TCP Dangerous Remote Access

Port 5901 (VNC-1)

Learn about port 5901 (VNC-1) - security risks, vulnerabilities, and common uses. Find devices with port 5901 open.

Quick Info

Port Number

5901

Protocol

TCP

Service

VNC-1

IANA Name

VNC-1

Service Description

TCP port 5901 is commonly associated with the Virtual Network Computing (VNC) protocol, specifically the first display number (display :1). VNC enables remote access and control of a graphical desktop environment on a server or workstation. The protocol operates on a client-server model. The VNC server listens on port 5900 + display number (e.g., 5901 for display :1, 5902 for display :2, etc.). The VNC client connects to this port to establish a session. The protocol involves the server sending framebuffer updates to the client whenever the screen changes. The client then transmits user input (mouse movements, keyboard strokes) back to the server, which processes them and updates the framebuffer accordingly. The protocol uses Remote Frame Buffer (RFB) as the underlying protocol for transferring screen data and input events. Authentication methods range from simple passwords to more sophisticated mechanisms like TLS encryption, but these are not always implemented or configured correctly by default. Older versions and configurations often used unencrypted connections, making them susceptible to eavesdropping and man-in-the-middle attacks.

## Firewall Recommendations

It is generally recommended to block port 5901 (and other VNC ports) at the firewall unless remote access is absolutely necessary. If remote access is required, it should be secured with strong authentication, encryption (preferably using TLS or SSH tunneling), and access control lists to restrict access to authorized users and IP addresses only. Consider using a VPN connection for secure remote access instead of directly exposing VNC to the internet. Regularly update VNC server software to patch known vulnerabilities. Monitor network traffic for suspicious activity on port 5901, such as brute-force attacks or connections from unauthorized IP addresses. If VNC is not being used, disable the service altogether.

Security Information

Port 5901, when running VNC, presents significant security risks if not properly secured. The primary vulnerability stems from the potential for unencrypted communication. If the connection between the client and server is not encrypted, attackers can eavesdrop on the traffic and capture sensitive information, including login credentials and displayed data. Brute-force attacks targeting weak or default passwords are also a common threat. Furthermore, vulnerabilities in the VNC server software itself can be exploited to gain unauthorized access to the system. Attackers may target this port to gain remote control of the system, steal data, install malware, or use the compromised system as a launchpad for further attacks within the network. The allure of remote access makes VNC a prime target for malicious actors seeking to compromise systems with minimal effort.

Known Vulnerabilities

CVE	Name	Severity	Description
CVE-2018-20421	RealVNC VNC Server 6.3.2 Buffer Overflow	High	Уязвимость переполнения буфера в RealVNC VNC Server 6.3.2 позволяет злоумышленнику удаленно выполнять код.
CVE-2015-8993	TightVNC Authentication Bypass	High	Уязвимость обхода аутентификации в TightVNC позволяет злоумышленнику получить несанкционированный доступ к VNC-серверу.
CVE-2020-25715	TigerVNC Out-of-bounds Read	Medium	TigerVNC suffers from an out-of-bounds read vulnerability. The fix is available in version 1.11.0.
CVE-2018-6848	LibVNCServer Memory Corruption	High	A heap-based buffer overflow vulnerability exists in LibVNCServer 0.9.11 and earlier. A malicious client can cause a crash or execute arbitrary code on the server by sending a crafted Tight encoding data stream.