CVE-2020-16126 in AccountsService
Summary
by MITRE • 11/11/2020
An Ubuntu-specific modification to AccountsService in versions before 0.6.55-0ubuntu13.2, among other earlier versions, improperly dropped the ruid, allowing untrusted users to send signals to AccountService, thus stopping it from handling D-Bus messages in a timely fashion.
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Analysis
by VulDB Data Team • 12/05/2020
The vulnerability identified as CVE-2020-16126 represents a critical security flaw within the AccountsService component of Ubuntu systems, specifically affecting versions prior to 0.6.55-0ubuntu13.2. This issue stems from an improper implementation of user ID handling within the system's account management service, creating a pathway for unauthorized signal injection that fundamentally undermines the service's operational integrity. The flaw manifests through a specific modification to the AccountsService that fails to properly maintain the real user ID context during signal processing, creating a vector for malicious actors to disrupt service functionality.
The technical exploitation of this vulnerability occurs through the manipulation of process identifiers within the D-Bus communication framework that AccountsService utilizes for system account management. When the service processes incoming signals, it incorrectly drops the real user ID, allowing untrusted user processes to send signals that can interrupt or terminate the service's ability to properly handle D-Bus messages. This behavior creates a denial of service condition where legitimate system operations relying on AccountsService become disrupted, as the service cannot maintain its expected message handling capabilities. The vulnerability directly relates to CWE-284, which addresses improper access control mechanisms, and specifically demonstrates weak privilege management in system services.
The operational impact of CVE-2020-16126 extends beyond simple service disruption to potentially compromise the overall system security posture. When AccountsService becomes unresponsive or unable to process D-Bus messages correctly, it affects user account management functionalities including authentication, session handling, and system user privilege management. Attackers can leverage this vulnerability to maintain persistent access or escalate privileges by disrupting the service that normally enforces proper user account controls. The attack vector operates through standard user-space processes that can send signals to the AccountsService, making exploitation relatively straightforward and accessible to attackers with basic system access.
Mitigation strategies for this vulnerability require immediate system updates to the patched versions of AccountsService, specifically targeting Ubuntu versions 0.6.55-0ubuntu13.2 and later. System administrators should implement comprehensive monitoring for unauthorized D-Bus signal activity and establish baseline behaviors for AccountsService to detect abnormal signal processing patterns. Security configurations should enforce stricter D-Bus access controls and privilege separation mechanisms, ensuring that only authorized processes can interact with the AccountsService through proper authentication channels. The remediation approach aligns with ATT&CK technique T1068, which addresses local privilege escalation through service manipulation, and requires systematic patch management protocols to prevent exploitation. Organizations should also consider implementing additional security controls such as mandatory access controls and process monitoring to detect potential exploitation attempts before they can cause significant service disruption.