CVE-2021-29970 – This vulnerability allows a malicious webpage t... (How to Fix)

Q: How do I fix CVE-2021-29970?

1. Open browser/Thunderbird. 2. Go to Help > About. 3. Allow automatic update. 4. Restart when prompted. For enterprise: Deploy updated packages via your management system.

Q: What is the severity of CVE-2021-29970?

CVE-2021-29970 has a CVSS score of 8.8 (HIGH). This vulnerability allows a malicious webpage to trigger a use-after-free memory corruption in Mozilla browsers when accessibility features are enabled, leading to a potentially exploitable crash. It affects Thunderbird, Firefox ESR, and Firefox. Attackers could potentially execute arbitrary code or cause denial of service.

📋 TL;DR

This vulnerability allows a malicious webpage to trigger a use-after-free memory corruption in Mozilla browsers when accessibility features are enabled, leading to a potentially exploitable crash. It affects Thunderbird, Firefox ESR, and Firefox. Attackers could potentially execute arbitrary code or cause denial of service.

💻 Affected Systems

Products:

Mozilla Thunderbird
Mozilla Firefox ESR
Mozilla Firefox

Versions: Thunderbird < 78.12, Firefox ESR < 78.12, Firefox < 90

Operating Systems: Windows, Linux, macOS

Default Config Vulnerable: ✅ No

Notes: This vulnerability ONLY triggers when accessibility features are enabled in the browser.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution leading to full system compromise, data theft, or malware installation.

🟠

Likely Case

Application crash (denial of service) or limited memory corruption without reliable code execution.

🟢

If Mitigated

No impact if accessibility features are disabled or patched versions are used.

🌐 Internet-Facing: HIGH - Web browsers are internet-facing by design and can be targeted by malicious websites.

🏢 Internal Only: MEDIUM - Internal users could be targeted via phishing or compromised internal sites.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Requires user interaction (visiting malicious webpage) and accessibility features enabled. Memory corruption vulnerabilities can be challenging to exploit reliably.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Thunderbird 78.12, Firefox ESR 78.12, Firefox 90

Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2021-28/

Restart Required: Yes

Instructions:

1. Open browser/Thunderbird. 2. Go to Help > About. 3. Allow automatic update. 4. Restart when prompted. For enterprise: Deploy updated packages via your management system.

🔧 Temporary Workarounds

Disable Accessibility Features

all

Turn off accessibility features in browser settings to prevent exploitation.

🧯 If You Can't Patch

Disable accessibility features in all affected browsers
Implement web filtering to block known malicious sites and restrict browsing

🔍 How to Verify

Check if Vulnerable:

Check browser version in Help > About menu. If version is below patched versions and accessibility features are enabled, system is vulnerable.

Check Version:

On Linux: thunderbird --version, firefox --version. On Windows: Check in Help > About.

Verify Fix Applied:

Confirm version is Thunderbird ≥78.12, Firefox ESR ≥78.12, or Firefox ≥90 in Help > About menu.

📡 Detection & Monitoring

Log Indicators:

Browser crash logs with memory corruption errors
Accessibility service crash events

Network Indicators:

Connections to suspicious domains followed by browser crashes

SIEM Query:

source="browser_crash_logs" AND (event_id="1000" OR error="memory_corruption")

📊 Metadata

CVE ID: CVE-2021-29970

CVSS v3 Score: 8.8 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CWE: CWE-416

Published: August 5, 2021

Last Updated: November 21, 2024

🔗 References

https://bugzilla.mozilla.org/show_bug.cgi?id=1709976 Exploit,Issue Tracking,Vendor Advisory
https://security.gentoo.org/glsa/202202-03 Third Party Advisory
https://security.gentoo.org/glsa/202208-14 Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-28/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-29/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-30/ Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1709976 Exploit,Issue Tracking,Vendor Advisory
https://security.gentoo.org/glsa/202202-03 Third Party Advisory
https://security.gentoo.org/glsa/202208-14 Third Party Advisory
https://www.mozilla.org/security/advisories/mfsa2021-28/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-29/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-30/ Vendor Advisory

📤 Share & Export

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🔗 Related Vulnerabilities

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