Source: cve@mitre.org
Heap-based buffer overflow in rfbproto.c in LibVNCClient in LibVNCServer before 0.9.11 allows remote servers to cause a denial of service (application crash) or possibly execute arbitrary code via a crafted FramebufferUpdate message containing a subrectangle outside of the client drawing area.
LibVNCServer versions prior to 0.9.11 are vulnerable to a heap-based buffer overflow, allowing a malicious VNC server to crash the client or potentially execute arbitrary code. This vulnerability stems from improper bounds checking within the FramebufferUpdate message handling, enabling an attacker to write data beyond allocated memory. Successful exploitation could lead to a denial of service or complete system compromise.
Step 1: Connection Establishment: The attacker establishes a VNC connection with a vulnerable LibVNCClient.
Step 2: Malicious FramebufferUpdate: The attacker, acting as a VNC server, sends a crafted FramebufferUpdate message. This message contains a subrectangle definition.
Step 3: Subrectangle Manipulation: The attacker crafts the subrectangle's dimensions to be outside the bounds of the client's drawing area, triggering the vulnerability.
Step 4: Data Overflow: The client, processing the malicious message, attempts to write pixel data for the oversized subrectangle. This write operation overflows the allocated buffer on the heap.
Step 5: Exploitation (DoS/RCE): The buffer overflow corrupts memory. This can lead to a crash (denial of service) or, if carefully crafted, allow the attacker to overwrite critical data and potentially execute arbitrary code.
The vulnerability lies within the rfbproto.c file of LibVNCClient, specifically in the handling of FramebufferUpdate messages. The flaw occurs when processing subrectangles within the update. The code fails to properly validate the size and position of these subrectangles against the client's drawing area. This lack of bounds checking allows an attacker to craft a malicious FramebufferUpdate message containing a subrectangle that extends beyond the allocated buffer on the heap. When the client attempts to process this malformed data, it writes data outside of the allocated memory, leading to a heap-based buffer overflow. This overwrite can corrupt critical data structures, causing a crash (denial of service) or, with careful crafting, potentially allowing for arbitrary code execution. The root cause is the missing or inadequate validation of the subrectangle dimensions against the client's framebuffer size before writing pixel data.
While no specific APT groups are definitively linked to active exploitation of this CVE, the ease of exploitation and the potential for remote code execution make it attractive to various threat actors. The vulnerability is a known quantity and could be incorporated into existing exploit kits. Not listed on CISA KEV.
Monitor network traffic for unusual VNC server behavior, such as sending malformed FramebufferUpdate messages with excessively large subrectangles.
Analyze client-side VNC logs for crashes or unexpected behavior after receiving FramebufferUpdate messages.
Implement intrusion detection systems (IDS) with signatures specifically designed to detect malicious FramebufferUpdate messages.
Monitor memory usage and look for unexpected heap allocations or memory corruption within the VNC client process.
Examine core dumps or crash reports for evidence of buffer overflows in rfbproto.c or related functions.
Upgrade LibVNCServer to version 0.9.11 or later. This is the primary and most effective remediation.
Implement input validation on the client side to ensure that subrectangle dimensions in FramebufferUpdate messages are within acceptable bounds.
Apply security patches provided by the vendor.
Consider using a network-based intrusion detection and prevention system (IDPS) to filter malicious VNC traffic.
Isolate VNC clients from untrusted networks.
Regularly audit VNC client configurations and security settings.