CVE-2018-12550 in Mosquitto
Summary
by MITRE
When Eclipse Mosquitto version 1.0 to 1.5.5 (inclusive) is configured to use an ACL file, and that ACL file is empty, or contains only comments or blank lines, then Mosquitto will treat this as though no ACL file has been defined and use a default allow policy. The new behaviour is to have an empty ACL file mean that all access is denied, which is not a useful configuration but is not unexpected.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 08/08/2023
The vulnerability identified as CVE-2018-12550 affects Eclipse Mosquitto versions ranging from 1.0 through 1.5.5, specifically when the broker is configured to utilize Access Control List files for authentication and authorization purposes. This represents a significant security weakness that stems from the broker's handling of malformed or empty ACL configuration files, creating a potential privilege escalation vector that could allow unauthorized access to MQTT messaging services. The issue is classified under CWE-284 Access Control Bypass, which directly relates to improper access control mechanisms within software systems.
When Mosquitto encounters an ACL file that is either completely empty or consists solely of comments and blank lines, the broker's default behavior incorrectly interprets this condition as if no ACL file had been specified at all. This misinterpretation results in the broker automatically falling back to a permissive default policy that allows all access to MQTT resources, effectively bypassing any intended access restrictions. The vulnerability demonstrates a critical flaw in the broker's configuration validation logic, where the system fails to properly validate the integrity and content of ACL files before applying access control decisions.
The operational impact of this vulnerability extends beyond simple security concerns to encompass potential service disruption and data exposure risks. Organizations relying on Mosquitto for secure IoT communications, industrial automation, or messaging infrastructure may unknowingly expose their systems to unauthorized access when using empty or improperly formatted ACL files. This behavior creates a false sense of security where administrators believe access controls are properly enforced, while in reality, all connections are permitted, potentially allowing malicious actors to access sensitive messaging queues, publish unauthorized messages, or subscribe to confidential topics. The vulnerability is particularly concerning in environments where Mosquitto serves as a critical messaging component for industrial control systems, smart city infrastructure, or enterprise IoT deployments.
Mitigation strategies for CVE-2018-12550 require immediate attention from system administrators and security teams responsible for Mosquitto deployments. The most effective approach involves upgrading to version 1.5.6 or later, where the behavior has been corrected to treat empty ACL files as explicit denials rather than allowing all access. Organizations should also implement comprehensive configuration management practices that include regular validation of ACL files to ensure they contain proper access control directives. Additionally, security teams should conduct thorough audits of existing Mosquitto configurations to identify any instances where empty or comment-only ACL files might be in use, and replace them with properly configured access control lists that explicitly define allowed and denied permissions for different user groups and topics. The vulnerability aligns with ATT&CK technique T1078 Valid Accounts, as it can lead to unauthorized access through default permissive configurations that bypass proper authentication mechanisms.