CVE-2024-5691: Mozilla Firefox Auth Bypass Vulnerability

CVE-2024-5691 Overview

CVE-2024-5691 is a sandbox bypass vulnerability affecting Mozilla Firefox, Firefox ESR, and Thunderbird. A sandboxed iframe could trick the browser using an X-Frame-Options header and present a button that, when clicked by the user, bypassed sandbox restrictions to open a new window. The flaw is categorized under [CWE-693] Protection Mechanism Failure and requires user interaction to exploit. Mozilla resolved the issue in Firefox 127, Firefox ESR 115.12, and Thunderbird 115.12.

Critical Impact
A malicious site embedded in a sandboxed iframe could escape sandbox restrictions with a single user click, opening arbitrary new windows and undermining the isolation guarantees of the HTML5 iframe sandbox.

Affected Products

Mozilla Firefox versions prior to 127
Mozilla Firefox ESR versions prior to 115.12
Mozilla Thunderbird versions prior to 115.12

Discovery Timeline

2024-06-11 - CVE-2024-5691 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-5691

Vulnerability Analysis

The HTML5 iframe sandbox attribute restricts capabilities available to embedded content, including navigation, script execution, and popup creation. This vulnerability allowed a sandboxed frame to circumvent those restrictions by manipulating how the browser processed the X-Frame-Options response header. When exploited, the sandboxed frame could render an interactive element such as a button that, upon user activation, opened a new window despite the absence of the allow-popups sandbox token.

The issue is classified as a Protection Mechanism Failure. The sandbox policy enforcement path failed to correctly evaluate framing rules under specific X-Frame-Options conditions, allowing untrusted content to escape the sandbox scope. Exploitation requires user interaction, which limits automated abuse but does not eliminate risk in social-engineering scenarios.

Root Cause

The root cause lies in inconsistent enforcement between the X-Frame-Options framing check and the iframe sandbox restriction logic in Mozilla's rendering engine. The sandbox policy did not correctly propagate to windows spawned through the bypass path, leaving popup creation unrestricted. Refer to Mozilla Bug Report #1888695 for the specific implementation details tracked by Mozilla engineers.

Attack Vector

An attacker hosts a page containing a sandboxed iframe that loads attacker-controlled content served with a crafted X-Frame-Options header. The sandboxed content presents a button crafted to look like a legitimate site action. When the user clicks the button, the browser opens a new window that operates outside the intended sandbox restrictions. This new window can then be used for phishing, credential harvesting, or delivery of secondary payloads.

The vulnerability describes a Sandbox Bypass and Security Feature Bypass. Because Thunderbird processes remote HTML content in email messages, the flaw is also reachable in email-based delivery scenarios where users interact with rendered messages.

Detection Methods for CVE-2024-5691

Indicators of Compromise

Endpoints running Firefox versions below 127, Firefox ESR below 115.12, or Thunderbird below 115.12
Outbound HTTP responses containing unusual X-Frame-Options header values paired with sandboxed iframe rendering
Unexpected browser popup windows or new tabs spawned immediately after user clicks on embedded third-party content

Detection Strategies

Inventory browser and email client versions across managed endpoints to identify systems below the patched builds
Correlate browser process telemetry with new window creation events that lack expected parent-frame permissions
Monitor web proxy or secure web gateway logs for HTML responses that combine X-Frame-Options headers with iframe embedding to identify potential exploitation attempts

Monitoring Recommendations

Alert on Firefox and Thunderbird process spawn events where the installed version predates the June 2024 patch level
Track user-reported phishing incidents that originated from unexpected browser popups
Review email gateway logs for HTML messages with embedded iframes rendered by unpatched Thunderbird clients

How to Mitigate CVE-2024-5691

Immediate Actions Required

Update Firefox to version 127 or later on all managed endpoints
Update Firefox ESR to version 115.12 or later where enterprise deployments require the extended support release
Update Thunderbird to version 115.12 or later on all systems that process email
Deploy Debian package updates referenced in the Debian LTS Announcement - June 2024 for affected Debian systems

Patch Information

Mozilla addressed this vulnerability in the June 2024 security release cycle. See Mozilla Security Advisory MFSA-2024-25 for Firefox 127, Mozilla Security Advisory MFSA-2024-26 for Firefox ESR 115.12, and Mozilla Security Advisory MFSA-2024-28 for Thunderbird 115.12. Debian users should apply the updates announced in the Debian LTS Announcement - June 2024 (msg 10).

Workarounds

Disable remote HTML content rendering in Thunderbird by configuring the client to display messages in plain text
Restrict browser access to untrusted third-party content through enterprise web filtering policies until patches are deployed
Educate users to avoid interacting with unexpected buttons or prompts inside embedded frames on untrusted sites

bash

# Verify installed Firefox and Thunderbird versions on Linux endpoints
firefox --version
thunderbird --version

# Example Debian update commands
sudo apt update
sudo apt install --only-upgrade firefox-esr thunderbird