CVE-2018-20021 in LibVNC
Summary
by MITRE
LibVNC before commit c3115350eb8bb635d0fdb4dbbb0d0541f38ed19c contains a CWE-835: Infinite loop vulnerability in VNC client code. Vulnerability allows attacker to consume excessive amount of resources like CPU and RAM
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 06/19/2023
The vulnerability identified as CVE-2018-20021 represents a critical infinite loop condition within the LibVNC client implementation that manifests as a resource exhaustion attack vector. This flaw exists in the VNC client codebase prior to commit c3115350eb8bb635d0fdb4dbbb0d0541f38ed19c, where the library fails to properly handle certain malformed or malicious input sequences that trigger iterative processing loops without proper termination conditions. The vulnerability specifically maps to CWE-835, which defines infinite loops or iterations that do not terminate correctly, creating a scenario where computational resources become indefinitely consumed by the processing system.
The technical implementation of this vulnerability occurs when the VNC client encounters specific packet structures or data sequences that cause the internal state machine to enter a loop where it repeatedly processes the same input without advancing to a termination condition. This type of flaw typically arises from insufficient boundary checks or state validation within the protocol parsing logic, where the client fails to properly validate the length or structure of received data before attempting to process it in iterative fashion. The vulnerability is particularly concerning because it can be triggered through network-based attacks where an unauthenticated remote attacker can send specifically crafted VNC protocol messages that cause the client application to enter an infinite processing loop.
The operational impact of this vulnerability extends beyond simple resource consumption to potentially enable denial of service attacks against VNC client applications. When exploited, the infinite loop consumes excessive CPU cycles and may also cause memory allocation patterns that lead to memory exhaustion, effectively rendering the target system unusable or causing application crashes. This vulnerability affects any system running LibVNC client software that connects to potentially malicious VNC servers or that processes untrusted VNC protocol data. The impact is particularly severe in environments where VNC clients are used for critical infrastructure management, as the resource exhaustion could prevent legitimate remote access operations and create extended downtime for system administrators.
Mitigation strategies for this vulnerability require immediate patching of affected LibVNC installations to the version containing commit c3115350eb8bb635d0fdb4dbbb0d0541f38ed19c or later, which implements proper loop termination conditions and input validation. System administrators should also implement network segmentation and access controls to limit exposure of VNC client applications to untrusted networks. From a defensive perspective, this vulnerability aligns with ATT&CK technique T1499.004 for resource exhaustion attacks and demonstrates the importance of implementing proper input validation and boundary checking in protocol implementations. Organizations should conduct vulnerability assessments to identify all systems running vulnerable LibVNC versions and establish monitoring procedures to detect unusual CPU or memory consumption patterns that might indicate exploitation attempts. The fix typically involves adding proper timeout mechanisms and ensuring that iterative processing loops include adequate termination conditions based on data validation and state transitions.