupting adjacent memory.
5. In the modem's execution context, this corruption can be leveraged to escalate privileges.

Notably absent from the description: any named function, struct, message type, or protocol layer. The description does not specify whether the flaw is in NAS, RRC, or another modem processing layer, nor which buffer is affected. No further code-level detail can be asserted without fabrication. What is certain is the vulnerability class (OOB write), the missing control (bounds check), the attack vector (rogue base station), and the outcome (remote privilege escalation with no additional execution privileges and no user interaction).

The CVSS vector — AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H — reflects that: the attack is adjacent (AV:A, the attacker must be within radio range and control the serving cell), high complexity (AC:H, both because operating rogue base station infrastructure and precisely triggering a memory-safety bug in baseband are non-trivial), requires no privileges (PR:N), requires no user interaction (UI:N), and yields high confidentiality, integrity, and availability impact on the affected component's scope (S:U).

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred strictly from the vulnerability class and the NVD description.

1. Reconnaissance / target identification. The attacker identifies devices operating on the MT6833 platform (Dimensity 700) within radio range, or targets a geographic area with known deployment of such devices. Because exploitation requires the target UE to attach to the attacker's cell, the attacker must first stand up or spoof base-station infrastructure capable of attracting the UE's connection.

2. Establishing control of the serving cell. The attacker deploys a rogue base station so that the victim UE connects to it. This is the prerequisite the description explicitly calls out — the vulnerability is only reachable "if a UE has connected to a rogue base station controlled by the attacker." This is the reconnaissance/positioning stage, not the exploit itself.

3. Triggering the flawed write. With the UE attached, the attacker transmits crafted signaling/radio data. The modem processes this data and reaches the vulnerable write path. Because the bounds check is missing, the size or offset validation that would normally reject malformed input does not fire, and the write proceeds beyond the buffer boundary. No user interaction is required — the UE processes incoming radio signaling automatically.

4. Achieving privilege escalation. The resulting out-of-bounds write corrupts memory in the modem context. Per the description, this "could lead to remote escalation of privilege" with "no additional execution privileges needed." The description does not specify the exact exploitation primitive (e.g., whether corruption of an adjacent structure, pointer, or control field is used), so the precise escalation technique cannot be asserted. The attacker gains elevated control within the affected modem software context.

5. Post-exploitation impact. Given high confidentiality, integrity, and availability impact, plausible outcomes include interception or manipulation of communications handled by the modem, degradation of service, or use of the corrupted context as a foothold. The NVD description does not enumerate post-exploitation behavior; no specific follow-on capability should be assumed beyond the stated escalation of privilege.

Impact Assessment

• Confidentiality (High): compromise in the modem context could expose communications and data processed by the baseband.
• Integrity (High): an attacker with elevated modem control may alter communications or device state.
• Availability (High): memory corruption in baseband processing can crash or destabilize the modem, affecting connectivity.
• Scope (Unchanged): impact is bounded to the vulnerable component.
• Attack Vector (Adjacent, AV:A): exploitation requires proximity in the radio sense and control of the serving cell — not internet-wide reachability. This materially narrows exposure relative to a network-vector bug. High attack complexity (AC:H) further reduces practical exploitability: reliably triggering a bounds-check bypass over the air in a baseband stack is difficult.
• Affected systems / blast radius: devices built on the MediaTek MT6833 (Dimensity 700) platform. This is a widely used mid-range SoC deployed across numerous Android handset models from multiple OEMs, so the potential affected population is large. However, exploitation is targeted and local-radio rather than mass-remote, so the realistic blast radius per attack is the set of UEs within range of a specific rogue cell.
• EPSS 0.00228 (12.5th percentile): current exploitation likelihood is assessed as very low, consistent with the absence of public PoC and the infrastructure burden of a rogue-base-station attack. CISA KEV: No — no evidence of known exploitation in the wild.

Detection Guidance

Network-level detection for baseband/radio-layer memory-safety bugs is inherently limited: modem signaling is largely opaque to host-based sensors, and the malicious traffic is RF, not IP. The rules below target the closest host- and telemetry-observable signals (modem crashes, malformed signaling anomalies logged by the device, and rogue-cell indicators) rather than the exploit itself. They are best-effort and require tuning.

Sigma Rule

title: Possible Modem Baseband Memory Corruption - MT6833 OOB Write (CVE-2026-20519)
id: 3f2a9c14-7b6e-4d21-9a5c-202620519001
status: experimental
description: Detects modem/baseband crash or abnormal reset consistent with an out-of-bounds write in the MediaTek MT6833 modem, potentially triggered via a rogue base station.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-20519
author: valtersit.com
date: 2026-01-01
logsource:
  category: system
  product: android
detection:
  keywords_modem:
    - "modem crash"
    - "modem reset"
    - "RILD"
    - "baseband"
  keywords_oob:
    - "out of bounds"
    - "buffer overflow"
    - "memory corruption"
  condition: keywords_modem and keywords_oob
falsepositives:
  - Legitimate modem firmware updates or carrier provisioning
  - Known benign modem resets on poor signal
level: medium
tags:
  - attack.t1068
  - cve.2026.20519

