CVE-2026-27736: BigBlueButton Open Redirect Vulnerability

CVE-2026-27736 Overview

CVE-2026-27736 is an open redirect vulnerability in BigBlueButton, an open-source virtual classroom platform. The flaw affects versions on the 3.x branch prior to 3.0.20. The application accepts a user-controlled errorRedirectUrl parameter and passes it directly to the respondWithRedirect function without validation. Attackers can craft URLs that redirect authenticated or unauthenticated users to attacker-controlled domains. This weakness maps to CWE-601: URL Redirection to Untrusted Site. BigBlueButton 3.0.20 patches the issue, and no workarounds are available.

Critical Impact

Attackers can leverage trusted BigBlueButton URLs to redirect victims to malicious sites, enabling phishing campaigns and credential theft against virtual classroom users.

Affected Products

• BigBlueButton 3.x branch versions prior to 3.0.20
• BigBlueButton virtual classroom server deployments
• Educational and remote-collaboration platforms embedding BigBlueButton

Discovery Timeline

• 2026-02-25 - CVE-2026-27736 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-27736

Vulnerability Analysis

The vulnerability resides in BigBlueButton's error-handling redirect flow. The application reads the errorRedirectUrl query string parameter from the incoming request. It then forwards the value to the respondWithRedirect function without performing host allowlisting or scheme validation. The function issues an HTTP redirect response that points the victim's browser to the attacker-supplied destination.

Because the initial request originates from a legitimate BigBlueButton domain, the link appears trustworthy to users and to many email security gateways. Attackers exploit this trust to deliver phishing pages, malware downloads, or fake login portals that mimic the BigBlueButton interface.

Root Cause

The root cause is missing input validation on a redirect target parameter. The respondWithRedirect function trusts the errorRedirectUrl value supplied by the client. The code does not compare the destination against an allowlist of approved hosts. It also does not enforce that the target remains within the same origin as the BigBlueButton deployment.

Attack Vector

An attacker crafts a URL containing a malicious errorRedirectUrl parameter that points to an external domain. The attacker distributes the link through email, chat, or social media, often impersonating a class invitation or session-recovery notice. When the victim clicks the link and interacts with the page, BigBlueButton issues a redirect to the attacker-controlled site. The victim sees a legitimate BigBlueButton domain in the original link, increasing the success rate of follow-on phishing or drive-by download attacks. User interaction is required, consistent with the UI:R component of the CVSS vector.

No verified public exploit code is available. The mechanics follow the standard open redirect pattern described in the GitHub Security Advisory GHSA-65cv-rg9f-qqrx.

Detection Methods for CVE-2026-27736

Indicators of Compromise

• HTTP requests to BigBlueButton endpoints containing an errorRedirectUrl parameter whose value references an external domain or non-HTTPS scheme
• Web server access logs showing 302 or 303 responses with Location headers pointing outside the BigBlueButton host
• Referrer headers on third-party domains that originate from BigBlueButton URLs containing errorRedirectUrl

Detection Strategies

• Inspect web application firewall and reverse proxy logs for errorRedirectUrl query parameters and flag values not matching the deployment's own hostname
• Correlate outbound redirect responses with user-agent and session telemetry to identify phishing campaigns abusing BigBlueButton URLs
• Hunt across email gateway logs for inbound messages containing BigBlueButton links with embedded errorRedirectUrl parameters

Monitoring Recommendations

• Enable verbose logging on the BigBlueButton HTML5 frontend and the nginx proxy layer to capture full request URIs
• Forward web access logs to a centralized SIEM and build alerts on suspicious redirect parameters
• Track BigBlueButton release notes and the vendor commit history for follow-up advisories

How to Mitigate CVE-2026-27736

Immediate Actions Required

• Upgrade all BigBlueButton 3.x deployments to version 3.0.20 or later
• Audit running instances using bbb-conf --check to confirm the installed package version
• Notify users to verify destination URLs before submitting credentials on pages reached through BigBlueButton links

Patch Information

BigBlueButton 3.0.20 fixes the issue by validating the errorRedirectUrl before passing it to respondWithRedirect. Review the upstream fix in the GitHub commit 691f92f3 and the security advisory GHSA-65cv-rg9f-qqrx for full remediation details.

Workarounds

• No official workarounds are available; upgrading to 3.0.20 is the only supported remediation
• As a temporary defense-in-depth measure, configure the upstream reverse proxy to strip or reject requests containing an errorRedirectUrl parameter whose value does not match the deployment hostname

# Example nginx snippet to block external errorRedirectUrl values
# Place inside the BigBlueButton server block
if ($arg_errorRedirectUrl ~* "^https?://(?!your-bbb-host\.example\.com)") {
    return 400;
}