CVE-2021-47891: Unified Remote 3.9 RCE Vulnerability

CVE-2021-47891 Overview

CVE-2021-47891 is a critical remote code execution vulnerability affecting Unified Remote version 3.9.0.2463. This vulnerability allows unauthenticated attackers to send specially crafted network packets to the Unified Remote service, enabling arbitrary command execution on target systems. The flaw stems from missing authentication for critical functions (CWE-306), allowing attackers to connect to the service on port 9512 and execute malicious payloads without any authentication requirements.

Critical Impact

Attackers can remotely execute arbitrary commands on vulnerable systems, potentially leading to complete system compromise, data theft, malware deployment, and lateral movement within affected networks.

Affected Products

• Unified Remote 3.9.0.2463
• Unified Remote Server (Windows/Linux/Mac installations)
• Systems with port 9512 exposed to untrusted networks

Discovery Timeline

• 2026-01-23 - CVE CVE-2021-47891 published to NVD
• 2026-01-26 - Last updated in NVD database

Technical Details for CVE-2021-47891

Vulnerability Analysis

This vulnerability exists due to the Unified Remote service failing to implement proper authentication controls for its network protocol. The service listens on TCP port 9512 and processes incoming packets without verifying the identity or authorization of connecting clients. This missing authentication mechanism allows any network-accessible attacker to interact with the service as if they were a legitimate, authorized client.

The impact is severe: attackers can leverage this flaw to open command prompts and execute arbitrary system commands. This includes the ability to download additional malicious payloads from attacker-controlled servers and execute them on the compromised system, effectively establishing a foothold for further attacks.

Root Cause

The root cause is classified as CWE-306: Missing Authentication for Critical Function. Unified Remote 3.9.0.2463 exposes administrative command execution capabilities through its network service without requiring any form of authentication. The service was designed to accept remote control commands but failed to implement security controls to verify that incoming connections originate from authorized clients.

Attack Vector

The attack is network-based and requires no user interaction or prior authentication. An attacker can exploit this vulnerability by:

1. Identifying systems running Unified Remote with port 9512 accessible
2. Establishing a TCP connection to the vulnerable service
3. Sending specially crafted packets that instruct the service to open a command prompt
4. Sending additional payloads to download and execute malicious files

The vulnerability mechanism involves sending crafted network packets to the Unified Remote service on port 9512. The service processes these packets and executes the embedded commands without authentication validation. Technical details and proof-of-concept information are available in the Exploit-DB #49587 entry and the VulnCheck Advisory.

Detection Methods for CVE-2021-47891

Indicators of Compromise

• Unexpected network connections to port 9512 from external or untrusted IP addresses
• Suspicious command prompt (cmd.exe) or shell processes spawned by the Unified Remote service
• Unusual outbound connections following Unified Remote service activity, indicating payload downloads
• Evidence of powershell.exe or certutil.exe being invoked by the Unified Remote process

Detection Strategies

• Monitor network traffic for connections to TCP port 9512, particularly from external sources
• Implement process monitoring to detect command execution spawned by Unified Remote service processes
• Deploy network intrusion detection signatures for known Unified Remote exploitation patterns
• Review endpoint detection logs for suspicious parent-child process relationships involving Unified Remote

Monitoring Recommendations

• Enable enhanced logging on systems running Unified Remote to capture all service interactions
• Configure network monitoring to alert on traffic to port 9512 from unauthorized networks
• Implement file integrity monitoring on systems with Unified Remote installed
• Set up alerts for any new executable downloads or suspicious process creation events

How to Mitigate CVE-2021-47891

Immediate Actions Required

• Restrict network access to port 9512 using firewall rules, allowing only trusted IP addresses
• Disable or uninstall Unified Remote on systems where it is not required
• Isolate systems running vulnerable versions from untrusted networks
• Update Unified Remote to the latest available version from the official download page

Patch Information

Users should check the Unified Remote Homepage for security updates and upgrade to the latest version available. Review the VulnCheck Advisory for additional remediation guidance.

Workarounds

• Implement strict firewall rules to block external access to port 9512
• Use a VPN or network segmentation to limit access to Unified Remote services
• Disable the Unified Remote service when not actively in use
• Deploy host-based firewall rules on individual systems to restrict service access to localhost only

# Configuration example - Firewall rule to restrict Unified Remote access
# Windows Firewall (PowerShell)
New-NetFirewallRule -DisplayName "Block Unified Remote External" -Direction Inbound -LocalPort 9512 -Protocol TCP -RemoteAddress "!LocalSubnet" -Action Block

# Linux iptables
iptables -A INPUT -p tcp --dport 9512 -s ! 127.0.0.1 -j DROP