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    CVE-2026-5731

    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.

    Published:Apr 7, 2026
    Last Modified:May 26, 2026
    EPS:Apr 7, 2026
    EPSS Score:0.00071
    CVSS Score:9.8

    Affected Products

    Vendor
    Mozilla
    Product
    Firefox
    Vendor
    Mozilla
    Product
    Firefox Esr
    Vendor
    Mozilla
    Product
    Thunderbird

    Exploits

    No exploit reference

    Related CVEs

    CVE-2026-84144 (CVSS 7.5) — Internally found bugs present in Thunderbird 154 and Thunderbird ESR 153.1. Some of these bugs showed evidence…
    CVE-2026-84143 (CVSS 9.8) — Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14. Some of th…
    CVE-2026-84142 (CVSS 9.8) — Internally found bugs present in Thunderbird 154. Some of these bugs showed evidence of memory corruption or a…
    CVE-2026-84141 (CVSS 9.8) — Integer overflow in the Graphics: ImageLib component. This vulnerability was fixed in Firefox 155, Firefox ESR…
    CVE-2026-84140 (CVSS 9.8) — Site isolation issue in the DOM: Navigation component. This vulnerability was fixed in Firefox 155, Firefox ES…
    CVE-2026-84139 (CVSS 6.1) — Clickjacking issue in the DOM: Events component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.…
    CVE-2026-84138 (CVSS 6.5) — Denial-of-service in the PDF Viewer component. This vulnerability was fixed in Firefox 155 and Thunderbird 155…
    CVE-2026-84137 (CVSS 4.3) — Spoofing issue in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 155, Firefox ESR 153…
    CVE-2026-84136 (CVSS 6.1) — Other issue in the DOM: Navigation component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.2, …
    CVE-2026-84135 (CVSS 9.8) — Other issue in Firefox Focus for Android. This vulnerability was fixed in Firefox 155.…

    References

    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
    https://www.mozilla.org/security/advisories/mfsa2026-25/
    https://www.mozilla.org/security/advisories/mfsa2026-26/
    https://www.mozilla.org/security/advisories/mfsa2026-27/
    https://www.mozilla.org/security/advisories/mfsa2026-28/
    https://www.mozilla.org/security/advisories/mfsa2026-29/
    https://access.redhat.com/errata/RHSA-2026:11805
    https://access.redhat.com/errata/RHSA-2026:11813
    https://access.redhat.com/errata/RHSA-2026:12264
    https://access.redhat.com/errata/RHSA-2026:13342
    https://access.redhat.com/errata/RHSA-2026:13412
    https://access.redhat.com/errata/RHSA-2026:13533
    https://access.redhat.com/errata/RHSA-2026:13582
    https://access.redhat.com/errata/RHSA-2026:13583
    https://access.redhat.com/errata/RHSA-2026:13596
    https://access.redhat.com/errata/RHSA-2026:13600
    https://access.redhat.com/errata/RHSA-2026:13665
    https://access.redhat.com/errata/RHSA-2026:13682
    https://access.redhat.com/errata/RHSA-2026:13683
    https://access.redhat.com/errata/RHSA-2026:13922
    https://access.redhat.com/errata/RHSA-2026:13977
    https://access.redhat.com/errata/RHSA-2026:14223
    https://access.redhat.com/errata/RHSA-2026:14303
    https://access.redhat.com/errata/RHSA-2026:15889
    https://access.redhat.com/errata/RHSA-2026:7671
    https://access.redhat.com/errata/RHSA-2026:7672
    https://access.redhat.com/errata/RHSA-2026:8052
    https://access.redhat.com/errata/RHSA-2026:8459
    https://access.redhat.com/errata/RHSA-2026:9345
    https://access.redhat.com/errata/RHSA-2026:9638
    https://access.redhat.com/security/cve/CVE-2026-5731
    https://bugzilla.redhat.com/show_bug.cgi?id=2455901
    https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-5731.json

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High