Full Report
LibVNCServer before a 0.9.12 release contains a heap use-after-free vulnerability in the server code of the file transfer extension, which can result in remote code execution.
Analysis Summary
# Vulnerability: LibVNCServer Heap Use-After-Free in File Transfer Extension
## CVE Details
- **CVE ID:** CVE-2018-6307
- **CVSS Score:** 8.8 (High) - *Calculated from vector CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H*
- **CWE:** CWE-416 (Use After Free)
## Affected Systems
- **Products:** LibVNCServer
- **Versions:** All versions prior to release 0.9.12
- **Configurations:** The vulnerability is specifically triggered when the **Tight file transfer extension** is enabled on the server.
## Vulnerability Description
A heap-based use-after-free (UAF) vulnerability exists within the server-side implementation of the LibVNCServer file transfer extension. The flaw occurs due to improper memory management when handling file transfer requests. An attacker can leverage this to reference memory after it has been freed, leading to memory corruption, unexpected program behavior, or the execution of arbitrary code in the context of the VNC server process.
## Exploitation
- **Status:** Proof of Concept (PoC) available
- **Complexity:** Low
- **Attack Vector:** Network (Remote)
- **Prerequisites:** Successful exploitation requires the attacker to have basic user privileges/authentication to the VNC session.
## Impact
- **Confidentiality:** High (Full access to memory/data)
- **Integrity:** High (Arbitrary code execution)
- **Availability:** High (Service crash or system takeover)
## Remediation
### Patches
- **LibVNCServer 0.9.12:** Users should upgrade to version 0.9.12 or any subsequent newer release which contains the security fix.
### Workarounds
- **Disable File Transfer:** If upgrading is not immediately possible, disable the Tight file transfer extension in the VNC server configuration to mitigate the attack vector.
- **Access Control:** Restrict VNC access to trusted networks or via VPN to limit exposure to potential attackers.
## Detection
- **Indicators of Compromise:** Monitor for unusual crash logs associated with the LibVNCServer process, specifically those indicating heap corruption or segmentation faults.
- **Detection methods and tools:** Use network intrusion detection systems (NIDS) to monitor for anomalous VNC file transfer traffic. Static and dynamic analysis tools can be used to scan older binaries for the presence of this specific UAF flaw.
## References
- **Vendor Advisory:** hxxps[://]ics-cert[.]kaspersky[.]com/advisories/2018/12/19/klcert-18-026-libvnc-heap-use-after-free/
- **NVD Entry:** hxxps[://]nvd[.]nist[.]gov/vuln/detail/CVE-2018-6307
- **Project Repository:** hxxps[://]github[.]com/LibVNC/libvncserver/releases/tag/LibVNCServer-0.9.12