CVE-2014-6053 in LibVNCServer
Summary
by MITRE
The rfbProcessClientNormalMessage function in libvncserver/rfbserver.c in LibVNCServer 0.9.9 and earlier does not properly handle attempts to send a large amount of ClientCutText data, which allows remote attackers to cause a denial of service (memory consumption or daemon crash) via a crafted message that is processed by using a single unchecked malloc.
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Analysis
by VulDB Data Team • 04/07/2022
The vulnerability identified as CVE-2014-6053 resides within the LibVNCServer library version 0.9.9 and earlier, specifically within the rfbProcessClientNormalMessage function located in libvncserver/rfbserver.c. This flaw represents a critical security weakness that manifests when the server processes ClientCutText data, which is part of the RFB protocol used for remote desktop access. The vulnerability stems from inadequate input validation and memory management practices within the VNC server implementation, creating a pathway for malicious actors to exploit the system through carefully crafted network messages.
The technical exploitation of this vulnerability occurs when an attacker sends a specially crafted ClientCutText message containing an excessive amount of data that exceeds normal operational parameters. The rfbProcessClientNormalMessage function fails to properly validate the size of incoming ClientCutText data before attempting to allocate memory for processing. This unchecked malloc operation creates a scenario where the server allocates memory based on attacker-controlled values, leading to either excessive memory consumption that can exhaust system resources or trigger a daemon crash due to malformed memory allocation requests. The flaw operates at the protocol level, making it particularly dangerous as it can be exploited without requiring authentication or prior access to the system.
The operational impact of CVE-2014-6053 extends beyond simple service disruption, as it enables remote attackers to perform denial of service attacks against VNC servers running vulnerable versions of LibVNCServer. This vulnerability can be exploited by any remote attacker who can establish a connection to the VNC service, making it particularly concerning for publicly exposed VNC servers or those accessible over untrusted networks. The memory consumption aspect of the attack can lead to system instability, while the daemon crash component can result in complete service unavailability, effectively preventing legitimate users from accessing the remote desktop services. This vulnerability directly maps to CWE-122, which describes improper restriction of operations within a limited memory buffer, and aligns with ATT&CK technique T1499.004 for network denial of service attacks.
Mitigation strategies for CVE-2014-6053 primarily involve upgrading to a patched version of LibVNCServer, as version 0.9.10 and later contain the necessary fixes to properly validate ClientCutText data size limits before memory allocation occurs. Organizations should implement network segmentation and access controls to limit exposure of VNC services to trusted networks only, while also deploying intrusion detection systems to monitor for suspicious ClientCutText traffic patterns. Additionally, implementing rate limiting and connection monitoring can help detect and prevent exploitation attempts. The fix typically involves adding proper bounds checking and input validation to ensure that memory allocation requests are within reasonable limits and that the server can gracefully handle unexpected data sizes without crashing or consuming excessive resources.