CVE-2021-40343 – CVE-2021-40343 is a privilege escalation vulner... (How to Fix)

Q: What is the severity of CVE-2021-40343?

CVE-2021-40343 has a CVSS score of 7.8 (HIGH). CVE-2021-40343 is a privilege escalation vulnerability in Nagios XI where insecure file permissions on nagios_unbundler.py allow the nagios user to execute arbitrary code as root. This affects Nagios XI installations where the vulnerable file exists with incorrect permissions. Attackers with access to the nagios user account can gain full root privileges on the system.

📋 TL;DR

CVE-2021-40343 is a privilege escalation vulnerability in Nagios XI where insecure file permissions on nagios_unbundler.py allow the nagios user to execute arbitrary code as root. This affects Nagios XI installations where the vulnerable file exists with incorrect permissions. Attackers with access to the nagios user account can gain full root privileges on the system.

💻 Affected Systems

Products:

Nagios XI

Versions: 5.8.5 and potentially earlier versions

Operating Systems: Linux

Default Config Vulnerable: ⚠️ Yes

Notes: Affects systems where nagios_unbundler.py exists with insecure permissions. The vulnerability is in the file permissions configuration.

📦 What is this software?

Nagios Xi by Nagios

View all CVEs affecting Nagios Xi →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete system compromise with root access, allowing installation of persistent backdoors, data theft, lateral movement, and full control of the monitoring infrastructure.

🟠

Likely Case

Privilege escalation from nagios user to root, enabling attackers to modify monitoring configurations, disable alerts, and access sensitive system information.

🟢

If Mitigated

Limited impact if proper access controls restrict nagios user access and file permissions are properly managed.

🌐 Internet-Facing: MEDIUM - Requires initial access to nagios user account, which typically requires some level of authentication or other vulnerability.

🏢 Internal Only: HIGH - Internal attackers with nagios user access can easily exploit this to gain root privileges.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ✅ No

Complexity: LOW

Exploitation requires access to the nagios user account. The vulnerability is straightforward to exploit once initial access is obtained.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Nagios XI 5.8.6 and later

Vendor Advisory: https://assets.nagios.com/downloads/nagiosxi/CHANGES-5.TXT

Restart Required: No

Instructions:

1. Update Nagios XI to version 5.8.6 or later. 2. Apply the official patch from Nagios. 3. Verify file permissions on nagios_unbundler.py are corrected.

🔧 Temporary Workarounds

Fix file permissions manually

linux

Manually correct the insecure file permissions on nagios_unbundler.py to prevent privilege escalation

chmod 750 /usr/local/nagiosxi/scripts/nagios_unbundler.py
chown root:root /usr/local/nagiosxi/scripts/nagios_unbundler.py

Remove unnecessary SUID/SGID bits

linux

Remove any unnecessary special permissions from the vulnerable file

chmod u-s,g-s /usr/local/nagiosxi/scripts/nagios_unbundler.py

🧯 If You Can't Patch

Restrict access to the nagios user account and monitor for unauthorized access attempts
Implement strict file integrity monitoring on nagios_unbundler.py and related system files

🔍 How to Verify

Check if Vulnerable:

Check file permissions on nagios_unbundler.py: ls -la /usr/local/nagiosxi/scripts/nagios_unbundler.py. If the file is writable by the nagios user or has insecure permissions, the system is vulnerable.

Check Version:

cat /usr/local/nagiosxi/var/xiversion

Verify Fix Applied:

Verify file permissions: ls -la /usr/local/nagiosxi/scripts/nagios_unbundler.py should show permissions like -rwxr-x--- and owned by root:root. Also check Nagios XI version is 5.8.6 or higher.

📡 Detection & Monitoring

Log Indicators:

Unusual activity by nagios user, privilege escalation attempts, modifications to nagios_unbundler.py file

Network Indicators:

Unexpected outbound connections from Nagios server, unusual SSH or remote access patterns

SIEM Query:

source="nagios" AND (event="privilege_escalation" OR event="file_modification" AND file="/usr/local/nagiosxi/scripts/nagios_unbundler.py")

📊 Metadata

CVE ID: CVE-2021-40343

CVSS v3 Score: 7.8 (HIGH)

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

CWE: CWE-732

Published: October 26, 2021

Last Updated: November 21, 2024

🔗 References

https://assets.nagios.com/downloads/nagiosxi/CHANGES-5.TXT Release Notes,Vendor Advisory
https://synacktiv.com Not Applicable
https://www.synacktiv.com/sites/default/files/2021-10/Nagios_XI_multiple_vulnerabilities_0.pdf Exploit,Third Party Advisory
https://assets.nagios.com/downloads/nagiosxi/CHANGES-5.TXT Release Notes,Vendor Advisory
https://synacktiv.com Not Applicable
https://www.synacktiv.com/sites/default/files/2021-10/Nagios_XI_multiple_vulnerabilities_0.pdf Exploit,Third Party Advisory

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🔗 Related Vulnerabilities

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