CVE-2019-8261 in UltraVNC
Summary
by MITRE
UltraVNC revision 1199 has a out-of-bounds read vulnerability in VNC code inside client CoRRE decoder, caused by multiplication overflow. This attack appears to be exploitable via network connectivity. This vulnerability has been fixed in revision 1200.
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Analysis
by VulDB Data Team • 07/26/2023
The vulnerability identified as CVE-2019-8261 affects UltraVNC version 1199 and represents a critical out-of-bounds read condition within the client-side CoRRE (Compression RLE Encoding) decoder component. This issue arises from a multiplication overflow error that occurs during the processing of remote desktop protocol data streams, specifically when handling compressed graphics data transmitted between VNC client and server components. The flaw exists in the core VNC protocol implementation where the decoder fails to properly validate input parameters before performing arithmetic operations, creating a scenario where maliciously crafted data can trigger memory access violations.
The technical nature of this vulnerability stems from improper bounds checking in the CoRRE decoder's handling of compressed data structures. When the decoder processes incoming data, it performs mathematical operations to calculate buffer sizes and memory allocation requirements, but a multiplication overflow occurs during these calculations. This overflow results in an incorrect buffer size being computed, leading to attempts to read memory locations beyond the allocated buffer boundaries. The vulnerability manifests as an out-of-bounds read operation that can potentially expose sensitive memory contents or cause application crashes, representing a classic memory safety issue that aligns with CWE-129 and CWE-787 categories.
The operational impact of CVE-2019-8261 extends beyond simple application instability, as it represents a network-based attack vector that can be exploited by remote adversaries without requiring local system access. Attackers can leverage this vulnerability by establishing a connection to a vulnerable UltraVNC client and sending specially crafted compressed graphics data that triggers the multiplication overflow condition. The attack surface is particularly concerning given the widespread use of VNC protocols for remote desktop access, making this vulnerability exploitable in numerous enterprise and home network environments. This type of vulnerability falls under ATT&CK technique T1059.007 for remote code execution through protocol manipulation and represents a significant risk to network security infrastructure.
The exploitation of this vulnerability can lead to several security consequences including denial of service conditions that render VNC clients unusable, potential information disclosure through memory dumps, and in some cases, the possibility of executing arbitrary code on the affected system. The multiplication overflow specifically affects the CoRRE decoder's ability to handle compressed data streams properly, creating opportunities for attackers to manipulate memory access patterns. Organizations using UltraVNC versions prior to 1200 should consider immediate remediation efforts as the vulnerability has been confirmed to be exploitable over network connections. The fix implemented in revision 1200 addresses the root cause by introducing proper input validation and bounds checking mechanisms that prevent the multiplication overflow condition from occurring during data processing operations.