Full Report
UltraVNC before 1.2.2.4 has a heap buffer overflow vulnerability in VNC server code inside file transfer handler, which can potentially result in code execution.
Analysis Summary
# Vulnerability: UltraVNC Heap-based Buffer Overflow in File Transfer Handler
## CVE Details
- **CVE ID:** CVE-2019-8271
- **CVSS Score:** 9.9 (Critical) - *Note: Based on CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H*
- **CWE:** CWE-122 (Heap-based Buffer Overflow)
## Affected Systems
- **Products:** UltraVNC
- **Versions:** All versions prior to 1.2.2.4
- **Configurations:** Systems running the VNC server component with the file transfer handler enabled.
## Vulnerability Description
A heap-based buffer overflow exists within the UltraVNC server-side code specifically inside the file transfer handler functionality. The flaw originates from improper bounds checking when processing file transfer data. An attacker can send specially crafted packets to the VNC server, leading to memory corruption in the heap, which can be leveraged to achieve arbitrary code execution in the context of the user running the VNC server.
## Exploitation
- **Status:** PoC (Proof of Concept) available
- **Complexity:** Low
- **Attack Vector:** Network (Remote)
- **Note:** Successful exploitation requires the attacker to have network access to the affected system and for a connection to be established where file transfer functions are accessible.
## Impact
- **Confidentiality:** High (Full access to memory and local files)
- **Integrity:** High (Ability to modify files and execute unauthorized code)
- **Availability:** High (Potential for service crashes or full system takeover)
## Remediation
### Patches
- **Update to UltraVNC version 1.2.2.4 or newer.** The vendor released this patch in February 2019 to address the memory handling logic in the file transfer module.
### Workarounds
- **Disable File Transfer:** If updating is not immediately possible, disable the file transfer feature in the UltraVNC server settings.
- **Access Control:** Restrict VNC access to authorized IP addresses and ensure the service is behind a firewall or VPN.
- **Disable VNC:** Turn off the VNC service on systems where it is not strictly required for administrative tasks.
## Detection
- **Indicators of Compromise:** Unusual service crashes of `winvnc.exe` followed by unexpected outbound network traffic or the creation of suspicious processes.
- **Detection methods and tools:**
- Use Network Intrusion Detection Systems (NIDS) to monitor for malformed VNC file transfer packets.
- Implement endpoint detection (EDR) to monitor for heap corruption or buffer overflow attempts targeting the UltraVNC process.
## References
- **Kaspersky ICS CERT:** hxxps[://]ics-cert[.]kaspersky[.]com/advisories/2019/03/01/klcert-19-018-ultravnc-heap-based-buffer-overflow/
- **NVD Entry:** hxxps[://]nvd[.]nist[.]gov/vuln/detail/CVE-2019-8271
- **UltraVNC Official Site:** hxxp[://]www[.]uvnc[.]com/