CVE-2021-42072 – Barrier versions before 2.4.0 have insufficient... (How to Fix)

Q: What is the severity of CVE-2021-42072?

CVE-2021-42072 has a CVSS score of 8.8 (HIGH). Barrier versions before 2.4.0 have insufficient client identity verification, allowing attackers to spoof connections to the server. This vulnerability enables denial-of-service attacks and could lead to information leaks or data corruption. Users running Barrier server components are affected.

📋 TL;DR

Barrier versions before 2.4.0 have insufficient client identity verification, allowing attackers to spoof connections to the server. This vulnerability enables denial-of-service attacks and could lead to information leaks or data corruption. Users running Barrier server components are affected.

💻 Affected Systems

Products:

Barrier

Versions: All versions before 2.4.0

Operating Systems: Linux, Windows, macOS

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects Barrier server components (barriers). Client components are not vulnerable.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Attackers could gain unauthorized access to the Barrier server, intercept or manipulate shared keyboard/mouse data, and potentially compromise connected systems.

🟠

Likely Case

Denial-of-service attacks disrupting Barrier functionality, with potential for unauthorized access to shared input streams.

🟢

If Mitigated

Limited impact with proper network segmentation and access controls, though vulnerability remains exploitable within allowed network paths.

🌐 Internet-Facing: HIGH - Barrier servers exposed to the internet are highly vulnerable to exploitation due to the authentication bypass.

🏢 Internal Only: MEDIUM - Internal networks still pose risk from compromised or malicious internal clients.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

The CWE-287 (Improper Authentication) classification suggests straightforward exploitation once protocol weaknesses are understood.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 2.4.0

Vendor Advisory: https://github.com/debauchee/barrier/releases/tag/v2.4.0

Restart Required: Yes

Instructions:

1. Download Barrier 2.4.0 or later from official repository. 2. Stop Barrier service. 3. Install new version. 4. Restart Barrier service.

🔧 Temporary Workarounds

Network Segmentation

linux

Restrict Barrier server access to trusted networks only

iptables -A INPUT -p tcp --dport 24800 -s trusted_network -j ACCEPT
iptables -A INPUT -p tcp --dport 24800 -j DROP

Firewall Rules

windows

Block Barrier port from untrusted networks

netsh advfirewall firewall add rule name="Block Barrier" dir=in action=block protocol=TCP localport=24800

🧯 If You Can't Patch

Implement strict network access controls to limit Barrier server exposure
Monitor Barrier logs for unauthorized connection attempts and investigate anomalies

🔍 How to Verify

Check if Vulnerable:

Check Barrier version: barrier --version or check installed package version

Check Version:

barrier --version

Verify Fix Applied:

Confirm version is 2.4.0 or later and test client authentication

📡 Detection & Monitoring

Log Indicators:

Failed authentication attempts
Unexpected client connections
Protocol errors

Network Indicators:

Unauthorized connections to port 24800
Suspicious traffic patterns to Barrier server

SIEM Query:

source="barrier.log" AND (event="authentication_failed" OR event="unexpected_client")

📊 Metadata

CVE ID: CVE-2021-42072

CVSS v3 Score: 8.8 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

CWE: CWE-287

Published: November 8, 2021

Last Updated: November 21, 2024

🔗 References

http://www.openwall.com/lists/oss-security/2021/11/02/4 Exploit,Mailing List,Third Party Advisory
https://github.com/debauchee/barrier/releases/tag/v2.4.0 Release Notes,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CIEVNCFEFO7L3NTM4VUZB3WKYYCBTFCI/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/XMU3STOKHPEZSC54MZ42YBFFC2R3BU2Q/
http://www.openwall.com/lists/oss-security/2021/11/02/4 Exploit,Mailing List,Third Party Advisory
https://github.com/debauchee/barrier/releases/tag/v2.4.0 Release Notes,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CIEVNCFEFO7L3NTM4VUZB3WKYYCBTFCI/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/XMU3STOKHPEZSC54MZ42YBFFC2R3BU2Q/

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