Source: cve@mitre.org
Heap-based buffer overflow in ultra.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 with the Ultra type tile, such that the LZO payload decompressed length exceeds what is specified by the tile dimensions.
LibVNCServer versions prior to 0.9.11 are vulnerable to a heap-based buffer overflow, allowing a malicious VNC server to potentially execute arbitrary code or cause a denial of service. This vulnerability is triggered by a crafted FramebufferUpdate message containing an Ultra type tile, exploiting a flaw in the LZO decompression process, leading to a buffer overflow when the decompressed data exceeds the allocated memory.
Step 1: Connection Establishment: The attacker establishes a VNC connection to a vulnerable LibVNCServer instance.
Step 2: Crafted FramebufferUpdate Message: The attacker sends a malicious FramebufferUpdate message. This message contains an Ultra type tile.
Step 3: Malicious LZO Payload: The Ultra tile contains a specially crafted LZO-compressed payload. This payload is designed to decompress to a size larger than what is specified by the tile dimensions.
Step 4: Decompression and Overflow: The LibVNCClient library attempts to decompress the LZO payload. Due to the lack of bounds checking, the decompressed data overflows the allocated buffer on the heap.
Step 5: Denial of Service or Code Execution: The buffer overflow corrupts adjacent memory, potentially leading to a denial of service (application crash) or, with careful exploitation, arbitrary code execution.
The vulnerability lies within the ultra.c file of LibVNCClient, specifically in how it handles the decompression of LZO-compressed data within FramebufferUpdate messages of the Ultra tile type. The code fails to properly validate the decompressed size against the tile dimensions before allocating memory. This leads to a heap-based buffer overflow when the LZO decompression results in a size larger than expected, overwriting adjacent memory. The root cause is a lack of bounds checking on the decompressed data size before memory allocation. The FramebufferUpdate message contains tile data, including the tile type (Ultra in this case), dimensions, and the compressed payload. The vulnerable code decompresses the LZO payload without adequately verifying that the decompressed size, determined by the LZO algorithm, aligns with the tile's width and height. This allows an attacker to craft a malicious payload that, when decompressed, overflows the allocated buffer.
While no specific APT groups are definitively linked to active exploitation, the availability of a PoC and the nature of the vulnerability make it attractive to various threat actors. CISA KEV status: Not listed.
Network traffic analysis: Monitor for unusual FramebufferUpdate messages with Ultra tiles and large LZO payloads.
IDS/IPS signatures: Implement signatures to detect malicious FramebufferUpdate messages.
Memory forensics: Analyze memory dumps for evidence of heap corruption or crashes related to LibVNCClient.
Log analysis: Review VNC server logs for crashes or errors related to frame buffer updates.
Upgrade to LibVNCServer version 0.9.11 or later.
Implement input validation: Ensure that the decompressed size of the LZO payload does not exceed the tile dimensions before allocating memory.
Apply security patches promptly.
Network segmentation: Isolate VNC servers from untrusted networks.
Regular security audits and penetration testing to identify and address vulnerabilities.