CriticalCVE-2026-84637

    firefox: Unrestricted Upload of File with Dangerous Type (CVE-2026-84637)

    Published Sources: BSI

    What is affected, what to do

    Malicious calendar invitations could use file URI attachments to launch local or network-hosted executables on Windows, bypassing Thunderbird's normal executable attachment protections. With the new invitation display enabled, the attachment could also appear under a misleading filename.

    Update Mozilla Thunderbird to version 153.2.0, Mozilla Thunderbird to version 155.0.

    Affected
    • Mozilla Thunderbird lt *..153.2.0
    • Mozilla Thunderbird lt 154.0..155.0
    Fixed in
    • Mozilla Thunderbird 153.2.0
    • Mozilla Thunderbird 155.0

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: BSI

    0.6 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 50.0

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    79.22 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    9.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.6 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    50.0
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CISA assessment (SSVC)
    Exploitation: none known · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 2 September 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-434

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Malicious calendar invitations could use file URI attachments to launch local or network-hosted executables on Windows, bypassing Thunderbird's normal executable attachment protections. With the new invitation display enabled, the attachment could also appear under a misleading filename. This vulnerability was fixed in Thunderbird 154 and Thunderbird 153.2.

    Source: BSI WID Portal

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    mozillafirefox
    • 116.0 – < 140.15.0
    • 141.0.0 – < 153.2.0
    • 154.0.0 – < 155.0.0
    • < 115.40.0
    • < 140.13.0
    • < 140.15.0
    • < 153.0.0
    • < 153.2.0
    • < 154.0.0
    • < 155.0.0
    • 140.15.0
    • 153.2.0
    • 155.0.0
    • 115.40.0
    • 140.13.0
    • 140.15.0
    • 153.0.0
    • 153.2.0
    • 154.0.0
    • 155.0.0
    mozillafirefox_mobile
    • < 155.0
    • 155.0
    mozillathunderbird
    • 141.0 – < 153.0
    • 141.0 – < 153.2.0
    • 154.0 – < 155.0
    • < 140.13.0
    • < 140.15.0
    • < 153.0
    • < 153.2.0
    • < 154.0
    • < 155.0
    • 153.0
    • 153.2.0
    • 155.0
    • 140.13.0
    • 140.15.0
    • 153.0
    • 153.2.0
    • 154.0
    • 155.0
    MozillaFirefox
    • < 155
    • 155
    MozillaFirefox ESR
    • < 115.40
    • < 140.15
    • < 153.2
    • 115.40
    • 140.15
    • 153.2
    MozillaThunderbird
    • < 140.15
    • < 153.2
    • < 155
    • 140.15
    • 153.2
    • 155
    From bundled advisories, not CVE-specific (10)

    Product list of a bundled advisory that does not state which product this CVE affects.

    Products from bundled advisories
    Affected versionsFixed in
    MozillaFirefox
    • < 153
    • < 154
    • 153
    • 154
    MozillaFirefox ESR
    • < 115.38
    • < 115.39
    • < 140.13
    • < 140.14
    • < 153.1
    • 115.38
    • 115.39
    • 140.13
    • 140.14
    • 153.1
    MozillaThunderbird
    • < 140.14
    • < 153.1
    • < 154
    • 140.14
    • 153.1
    • 154

    Vendors and products

    Affected operating systems

    Affected operating systems
    Linuxdebianfirefox-esrbookworm
    Linuxdebianfirefox-esrbullseye
    Linuxdebianfirefox-esrtrixie
    Linuxdebianthunderbirdbookworm
    Linuxdebianthunderbirdbullseye
    Linuxdebianthunderbirdtrixie

    Weakness classes (CWE)

    • CWE-434Unrestricted Upload of File with Dangerous Type

      Base

      The product allows the upload or transfer of dangerous file types that are automatically processed within its environment.

      Entry at MITRE CWE-434

    References & sources

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Initial analysis by NVDNVD

      Newly listed as affected:
      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Product configurationadded
      —
      OR *cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:* versions up to (excluding) 153.2.0 *cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:* versions from (including) 154.0 up to (excluding) 155.0
      Reference typeadded
      —
      Mozilla Corporation: https://bugzilla.mozilla.org/show_bug.cgi?id=2043878 Types: Permissions Required
      Reference typeadded
      —
      Mozilla Corporation: https://www.mozilla.org/security/advisories/mfsa2026-78/ Types: Vendor Advisory
      Reference typeadded
      —
      Mozilla Corporation: https://www.mozilla.org/security/advisories/mfsa2026-88/ Types: Vendor Advisory

