CVE-2023-25736 – This vulnerability involves an invalid downcast... (How to Fix)

Q: What is the severity of CVE-2023-25736?

CVE-2023-25736 has a CVSS score of 9.8 (CRITICAL). This vulnerability involves an invalid downcast from nsHTMLDocument to nsIContent in Firefox, which could lead to undefined behavior including potential memory corruption. It affects Firefox versions before 110. Attackers could exploit this to execute arbitrary code or cause crashes.

📋 TL;DR

This vulnerability involves an invalid downcast from nsHTMLDocument to nsIContent in Firefox, which could lead to undefined behavior including potential memory corruption. It affects Firefox versions before 110. Attackers could exploit this to execute arbitrary code or cause crashes.

💻 Affected Systems

Products:

Mozilla Firefox

Versions: All versions before 110

Operating Systems: Windows, Linux, macOS, Android

Default Config Vulnerable: ⚠️ Yes

Notes: All standard Firefox installations are vulnerable. Extensions or security settings don't mitigate this vulnerability.

📦 What is this software?

Firefox by Mozilla

View all CVEs affecting Firefox →

⚠️ Risk & Real-World Impact

🔴

Worst Case

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

🟠

Likely Case

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

🟢

If Mitigated

Browser crash with no data loss if sandboxing and other security controls contain the impact.

🌐 Internet-Facing: HIGH - Firefox browsers directly interact with untrusted web content, making them prime targets for exploitation.

🏢 Internal Only: MEDIUM - Internal users browsing malicious internal sites or compromised legitimate sites could be affected.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires crafting malicious web content that triggers the invalid downcast. No public exploit code is known.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Firefox 110 and later

Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2023-05/

Restart Required: Yes

Instructions:

1. Open Firefox. 2. Click menu → Help → About Firefox. 3. Firefox will check for updates and install Firefox 110+. 4. Restart Firefox when prompted.

🔧 Temporary Workarounds

Disable JavaScript

all

Prevents execution of malicious scripts that might trigger the vulnerability

about:config → javascript.enabled = false

Use alternative browser

all

Temporarily switch to a non-vulnerable browser until Firefox is updated

🧯 If You Can't Patch

Restrict browsing to trusted websites only
Implement network filtering to block malicious content delivery

🔍 How to Verify

Check if Vulnerable:

Check Firefox version: Open Firefox → Help → About Firefox. If version is less than 110, you are vulnerable.

Check Version:

firefox --version (Linux/macOS) or check About Firefox (Windows)

Verify Fix Applied:

Confirm Firefox version is 110 or higher using the same method.

📡 Detection & Monitoring

Log Indicators:

Firefox crash reports with memory corruption signatures
Unexpected browser termination events

Network Indicators:

Unusual outbound connections from Firefox post-crash
Requests to known exploit delivery domains

SIEM Query:

source="firefox.log" AND ("crash" OR "segfault" OR "memory corruption")

📊 Metadata

CVE ID: CVE-2023-25736

CVSS v3 Score: 9.8 (CRITICAL)

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

Published: June 19, 2023

Last Updated: December 11, 2024

🔗 References

https://bugzilla.mozilla.org/show_bug.cgi?id=1811331 Permissions Required
https://www.mozilla.org/security/advisories/mfsa2023-05/ Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1811331 Permissions Required
https://www.mozilla.org/security/advisories/mfsa2023-05/ Vendor Advisory

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