CVE-2017-7393 in TigerVNC
Summary
by MITRE
In TigerVNC 1.7.1 (VNCSConnectionST.cxx VNCSConnectionST::fence), an authenticated client can cause a double free, leading to denial of service or potentially code execution.
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Analysis
by VulDB Data Team • 11/24/2022
The vulnerability identified as CVE-2017-7393 resides within the TigerVNC 1.7.1 implementation, specifically in the VNCSConnectionST.cxx file within the VNCSConnectionST::fence method. This flaw represents a critical memory management issue that can be exploited by authenticated attackers who possess valid credentials to connect to a VNC server. The vulnerability manifests as a double free condition, which occurs when the same memory block is deallocated twice during the execution of the fence method, creating a scenario where the application attempts to free memory that has already been freed. Such memory corruption vulnerabilities are particularly dangerous as they can lead to unpredictable behavior in the affected application.
The technical nature of this vulnerability aligns with CWE-415, which describes improper double-free conditions in software implementations. The flaw occurs during the processing of fence requests within the VNC protocol implementation, where the application fails to properly track memory allocation and deallocation states. When an authenticated client sends a specially crafted fence request, the application's memory management routines execute the free operation twice on the same memory segment, potentially leading to heap corruption. This type of vulnerability can be leveraged by attackers to cause the VNC server to crash, resulting in a denial of service condition that prevents legitimate users from accessing the system.
The operational impact of CVE-2017-7393 extends beyond simple service disruption, as the double free condition creates opportunities for more severe exploitation scenarios. While the primary effect is denial of service, the memory corruption aspect introduces potential for code execution if an attacker can manipulate the heap layout to achieve arbitrary code execution. This vulnerability affects any system running TigerVNC 1.7.1 where VNC server functionality is enabled and accessible to authenticated users. The attack surface is particularly concerning in enterprise environments where VNC servers are commonly used for remote desktop access, as it provides a path for authenticated attackers to compromise system availability and potentially escalate privileges.
Mitigation strategies for this vulnerability should include immediate patching of TigerVNC installations to versions that address the double free condition in the fence method. System administrators should also implement network segmentation to limit access to VNC services and enforce strong authentication mechanisms. Monitoring for unusual fence request patterns and implementing intrusion detection systems can help identify potential exploitation attempts. The vulnerability demonstrates the importance of proper memory management in network services and aligns with ATT&CK technique T1070.004, which covers the use of legitimate credentials for privilege escalation and service disruption. Organizations should also consider implementing additional security controls such as disabling unnecessary VNC services, using VPNs for remote access, and regularly auditing VNC server configurations to prevent unauthorized access to vulnerable systems.