CVE-2021-41380 in Viewer
Summary
by MITRE • 09/18/2021
RealVNC Viewer 6.21.406 allows remote VNC servers to cause a denial of service (application crash) via crafted RFB protocol data.
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Analysis
by VulDB Data Team • 08/04/2024
The vulnerability identified as CVE-2021-41380 affects RealVNC Viewer version 6.21.406 and represents a significant denial of service flaw within the VNC protocol implementation. This vulnerability specifically targets the RFB protocol handling mechanism within the viewer application, where remote VNC servers can craft malicious protocol data that triggers application instability leading to complete system crashes. The issue stems from inadequate input validation and error handling within the RFB protocol parser, creating a condition where malformed data can cause the application to terminate unexpectedly without proper error recovery mechanisms.
The technical exploitation of this vulnerability occurs through the manipulation of RFB protocol messages that are exchanged between VNC servers and clients during the remote desktop connection process. When the RealVNC Viewer receives specially crafted RFB protocol data from a malicious server, the application fails to properly validate or sanitize the incoming data before processing it, resulting in a memory corruption or stack overflow condition that ultimately leads to application termination. This flaw falls under the category of improper input validation as classified by CWE-20, specifically involving the failure to properly handle malformed protocol data within network communication layers. The vulnerability demonstrates characteristics consistent with CWE-121, where buffer overflow conditions can occur when processing untrusted data, and CWE-125, which addresses out-of-bounds read conditions that may result from improper data handling.
The operational impact of this vulnerability extends beyond simple application disruption to potentially enable more sophisticated attack vectors within network environments where VNC viewers are commonly deployed. Attackers can leverage this flaw to remotely compromise the availability of desktop systems by forcing the VNC viewer application to crash repeatedly, effectively preventing legitimate users from accessing remote desktop sessions. This denial of service condition can be particularly damaging in enterprise environments where VNC viewers are used for remote administration, technical support, and system maintenance operations. The vulnerability affects systems where RealVNC Viewer is installed and actively used for remote desktop connections, creating a persistent threat that can be exploited by anyone with access to a VNC server that can establish a connection to the vulnerable client system. The impact is particularly severe in environments where VNC is used for critical system administration tasks, as the service disruption can halt important maintenance operations and technical support activities.
Organizations should implement immediate mitigations including updating to the latest version of RealVNC Viewer where this vulnerability has been patched, typically version 6.21.407 or later, which includes proper input validation and error handling for RFB protocol data. Network segmentation and access control measures should be enforced to limit exposure of VNC viewer applications to untrusted networks and servers. Implementing network monitoring solutions that can detect anomalous RFB protocol behavior may provide early warning of potential exploitation attempts. The mitigation strategy should also include disabling VNC viewer functionality when not actively needed and implementing secure remote access protocols such as SSH tunneling or VPN connections for VNC traffic to reduce exposure. Organizations should also consider implementing intrusion detection systems that can identify and alert on suspicious RFB protocol patterns that may indicate exploitation attempts. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving service stoppage and denial of service conditions, specifically mapping to T1499.004 for network disruption and T1566.002 for spearphishing with a malicious attachment, as attackers may use this vulnerability as part of broader attack campaigns targeting remote desktop infrastructure.