CVE-2020-26974 – This vulnerability involves a type confusion bu... (How to Fix)

Q: How do I fix CVE-2020-26974?

1. Open the affected application (Firefox/Thunderbird). 2. Go to Help > About Firefox/Thunderbird. 3. Allow the application to check for and install updates. 4. Restart the application when prompted.

Q: What is the severity of CVE-2020-26974?

CVE-2020-26974 has a CVSS score of 8.8 (HIGH). This vulnerability involves a type confusion bug in Firefox's CSS flexbox implementation where a StyleGenericFlexBasis object could be incorrectly cast, leading to a use-after-free condition. This can result in memory corruption and potentially exploitable crashes, affecting Firefox, Thunderbird, and Firefox ESR users.

📋 TL;DR

This vulnerability involves a type confusion bug in Firefox's CSS flexbox implementation where a StyleGenericFlexBasis object could be incorrectly cast, leading to a use-after-free condition. This can result in memory corruption and potentially exploitable crashes, affecting Firefox, Thunderbird, and Firefox ESR users.

💻 Affected Systems

Products:

Firefox
Thunderbird
Firefox ESR

Versions: Firefox < 84, Thunderbird < 78.6, Firefox ESR < 78.6

Operating Systems: Windows, Linux, macOS, Android

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

Application crash (denial of service) or limited memory corruption that could be leveraged for further exploitation.

🟢

If Mitigated

Minimal impact if systems are patched or isolated from untrusted content.

🌐 Internet-Facing: HIGH - Web browsers process untrusted content from the internet, making exploitation likely via malicious websites.

🏢 Internal Only: MEDIUM - Internal users could be targeted via phishing or compromised internal sites, but attack surface is reduced compared to internet-facing scenarios.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires triggering the specific CSS flexbox condition, but no public proof-of-concept has been released.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Firefox 84, Thunderbird 78.6, Firefox ESR 78.6

Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2020-54/

Restart Required: Yes

Instructions:

1. Open the affected application (Firefox/Thunderbird). 2. Go to Help > About Firefox/Thunderbird. 3. Allow the application to check for and install updates. 4. Restart the application when prompted.

🔧 Temporary Workarounds

Disable JavaScript

all

Prevents execution of malicious scripts that might trigger the vulnerability, but breaks most website functionality.

about:config -> javascript.enabled = false

Use Content Security Policy

all

Implement CSP headers to restrict script sources, reducing attack surface.

Content-Security-Policy: script-src 'self'

🧯 If You Can't Patch

Restrict browser usage to trusted websites only and disable untrusted content.
Implement network segmentation to isolate vulnerable systems from potential attack vectors.

🔍 How to Verify

Check if Vulnerable:

Check the application version in Help > About Firefox/Thunderbird and compare against affected versions.

Check Version:

firefox --version or thunderbird --version on command line

Verify Fix Applied:

Confirm the application version is Firefox ≥84, Thunderbird ≥78.6, or Firefox ESR ≥78.6.

📡 Detection & Monitoring

Log Indicators:

Application crash logs referencing heap corruption or use-after-free in CSS flexbox code

Network Indicators:

Unusual traffic patterns to/from browser instances, especially to known malicious domains

SIEM Query:

source="firefox.log" AND ("crash" OR "heap corruption" OR "use-after-free")

📊 Metadata

CVE ID: CVE-2020-26974

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-787

Published: January 7, 2021

Last Updated: November 21, 2024

🔗 References

https://bugzilla.mozilla.org/show_bug.cgi?id=1681022 Permissions Required
https://www.mozilla.org/security/advisories/mfsa2020-54/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-55/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-56/ Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1681022 Permissions Required
https://www.mozilla.org/security/advisories/mfsa2020-54/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-55/ Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2020-56/ Vendor Advisory

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🔗 Related Vulnerabilities

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