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CVE-2020-6581

7.3 HIGH
Remote Code Execution

📋 TL;DR

CVE-2020-6581 is a command injection vulnerability in Nagios NRPE 3.2.1 where insufficient filtering of escape sequences allows attackers to inject arbitrary commands. This affects systems running Nagios NRPE 3.2.1 with command execution enabled. Attackers can potentially execute arbitrary commands with the privileges of the NRPE daemon.

💻 Affected Systems

Products:
  • Nagios NRPE
Versions: 3.2.1
Operating Systems: Linux, Unix-like systems
Default Config Vulnerable: ⚠️ Yes
Notes: Systems with command execution enabled in NRPE configuration are vulnerable. The vulnerability exists in the nasty_metachars function's handling of escape sequences.

📦 What is this software?

Fedora by Fedoraproject

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Remote Plug In Executor by Nagios

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⚠️ Risk & Real-World Impact

🔴

Worst Case

Full system compromise through remote code execution, allowing attackers to install malware, exfiltrate data, or pivot to other systems.

🟠

Likely Case

Unauthorized command execution leading to service disruption, data leakage, or lateral movement within the network.

🟢

If Mitigated

Limited impact with proper network segmentation and least privilege configurations, potentially only affecting the NRPE service.

🌐 Internet-Facing: HIGH if NRPE is exposed to untrusted networks without proper filtering, as the vulnerability can be exploited remotely.
🏢 Internal Only: MEDIUM if NRPE is only accessible internally, but still poses risk from compromised internal systems or malicious insiders.

🎯 Exploit Status

Public PoC: ⚠️ Yes
Weaponized: LIKELY
Unauthenticated Exploit: ⚠️ Yes
Complexity: LOW

Exploitation requires network access to the NRPE service (default port 5666). The vulnerability is in input parsing, making exploitation straightforward once access is obtained.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 3.2.2 and later

Vendor Advisory: https://github.com/NagiosEnterprises/nrpe/releases

Restart Required: Yes

Instructions:

1. Download NRPE 3.2.2 or later from the official Nagios repository. 2. Stop the NRPE service. 3. Install the updated version. 4. Restart the NRPE service. 5. Verify the service is running correctly.

🔧 Temporary Workarounds

Network Access Restriction

linux

Restrict network access to NRPE service to trusted hosts only using firewall rules.

iptables -A INPUT -p tcp --dport 5666 -s trusted_ip -j ACCEPT
iptables -A INPUT -p tcp --dport 5666 -j DROP

Command Whitelisting

linux

Configure NRPE to only allow specific commands via the allowed_hosts and command configuration.

Edit /usr/local/nagios/etc/nrpe.cfg and set allowed_hosts=trusted_ip
Define specific commands in command[] directives

🧯 If You Can't Patch

  • Implement strict network segmentation to isolate NRPE services from untrusted networks.
  • Monitor NRPE logs for unusual command execution patterns and implement alerting.

🔍 How to Verify

Check if Vulnerable:

Check NRPE version with: nrpe --version | grep 'NRPE'

Check Version:

nrpe --version

Verify Fix Applied:

Verify version is 3.2.2 or later: nrpe --version | grep -E 'NRPE v3\.2\.[2-9]|NRPE v3\.[3-9]'

📡 Detection & Monitoring

Log Indicators:

  • Unusual command execution in NRPE logs
  • Failed authentication attempts to NRPE service
  • Unexpected process execution from NRPE daemon

Network Indicators:

  • Unusual traffic to port 5666 from untrusted sources
  • Command injection patterns in network packets

SIEM Query:

source="nrpe.log" AND (command="*;*" OR command="*|*" OR command="*`*" OR command="*$(*")

📊 Metadata

CVE ID: CVE-2020-6581
CVSS v3 Score: 7.3 (HIGH)
CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H
Published: March 16, 2020
Last Updated: November 21, 2024

🔗 References

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