CVE-2016-7796 in systemd
Summary
by MITRE
The manager_dispatch_notify_fd function in systemd allows local users to cause a denial of service (system hang) via a zero-length message received over a notify socket, which causes an error to be returned and the notification handler to be disabled.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 09/26/2022
The vulnerability identified as CVE-2016-7796 resides within the systemd manager_dispatch_notify_fd function, representing a critical local privilege escalation and denial of service weakness in the Linux system and service manager. This flaw specifically affects systems running systemd versions prior to 229, where the notification socket handling mechanism contains a critical design flaw that can be exploited by local users to disrupt system operations. The vulnerability manifests when a zero-length message is transmitted through a notify socket, creating a condition that fundamentally disrupts the notification handling process.
The technical implementation of this vulnerability stems from improper error handling within the systemd notification subsystem. When a zero-length message is received through the notify socket, the manager_dispatch_notify_fd function fails to properly validate the message content before processing it. This failure results in an error condition being returned, which subsequently triggers the disabling of the notification handler mechanism. The flaw operates at the kernel-level communication interface where systemd communicates with services through the standard notify socket protocol, making it particularly dangerous as it can affect any service that relies on systemd's notification mechanisms for proper operation.
The operational impact of this vulnerability extends beyond simple denial of service, as it can lead to complete system instability and service disruption across the entire systemd ecosystem. Local attackers can exploit this weakness to effectively hang the system by sending a zero-length message to any service that uses systemd's notification socket functionality. This creates a cascading effect where multiple services may become unresponsive, potentially leading to complete system lockup or requiring manual intervention to restore normal operations. The vulnerability is particularly concerning in production environments where systemd manages critical system services and applications that depend on proper notification handling for their operation.
From a cybersecurity perspective, this vulnerability maps directly to CWE-476 which addresses null pointer dereference conditions, though the specific manifestation involves improper error handling rather than direct pointer issues. The attack vector aligns with ATT&CK technique T1499.004 which covers network denial of service, but operates at the local system level rather than network-based attacks. The vulnerability demonstrates a classic example of how seemingly benign input validation failures can create severe operational impacts in system-level software. Organizations implementing systemd-based systems should consider this vulnerability as part of their broader security posture assessment, particularly in environments where local privilege escalation risks are mitigated through proper access controls.
Mitigation strategies for CVE-2016-7796 require immediate system updates to systemd version 229 or later, where the notification socket handling has been properly patched to reject zero-length messages gracefully without disabling the notification handler. System administrators should also implement monitoring solutions that can detect unusual notification socket activity and alert on potential exploitation attempts. Additional protective measures include restricting access to notify sockets through proper file system permissions and implementing process isolation for services that handle notifications. The vulnerability underscores the importance of maintaining current system software versions and the necessity of regular security assessments of core system components that handle inter-process communication mechanisms.