CVE-2026-44091: MQTT Broker Auth Bypass Vulnerability

CVE-2026-44091 Overview

CVE-2026-44091 is a high-severity vulnerability affecting an MQTT Broker implementation disclosed through CERT-VDE. An unauthenticated remote attacker can post a malicious ID to the MQTT Broker, which results in the creation of a new configuration entry in the system configuration. The flaw is categorized under [CWE-501] (Trust Boundary Violation) and enables an external actor to inject data across a trust boundary into the system's configuration store. Successful exploitation impacts the integrity and availability of the affected device without requiring authentication or user interaction.

Critical Impact
An unauthenticated network attacker can inject configuration entries into the target system, leading to integrity compromise and availability loss.

Affected Products

MQTT Broker component referenced in CERT-VDE advisory VDE-2026-008
Specific vendor and product identifiers were not published in the NVD record
Consult the CERT-VDE advisory for the authoritative list of affected devices and firmware versions

Discovery Timeline

2026-07-30 - CVE-2026-44091 published to NVD
2026-07-30 - Last updated in NVD database

Technical Details for CVE-2026-44091

Vulnerability Analysis

The vulnerability resides in the MQTT Broker's handling of message identifiers posted by remote clients. The broker accepts an attacker-supplied ID over the network and uses it to create a new configuration entry in the system configuration store. Because the ID crosses from an untrusted network context into a trusted configuration boundary without validation or authentication, an attacker can influence system behavior by injecting arbitrary configuration data.

The attack requires no credentials, no user interaction, and low attack complexity. The CVSS 4.0 vector reports high impact to integrity and availability of the vulnerable component, with no impact to confidentiality. Subsequent system components are not directly impacted according to the vector.

Root Cause

The root cause is a trust boundary violation [CWE-501]. The MQTT Broker treats data received from unauthenticated network peers as trusted input suitable for writing into the system configuration. Missing authentication controls and missing input validation on the posted ID allow an external client to introduce persistent configuration entries.

Attack Vector

Exploitation occurs over the network against the exposed MQTT service. An attacker publishes a crafted message containing a malicious ID to the broker. The broker processes the ID and writes a corresponding configuration entry into the system configuration. Repeated or targeted use of this primitive can alter device behavior, disrupt normal operations, or degrade availability of dependent services.

No verified proof-of-concept code has been published. See the CERT-VDE Security Advisory for vendor-supplied technical details.

Detection Methods for CVE-2026-44091

Indicators of Compromise

Unexpected new entries appearing in the system configuration store of the MQTT Broker or host device
MQTT PUBLISH messages originating from unauthenticated or untrusted sources on TCP port 1883 or 8883
Configuration changes with no corresponding administrative session or change ticket
Device instability, restarts, or service degradation following inbound MQTT traffic

Detection Strategies

Monitor MQTT broker logs for connection attempts from unauthenticated clients and for anomalous message identifiers
Baseline the expected set of configuration entries and alert on additions or modifications outside change windows
Inspect network flows to identify MQTT traffic reaching brokers from outside authorized network segments

Monitoring Recommendations

Forward MQTT broker and system configuration logs to a centralized analytics platform for correlation
Enable integrity monitoring on configuration files and persistent stores backing the broker
Alert on high-volume publish activity from single sources, which may indicate configuration flooding attempts

How to Mitigate CVE-2026-44091

Immediate Actions Required

Review the CERT-VDE Security Advisory VDE-2026-008 and identify affected devices in your environment
Restrict network exposure of the MQTT Broker to trusted management networks only
Enforce MQTT authentication and TLS for all broker connections
Audit the system configuration store for unauthorized entries created since the broker was first exposed

Patch Information

Refer to the CERT-VDE Security Advisory for vendor patch availability and firmware update instructions. No fixed version details are included in the current NVD record.

Workarounds

Block inbound access to MQTT ports 1883 and 8883 at the perimeter firewall where the broker does not need public reachability
Place the MQTT Broker behind a VPN or dedicated management VLAN accessible only to authorized operators
Require client certificate authentication or username and password authentication on the broker
Apply access control lists on the broker to restrict which clients can publish to configuration-related topics

bash

# Example: restrict MQTT broker exposure with iptables
iptables -A INPUT -p tcp --dport 1883 -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 1883 -j DROP
iptables -A INPUT -p tcp --dport 8883 -s 10.10.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 8883 -j DROP