YARA Rule

rule CVE_2026_20519_MediaTek_Modem_OOB_Write
{
    meta:
        author = "valtersit.com"
        description = "Identifies potential exploitation artifacts / crash dumps referencing the MT6833 modem OOB write (CVE-2026-20519)"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-20519"
        cve = "CVE-2026-20519"
        date = "2026-01-01"
    strings:
        $moly = "MOLY01778993" ascii wide
        $msv  = "MSV-8898" ascii wide
        $modem = "modem" ascii nocase
        $oob1 = "out of bounds" ascii nocase
        $oob2 = "out-of-bounds" ascii nocase
        $bb  = "baseband" ascii nocase
    condition:
        (uint16(0) == 0x5A4D or $modem or $bb) and
        ($moly or $msv or $oob1 or $oob2)
}

Suricata IDS Rule

Note: the exploit traffic in this CVE is radio-layer (non-IP) and is not observable by a standard IP-based Suricata sensor. The rule below targets IP-observable telemetry that may correlate with rogue base station operations or modem-related artifacts exposed on the host stack; it is a heuristic, not a direct detection of the RF exploit.

alert ip any any -> any any (msg:"CVE-2026-20519 Possible MediaTek MT6833 Modem Compromise - Rogue Base Station Correlation"; flow:to_client; content:"baseband"; nocase; content:"crash"; nocase; distance:0; within:200; classtype:attempted-user; reference:cve,2026-20519; metadata:affected_product MediaTek_MT6833, attack_target Client_Endpoint, created_at 2026_01_01, cve CVE_2026_20519, signature_severity Major; sid:2026205190; rev:1;)

Elastic Detection

sequence by host.id with maxspan=5m
  [any where process.name in ("rild", "rild2", "modemd") and
     event.action == "started"]
  [system where event.category == "process" and
     (message like~ "*out of bounds*" or message like~ "*memory corruption*") and
     process.name in ("rild", "rild2", "modemd")]

Splunk SPL Query

index=android_logs (sourcetype=logcat OR sourcetype=android) 
("modem crash" OR "modem reset" OR "RILD" OR "baseband")
("out of bounds" OR "out-of-bounds" OR "memory corruption" OR "buffer overflow")
| stats count min(_time) as first_seen max(_time) as last_seen 
    values(host) as hosts, values(message) as messages 
    by src_ip, dest_ip
| where count >= 1
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Wazuh OSSEC Rule

<group name="cve-2026-20519,mediaTek,modem,">
  <rule id="2026205190" level="10">
    <decoded_as>json</decoded_as>
    <match>modem crash|modem reset|baseband</match>
    <regex match="out of bounds|out-of-bounds|memory corruption|buffer overflow" type="pcre2">(?i)(out[\s-]of[\s-]bounds|memory corruption|buffer overflow)</regex>
    <description>CVE-2026-20519: Possible MediaTek MT6833 modem out-of-bounds write / baseband memory corruption (rogue base station).</description>
    <mitre>
      <id>T1068</id>
    </mitre>
    <group>cve_2026_20519,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch evidence is available from any tracked source. Vendor fix: N/A. Workarounds: N/A.

1. Immediate mitigations (compensating controls):

   • Restrict device exposure to rogue/unauthorized base stations where platform support exists; enable and enforce network authentication/integrity protections for cellular signaling where the carrier supports them.
   • Maintain baseband firmware at the latest vendor/OEM build and monitor MediaTek Product Security Bulletins for a fix referencing Patch ID MOLY01778993 / Issue ID MSV-8898.
   • On managed fleets, increase logging/telemetry of modem resets and baseband crashes; unexpected reset spikes may indicate over-the-air exploit attempts.
   • In high-risk environments, limit reliance on MT6833-based devices for security-sensitive mobile communications until a patch is confirmed.

2. Patch / upgrade steps:

   • No vendor patch is currently identified. Track the MediaTek Product Security Bulletin and OEM (handset maker) security bulletins for firmware updates for MT6833-based devices.
   • When a fixed firmware version is published, deploy via the OEM's standard update channel. Verify the build references Patch ID MOLY01778993.

3. Long-term hardening:

   • Adopt a vendor/OEM security-update cadence for baseband firmware as a tracked, risk-managed asset — baseband patching is frequently neglected relative to the application/main OS layer.
   • Where feasible, deploy devices with baseband isolation/memory-integrity features and support signaling-layer protections that make attaching to unauthenticated cells harder for attackers.
   • Continuously monitor the MediaTek and affected OEM bulletins for the assigned fix; revalidate this vulnerability's status whenever a firmware update is shipped.

References

• NVD — CVE-2026-20519: https://nvd.nist.gov/vuln/detail/CVE-2026-20519
• MediaTek Product Security Bulletins: https://www.mediatek.com/product-security-bulletin
• CWE-787 — Out-of-bounds Write: https://cwe.mitre.org/data/definitions/787.html
• MITRE ATT&CK T1068 — Exploitation for Privilege Escalation: https://attack.mitre.org/techniques/T1068/
• CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog (CVE-2026-20519 not listed)

Threat actor / in-the-wild evidence: None available. No public PoC, no exploit code, no Nuclei template, and not listed in CISA KEV. Exploitation likelihood is currently assessed as very low (EPSS 0.00228), with the principal barrier being the requirement to operate rogue base station infrastructure and to reliably trigger a baseband memory-safety flaw over the air.