CVE-2025-4918
📋 TL;DR
This vulnerability allows an attacker to perform out-of-bounds memory operations on JavaScript Promise objects, potentially leading to arbitrary code execution. It affects Firefox, Firefox ESR, and Thunderbird users running outdated versions. Attackers could exploit this by tricking users into visiting malicious websites.
💻 Affected Systems
- Firefox
- Firefox ESR
- Thunderbird
📦 What is this software?
Firefox by Mozilla
Firefox by Mozilla
Firefox by Mozilla
Thunderbird by Mozilla
Thunderbird by Mozilla
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution with system-level privileges, complete system compromise, data theft, and persistent backdoor installation.
Likely Case
Browser crash, memory corruption leading to information disclosure, or limited code execution within browser sandbox.
If Mitigated
Browser crash with no data loss if sandboxing works properly, or exploit blocked by security controls.
🎯 Exploit Status
Exploitation requires JavaScript execution in browser context. No authentication needed - visiting malicious website is sufficient.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firefox 138.0.4, Firefox ESR 128.10.1, Firefox ESR 115.23.1, Thunderbird 128.10.2, Thunderbird 138.0.2
Vendor Advisory: https://www.mozilla.org/security/advisories/mfsa2025-36/
Restart Required: Yes
Instructions:
1. Open Firefox/Thunderbird. 2. Click menu → Help → About Firefox/Thunderbird. 3. Browser will check for updates and prompt to install. 4. Restart browser when update completes. 5. Verify version matches patched versions above.
🔧 Temporary Workarounds
Disable JavaScript
allTemporarily disable JavaScript execution to prevent exploitation
about:config → javascript.enabled = false
Use Content Security Policy
allImplement strict CSP headers to restrict script execution
Content-Security-Policy: script-src 'self'
🧯 If You Can't Patch
- Restrict browser usage to trusted websites only
- Implement network filtering to block malicious domains and scripts
🔍 How to Verify
Check if Vulnerable:
Check browser version in About dialog. If version matches affected ranges, system is vulnerable.Check Version:
Firefox: about:support → Application Basics → Version. Thunderbird: Help → About ThunderbirdVerify Fix Applied:
Verify browser version matches patched versions: Firefox ≥138.0.4, Firefox ESR ≥128.10.1 or ≥115.23.1, Thunderbird ≥128.10.2 or ≥138.0.2📡 Detection & Monitoring
Log Indicators:
- Browser crash logs with memory access violations
- Unexpected process termination
- High memory usage patterns
Network Indicators:
- Requests to known malicious domains hosting exploit code
- Unusual JavaScript execution patterns
SIEM Query:
source="browser_logs" AND (event="crash" OR event="memory_violation") AND version<"138.0.4"🔗 References
- https://bugzilla.mozilla.org/show_bug.cgi?id=1966612
- https://www.mozilla.org/security/advisories/mfsa2025-36/
- https://www.mozilla.org/security/advisories/mfsa2025-37/
- https://www.mozilla.org/security/advisories/mfsa2025-38/
- https://www.mozilla.org/security/advisories/mfsa2025-40/
- https://www.mozilla.org/security/advisories/mfsa2025-41/
- https://lists.debian.org/debian-lts-announce/2025/05/msg00024.html
- https://lists.debian.org/debian-lts-announce/2025/05/msg00046.html
- https://www.vicarius.io/vsociety/posts/cve-2025-4918-detect-firefox-out-of-bounds-write
- https://www.vicarius.io/vsociety/posts/cve-2025-4918-mitigate-firefox-out-of-bounds-write
