Full Report
UltraVNC before 1.2.2.4 has a heap buffer overflow vulnerability in VNC server code inside file transfer offer 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-2019-8274
- **CVSS Score:** 9.9 (Critical) - *Note: Based on the vector CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H provided in the source.*
- **CWE:** CWE-122 (Heap-based Buffer Overflow)
## Affected Systems
- **Products:** UltraVNC
- **Versions:** All versions prior to 1.2.2.4
- **Configurations:** Systems running the UltraVNC server component with network access enabled.
## Vulnerability Description
A heap-based buffer overflow vulnerability exists within the UltraVNC server code, specifically inside the **file transfer offer handler**. The flaw is triggered when the server processes a specially crafted file transfer request. Due to insufficient validation of the size of incoming data before copying it to a heap-allocated buffer, an attacker can overwrite adjacent memory. This memory corruption can be leveraged to redirect execution flow and execute arbitrary code in the context of the VNC server process.
## Exploitation
- **Status:** PoC available (Proof of Concept exists)
- **Complexity:** Low
- **Attack Vector:** Network (Remote)
## Impact
- **Confidentiality:** High (Full access to data handled by the process)
- **Integrity:** High (Ability to modify system files and memory)
- **Availability:** High (Potential for application crash or total system takeover)
## Remediation
### Patches
- **Update to UltraVNC 1.2.2.4** or a more recent version. The vendor released this patch in February 2019 to address the overflow.
### Workarounds
- **Disable File Transfer:** If the file transfer feature is not required for operations, disable it within the UltraVNC server settings.
- **Access Control:** Restrict access to the VNC port (default TCP 5900) using firewalls or VPNs to ensure only trusted IP addresses can connect to the server.
- **Standard User Rights:** Run the VNC service with the least privilege necessary to mitigate the impact of potential code execution.
## Detection
- **Indicators of Compromise:** Monitor for unexpected crashes of the `WinVNC.exe` process, particularly those associated with heap corruption exceptions.
- **Detection Methods and Tools:**
- Use Network Intrusion Detection Systems (NIDS) to inspect VNC traffic for malformed file transfer negotiation packets.
- Employ vulnerability scanners to identify outdated versions of UltraVNC (1.2.2.3 and below) across the enterprise.
## References
- **Vendor Advisory:** hxxp[://]www[.]uvnc[.]com/
- **NVD Detail:** hxxps[://]nvd[.]nist[.]gov/vuln/detail/CVE-2019-8274
- **Kaspersky ICS CERT:** hxxps[://]ics-cert[.]kaspersky[.]com/advisories/2019/03/01/klcert-19-021-ultravnc-heap-based-buffer-overflow/