Executive Summary

[{'type': 'paragraph', 'content': "CVE-2026-27467 is a vulnerability in the BigBlueButton virtual classroom software versions 3.0.19 and below. When a user joins a session with their microphone muted, the client still sends audio data to the server despite the mute state. Although the server discards this audio so it is not audible to other participants, the client-side audio track is not properly disabled, causing a mismatch between the user's mute state and actual audio transmission."}, {'type': 'paragraph', 'content': 'This issue occurs because the client does not enforce the mute state correctly during the initial audio join phase, due to a change in how audio channels are muted on creation. The vulnerability allows a malicious server operator to potentially access audio data sent during the period between joining the meeting and the first time the user unmutes.'}, {'type': 'paragraph', 'content': 'The problem was fixed in version 3.0.20 by modifying the client to explicitly disable the local audio track when joining a session if the microphone is initially muted.'}] [1, 2]

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Impact Analysis

This vulnerability can impact users by unintentionally transmitting audio data to the server even when their microphone is muted upon joining a session.

While the server discards this audio so other participants cannot hear it, a malicious server operator could potentially access this audio data, leading to a low confidentiality loss.

The attack requires network access, high privileges on the server, and user interaction, making exploitation complex. There is no impact on data integrity or availability.

Users can mitigate the risk by updating to version 3.0.20 or later, or by briefly unmuting and muting their microphone again after joining to ensure the audio track is properly muted.

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Compliance Impact

I don't know

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Detection Guidance

This vulnerability involves the client sending audio data to the server even when the microphone is muted upon joining a session. Detection would involve monitoring network traffic for audio data packets sent from the client immediately after joining a BigBlueButton session with the microphone muted.

Since the issue is specific to versions 3.0.19 and below of the BigBlueButton HTML5 client, verifying the client version in use is a primary step.

Suggested commands or methods to detect this behavior could include:

• Using network packet capture tools like tcpdump or Wireshark to monitor RTP or WebRTC audio streams immediately after joining a session with the microphone muted.
• Example tcpdump command to capture traffic on the default WebRTC ports (adjust ports as needed):
• tcpdump -i <interface> -w capture.pcap port 3478 or port 10000
• Analyzing the capture with Wireshark to identify audio packets sent from the client before the user unmutes.
• Checking client logs or browser developer tools console for any audio track state inconsistencies or errors related to mute state enforcement.

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Mitigation Strategies

The vulnerability has been fixed in BigBlueButton version 3.0.20. The primary mitigation step is to upgrade the BigBlueButton HTML5 client to version 3.0.20 or later.

As a workaround, users can briefly unmute and then mute their microphone again after joining a session to prevent unintended audio transmission.

Additionally, restricting server operator privileges and monitoring server access can reduce the risk of malicious audio data access.

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