CVE-2021-38498 – This vulnerability allows a malicious document ... (How to Fix)

Q: How do I fix CVE-2021-38498?

1. Open browser/mail client. 2. Go to Help > About Firefox/Thunderbird. 3. Allow automatic update to latest version. 4. Restart application when prompted.

Q: What is the severity of CVE-2021-38498?

CVE-2021-38498 has a CVSS score of 7.5 (HIGH). This vulnerability allows a malicious document to trigger a use-after-free condition during browser shutdown, leading to memory corruption and potentially exploitable crashes. It affects Firefox, Thunderbird, and Firefox ESR users running outdated versions, potentially enabling arbitrary code execution.

📋 TL;DR

This vulnerability allows a malicious document to trigger a use-after-free condition during browser shutdown, leading to memory corruption and potentially exploitable crashes. It affects Firefox, Thunderbird, and Firefox ESR users running outdated versions, potentially enabling arbitrary code execution.

💻 Affected Systems

Products:

Firefox
Thunderbird
Firefox ESR

Versions: Firefox < 93, Thunderbird < 91.2, Firefox ESR < 91.2

Operating Systems: Windows, macOS, Linux, All supported platforms

Default Config Vulnerable: ⚠️ Yes

Notes: All default configurations are vulnerable. No special settings required for exploitation.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

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

🟠

Likely Case

Browser crash and denial of service, with potential for limited code execution in some scenarios.

🟢

If Mitigated

No impact if patched; crashes without exploitation if unpatched but with exploit mitigations.

🌐 Internet-Facing: HIGH - Attackers can exploit via malicious web content without user interaction beyond browsing.

🏢 Internal Only: MEDIUM - Requires user to open malicious documents, which could occur via internal phishing or compromised sites.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires triggering specific shutdown conditions with crafted documents. No public exploits known, but use-after-free vulnerabilities are commonly exploited.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Firefox 93+, Thunderbird 91.2+, Firefox ESR 91.2+

Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2021-43/

Restart Required: Yes

Instructions:

1. Open browser/mail client. 2. Go to Help > About Firefox/Thunderbird. 3. Allow automatic update to latest version. 4. Restart application when prompted.

🔧 Temporary Workarounds

Disable JavaScript

all

Prevents execution of malicious scripts that could trigger the vulnerability.

about:config → javascript.enabled = false

Use Content Security Policy

all

Restrict document sources to trusted origins only.

Header set Content-Security-Policy "default-src 'self'"

🧯 If You Can't Patch

Restrict user access to untrusted websites and documents
Implement application whitelisting to prevent execution of malicious payloads

🔍 How to Verify

Check if Vulnerable:

Check browser version in Help > About Firefox/Thunderbird. If version is below patched versions, system is vulnerable.

Check Version:

firefox --version || thunderbird --version

Verify Fix Applied:

Confirm version is Firefox 93+, Thunderbird 91.2+, or Firefox ESR 91.2+ after update.

📡 Detection & Monitoring

Log Indicators:

Browser crash logs with memory corruption signatures
Unexpected process termination during shutdown

Network Indicators:

Requests to known malicious domains serving crafted documents
Unusual document downloads triggering crashes

SIEM Query:

source="browser_crash_logs" AND (event_id="crash" OR memory_corruption="true")

📊 Metadata

CVE ID: CVE-2021-38498

CVSS v3 Score: 7.5 (HIGH)

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

CWE: CWE-416

Published: November 3, 2021

Last Updated: November 21, 2024

🔗 References

https://bugzilla.mozilla.org/show_bug.cgi?id=1729642 Permissions Required,Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-43/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-45/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-47/ Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1729642 Permissions Required,Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-43/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-45/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2021-47/ Vendor Advisory

📤 Share & Export

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