Full Report
LibVNCServer before a 0.9.12 release contains multiple heap out-of-bound write vulnerabilities in VNC client code, which can result in remote code execution.
Analysis Summary
# Vulnerability: LibVNC Memory Corruption (Heap Out-of-Bound Write)
## CVE Details
- **CVE ID:** CVE-2018-20019
- **CVSS Score:** 8.8 (High) - *Calculated from vector CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H*
- **CWE:** CWE-787 (Out-of-bounds Write) / CWE-122 (Heap-based Buffer Overflow)
## Affected Systems
- **Products:** LibVNCServer (specifically VNC client-side code components)
- **Versions:** All versions prior to release 0.9.12
- **Configurations:** Systems utilizing LibVNC library to build VNC client applications that connect to remote VNC servers.
## Vulnerability Description
LibVNC contains multiple heap out-of-bound write vulnerabilities within its client-side processing logic. These flaws occur when the client application processes specially crafted responses from a VNC server. Because the library fails to properly validate the boundaries of data received over the network before writing it to the heap memory, an attacker can overwrite adjacent memory structures. This memory corruption can be leveraged to bypass security mechanisms and achieve arbitrary code execution (RCE) in the context of the user running the VNC client.
## Exploitation
- **Status:** Proof of Concept (PoC) available.
- **Complexity:** Low (Technical requirements for execution are straightforward once a connection is established).
- **Attack Vector:** Network (Remote). The attack is triggered when a vulnerable client connects to a malicious or compromised VNC server.
## Impact
- **Confidentiality:** High (Potential for unauthorized access to data and memory contents).
- **Integrity:** High (An attacker can modify memory and execute unauthorized commands).
- **Availability:** High (Exploitation typically results in application crashes or system instability).
## Remediation
### Patches
- **LibVNCServer 0.9.12:** Users should update to version 0.9.12 or any subsequent newer releases where these vulnerabilities have been addressed.
### Workarounds
- **Restrict Connections:** Limit VNC client connections to known, trusted servers only.
- **Network Segmentation:** Use firewalls or VPCs to restrict outbound VNC traffic (typically port 5900+) to authorized IP addresses.
## Detection
- **Indicators of Compromise:** Unexpected crashes of VNC client applications (Segmentation Faults) when connecting to external servers.
- **Detection Methods and Tools:**
- Use Software Composition Analysis (SCA) tools to identify vulnerable versions of `libvncserver` or `libvncclient` in the environment.
- Monitor network traffic for unusual VNC handshaking patterns or connections to untrusted external IP addresses.
## References
- **Vendor Advisory:** KLCERT-18-029
- **NVD Entry:** hxxps[://]nvd[.]nist[.]gov/vuln/detail/CVE-2018-20019
- **Kaspersky ICS CERT:** hxxps[://]ics-cert[.]kaspersky[.]com/advisories/2018/12/19/klcert-18-029-libvnc-multiple-heap-out-of-bound-vulnerabilities/
- **LibVNC Project:** hxxps[://]github[.]com/LibVNC/libvncserver/releases/tag/LibVNCServer-0.9.12