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-2018-15126
- **CVSS Score:** 9.9 (Critical) - *Note: Based on the provided vector 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 triggered only when the **Tight file transfer extension** is enabled in the server configuration.
## 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 management of memory pointers during file transfer operations. When specific sequences of commands are sent by a client, the server may free a memory block on the heap and subsequently attempt to use that same memory address. This memory corruption can be leveraged to achieve arbitrary code execution (RCE) in the context of the VNC server process.
## Exploitation
- **Status:** PoC available
- **Complexity:** Low
- **Attack Vector:** Network (Remote)
- **Prerequisites:** Successful exploitation requires the attacker to have valid user privileges/authentication to the VNC server.
## Impact
- **Confidentiality:** High (Full access to memory and sensitive data)
- **Integrity:** High (Ability to modify files and execute arbitrary code)
- **Availability:** High (Can lead to service crashes or total system takeover)
## Remediation
### Patches
- **LibVNCServer 0.9.12:** Users should upgrade to version 0.9.12 or newer. Patches were officially released in November 2018.
### Workarounds
- **Disable File Transfer:** If upgrading is not immediately possible, disable the Tight file transfer extension in the VNC server configuration settings to mitigate the attack vector.
- **Access Control:** Restrict VNC access to trusted IP addresses using firewalls or VPNs to limit exposure to potential attackers.
## Detection
- **Indicators of Compromise:** Monitor for unusual crash logs in the VNC service associated with heap corruption or memory access violations.
- **Detection methods and tools:** Use network intrusion detection systems (NIDS) to identify non-standard or malformed file transfer requests targeting VNC ports (typically TCP 5900+). Vulnerability scanners (e.g., Nessus, OpenVAS) can be used to identify outdated versions of LibVNCServer.
## References
- **Vendor Advisory:** [https]://ics-cert.kaspersky[.]com/advisories/2018/12/19/klcert-18-027-libvnc-heap-use-after-free/
- **NVD Entry:** [https]://nvd.nist[.]gov/vuln/detail/CVE-2018-15126
- **Project Repository:** [https]://github[.]com/LibVNC/libvncserver/releases/tag/LibVNCServer-0.9.12