CVE-2026-27466: BigBlueButton ClamAV DoS Vulnerability

CVE-2026-27466 Overview

BigBlueButton, the widely-adopted open-source virtual classroom platform, contains a significant security flaw in its official server customization documentation. The instructions provided for configuring ClamAV as a presentation file scanner inadvertently expose the server to Denial of Service attacks by incorrectly exposing network ports to the internet.

Critical Impact

Remote attackers can exploit exposed ClamAV ports (3310 and 7357) to exhaust server resources or shutdown the clamd process, disrupting virtual classroom availability. Additionally, improper write permissions on mounted volumes could enable future file manipulation attacks.

Affected Products

• BigBlueButton versions 3.0.21 and below
• BigBlueButton servers following the official ClamAV documentation for presentation file scanning
• Deployments using Docker with ClamAV integration

Discovery Timeline

• February 21, 2026 - CVE-2026-27466 published to NVD
• February 26, 2026 - Last updated in NVD database

Technical Details for CVE-2026-27466

Vulnerability Analysis

This vulnerability stems from insecure default configuration guidance in BigBlueButton's official documentation. The flawed instructions for enabling ClamAV as a presentation file scanner expose both ClamAV ports (3310 for clamd and 7357 for freshclam) directly to the internet, contrary to ClamAV's own security recommendations. The clamd documentation explicitly warns against exposing these ports to untrusted networks due to the resource-intensive nature of antivirus scanning operations.

A critical compounding factor is that standard Ubuntu firewall (ufw) protections are ineffective against this exposure. Docker routes container traffic through the NAT table, which operates independently of ufw's filter table rules. This means administrators who believe they have firewall protection in place may still have these ports exposed to the internet.

Furthermore, the documentation example mounts /var/bigbluebutton with write permissions into the container, which is unnecessary for the scanning function and creates an additional attack surface for potential future clamd vulnerabilities.

Root Cause

The root cause is classified as CWE-668 (Exposure of Resource to Wrong Sphere). The vulnerability exists because the official documentation provides configuration instructions that expose internal service ports to external network access, bypassing intended security boundaries. The Docker networking behavior that circumvents ufw rules exacerbates this issue, creating a false sense of security for administrators who believe their firewall is protecting these services.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying BigBlueButton servers that followed the flawed documentation
2. Connecting directly to the exposed ClamAV ports (3310 or 7357)
3. Submitting large or computationally complex documents for scanning
4. Exhausting server CPU, memory, or disk resources through repeated malicious requests
5. Alternatively, sending specially crafted requests to crash the clamd process

The vulnerability manifests in the network port exposure configuration within Docker deployment settings. Attackers can send complex or large documents to the clamd service on port 3310, causing resource exhaustion. The clamd process can also be forcibly shutdown through targeted requests. For detailed technical information about the specific configuration issues, see the GitHub Security Advisory.

Detection Methods for CVE-2026-27466

Indicators of Compromise

• Unexpected network connections to ports 3310 or 7357 from external IP addresses
• High CPU or memory utilization by the clamd process
• Unusual volume of presentation file scanning requests
• clamd process crashes or restarts
• Network traffic anomalies showing large payloads directed at ClamAV ports

Detection Strategies

• Monitor for external connections to ports 3310 and 7357 using network flow analysis
• Implement alerting for clamd process crashes or abnormal resource consumption
• Review Docker container network configurations for exposed ports
• Audit server configurations against the vulnerable documentation patterns
• Deploy intrusion detection rules for ClamAV-specific attack signatures

Monitoring Recommendations

• Establish baseline resource utilization metrics for ClamAV processes and alert on deviations
• Configure logging for all connections to ClamAV ports with source IP tracking
• Implement rate limiting monitoring for presentation file submissions
• Set up automated health checks for clamd process availability
• Review Docker port mappings regularly as part of security audits

How to Mitigate CVE-2026-27466

Immediate Actions Required

• Upgrade to BigBlueButton version 3.0.22 or later immediately
• Review Docker container configurations to ensure ports 3310 and 7357 are not exposed to external networks
• Implement network-level firewall rules that operate at the Docker networking layer (not just ufw)
• Remove unnecessary write permissions for the /var/bigbluebutton mount in ClamAV containers
• Audit logs for any evidence of prior exploitation attempts

Patch Information

BigBlueButton has released version 3.0.22 which addresses this vulnerability with corrected documentation and secure default configurations. The fix is available in commit f3d33d94a9682e87c7d41f55700b19d61e1ab8b4. Organizations should upgrade immediately and review their existing configurations against the updated documentation. See the GitHub Commit Details for technical implementation details.

Workarounds

• Bind ClamAV ports to localhost (127.0.0.1) only, preventing external network access
• Implement iptables rules at the DOCKER-USER chain level to block external access to ClamAV ports
• Place ClamAV containers on an isolated Docker network without external port exposure
• Use a reverse proxy or application-level gateway to control access to scanning services
• If ClamAV scanning is not essential, consider temporarily disabling the feature until patching is complete

# Configuration example
# Block external access to ClamAV ports at Docker level
iptables -I DOCKER-USER -p tcp --dport 3310 -j DROP
iptables -I DOCKER-USER -p tcp --dport 7357 -j DROP
iptables -I DOCKER-USER -p tcp --dport 3310 -s 127.0.0.1 -j ACCEPT
iptables -I DOCKER-USER -p tcp --dport 7357 -s 127.0.0.1 -j ACCEPT