      Source identifier: nvd@nist.gov

    2. Record modifiedCISA ADP

      CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

      CISA assessment (SSVC): exploitation: none known, automatable: yes

      Weakness type: CWE-434

      Show 3 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      CVSSadded
      —
      AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
      Weakness type (CWE)added
      —
      CWE-434
      SSVC assessmentadded
      —
      {"id":"CVE-2026-84637","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-09-02T18:49:31.675786Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    3. ReceivedMozilla

      Description added or changed

      Show 5 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      Malicious calendar invitations could use file URI attachments to launch local or network-hosted executables on Windows, bypassing Thunderbird's normal executable attachment protections. With the new invitation display enabled, the attachment could also appear under a misleading filename. This vulnerability was fixed in Thunderbird 154 and Thunderbird 153.2.
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/84xxx/CVE-2026-84637.json">CVE-2026-84637</a>
      Referenceadded
      —
      https://bugzilla.mozilla.org/show_bug.cgi?id=2043878
      Referenceadded
      —
      https://www.mozilla.org/security/advisories/mfsa2026-78/
      Referenceadded
      —
      https://www.mozilla.org/security/advisories/mfsa2026-88/

      Source identifier: security@mozilla.org

    Linked CVEs

    Linked CVEs
    HighCVE-2026-84642Allowed UNC hostnames for attachments interpreted as a regular expression7.5—
    HighCVE-2026-84641Information disclosure due to malicious IMAP server response7.5—
    HighCVE-2026-84640One byte overflow read in mail parser7.5—
    CriticalCVE-2026-84639Uninitialized memory in MIME parsing9.1—
    CriticalCVE-2026-84637Calendar invitation attachments could launch local executables9.8—
    HighCVE-2026-84145Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.157.5—
    HighCVE-2026-84144Internally found bugs fixed in Thunderbird 155 and Thunderbird ESR 153.27.5—
    CriticalCVE-2026-84143Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.159.8—
    CriticalCVE-2026-84142Internally found bugs fixed in Thunderbird 1559.8—
    CriticalCVE-2026-84141Integer overflow in the Graphics: ImageLib component9.8—
    CriticalCVE-2026-84140Site isolation issue in the DOM: Navigation component9.8—
    CriticalCVE-2026-84139Clickjacking issue in the DOM: Events component9.8—
    HighCVE-2026-84138Denial-of-service in the PDF Viewer component7.5—
    CriticalCVE-2026-84137Spoofing issue in the DOM: Core & HTML component9.8—
    CriticalCVE-2026-84136Other issue in the DOM: Navigation component9.8—
    CriticalCVE-2026-84135Other issue in Firefox Focus for Android9.8—
    CriticalCVE-2026-84134Other issue in the Profile Backup component9.8—
    CriticalCVE-2026-84133Site isolation issue in the DOM: Push Subscriptions component9.8—
    HighCVE-2026-84132Information disclosure in the Networking: HTTP component7.5—
    HighCVE-2026-84131Privilege escalation due to invalid pointer in the Graphics component8.8—
    HighCVE-2026-84130Information disclosure in the Graphics: WebGPU component7.5—
    CriticalCVE-2026-84129Site isolation issue in the DOM: Navigation component9.8—
    HighCVE-2026-84128Privilege escalation in the WebDriver BiDi component8.8—
    MediumCVE-2026-84127Information disclosure in the WebExtensions component in Firefox for Android4.3—
    Show 14 more CVEs
    Linked CVEs
    MediumCVE-2026-84126Incorrect boundary conditions in the Layout: Grid component4.3—
    MediumCVE-2026-84125Use-after-free in the DOM: Core & HTML component5.4—
    MediumCVE-2026-84124Use-after-free in the DOM: Core & HTML component5.4—
    HighCVE-2026-84123Privilege escalation due to use-after-free in the Graphics: WebGPU component8.8—
    MediumCVE-2026-84122Use-after-free in the Audio/Video component5.4—
    CriticalCVE-2026-84121Sandbox escape due to use-after-free in the DOM: Security component9.6—
    MediumCVE-2026-84120Use-after-free in the Audio/Video component5.4—
    CriticalCVE-2026-84119Sandbox escape due to use-after-free in the DOM: Navigation component9.6—
    MediumCVE-2026-84118Use-after-free in the JavaScript: GC component5.4—
    HighCVE-2026-84117Privilege escalation in Firefox for Android8.8—
    CriticalCVE-2026-75874Sandbox escape in the Remote Settings Client component10.0—
    HighCVE-2026-74952Privilege escalation in the Application Update component8.8—
    HighCVE-2026-16371Privilege escalation in the DOM: Navigation component8.8—
    HighCVE-2026-16365Privilege escalation in the DOM: Workers component8.8—
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