Full Report
An attacker controlling a device with the UltraVNC Server running can perform remote code execution on the client devices to cause a denial-of-service condition, modify system's data and/or obtain sensitive information.
Analysis Summary
# Vulnerability: UltraVNC Client-Side Heap-based Buffer Overflow
## CVE Details
- **CVE ID:** CVE-2019-8258
- **CVSS Score:** 8.8 (High) - *Note: Based on the provided vector CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H*
- **CWE:** CWE-122 (Heap-based Buffer Overflow)
## Affected Systems
- **Products:** UltraVNC (VNC Client/Viewer)
- **Versions:** All versions prior to 1.2.2.4
- **Configurations:** Client devices connecting to a remote VNC server.
## Vulnerability Description
A heap-based buffer overflow vulnerability exists in the UltraVNC client. The flaw is triggered when a client connects to a malicious or compromised UltraVNC server. The server can send a specially crafted network packet that exceeds the allocated buffer size on the client's heap. This memory corruption can be leveraged to overwrite adjacent data, leading to arbitrary code execution (RCE) in the context of the user running the VNC viewer.
## Exploitation
- **Status:** PoC (Proof of Concept) available
- **Complexity:** Low
- **Attack Vector:** Network (Remote)
- **User Interaction:** Required (Target user must initiate a connection to the attacker's server)
## Impact
- **Confidentiality:** High (Potential to obtain sensitive system information)
- **Integrity:** High (Potential to modify system data)
- **Availability:** High (Potential to cause a Denial-of-Service condition or system crash)
## Remediation
### Patches
- **UltraVNC Version 1.2.2.4:** This version (released February 2019) contains the official fix for the heap overflow. Users are urged to upgrade to this version or newer.
### Workarounds
- **Strict Server Verification:** Only connect to trusted, authorized VNC servers.
- **Network Segmentation:** Limit outbound VNC traffic (typically TCP port 5900+) to known-safe IP addresses via firewall rules.
## Detection
- **Indicators of Compromise:** Unusual activity from the `vncviewer.exe` process, such as unexpected network connections or the spawning of shell processes (`cmd.exe`, `powershell.exe`).
- **Detection Methods and Tools:**
- Use Network Intrusion Detection Systems (NIDS) to monitor for malformed VNC protocol handshakes.
- Deploy Endpoint Detection and Response (EDR) to monitor for heap corruption events or crashes in UltraVNC.
## References
- **Vendor Advisory:** hxxps[://]www[.]uvnc[.]com/
- **Kaspersky ICS CERT:** hxxps[://]ics-cert[.]kaspersky[.]com/advisories/2019/03/01/klcert-19-004-ultravnc-heap-based-buffer-overflow/
- **NVD Entry:** hxxps[://]nvd[.]nist[.]gov/vuln/detail/CVE-2019-8258