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CVE Record

CVE-2026-8391: Other issue in the JavaScript Engine component

Other issue in the JavaScript Engine component. This vulnerability was fixed in Firefox 150.0.3, Firefox ESR 115.36, Firefox ESR 140.11, and Thunderbird 140.11.

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2026-8391 is a high-severity Mozilla JavaScript engine issue fixed in Firefox 150.0.3, Firefox ESR 115.36/140.11, and Thunderbird 140.11. A user would likely need to interact with attacker-controlled content. Successful exploitation could affect confidentiality, integrity, and availability, but the public bundle gives limited technical detail.

Executive priority

Treat as a prompt patching item for browser and email fleets. User interaction is required and active exploitation is not evidenced here, but JavaScript engine impact and high C/I/A scoring justify timely deployment through standard endpoint update channels.

Technical view

The CVSS 3.1 vector is 7.5: network-reachable, no privileges, user interaction required, high attack complexity, unchanged scope, and high C/I/A impact. Mozilla describes it only as an “Other issue” in the JavaScript Engine component. The bundle maps multiple CWEs, including memory and input-handling classes, but does not provide root-cause detail.

Likely exposure

Exposure is most likely on endpoints or servers using Mozilla Firefox, Firefox ESR, or Thunderbird versions older than the fixed releases. Red Hat advisories indicate downstream packaged products also require attention. The bundle does not enumerate every vulnerable earlier version.

Exploitation context

The provided bundle does not show CISA KEV listing or confirmed active exploitation. The vector suggests exploitation would require a user to open or render malicious web or mail content, with high complexity limiting broad automated exploitation confidence.

Researcher notes

Public detail is sparse. Avoid assuming exploitability mechanics beyond the CVSS vector and Mozilla component statement. The CWE set is broad and may reflect classification uncertainty. Focus validation on version state, vendor errata applicability, and whether Bugzilla or vendor advisories later disclose more detail.

Mitigation direction

  • Update Firefox to 150.0.3 or the applicable fixed ESR release.
  • Update Thunderbird to 140.11 where deployed.
  • Apply relevant Red Hat security advisories for packaged Mozilla components.
  • Prioritize managed browser and email-client fleets.
  • Check Mozilla and OS vendor guidance for any additional mitigations.

Validation and detection

  • Inventory Firefox, Firefox ESR, and Thunderbird versions across endpoints.
  • Confirm installed versions meet or exceed the fixed releases.
  • Check Red Hat systems for the listed RHSA errata status.
  • Review vulnerability scanner findings against vendor advisory versions.
  • Monitor Mozilla, Red Hat, and KEV sources for status changes.
Prepared
Confidence
medium
Sources
11

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-119: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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cwe · low confidence lookup

CWE-20: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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cwe · low confidence lookup

CWE-475: Exact CWE lookup

Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cwe · medium confidence lookup

CWE-79: User-session and phishing behavior lookup

Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

Open ATT&CK lookup
cve · low confidence lookup

CVE-2026-8391 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.5 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

2CVSS vectors
5Timeline events
2ADP providers
30Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: partial

CVSS vector scores

2 official scores

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H1.65.9redhat-SADP
5.3CVSS 3.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N3.91.4CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

7.5High
CVSS 3.1 vector shape for CVE-2026-8391Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. ADP timelineredhat-SADP

    Made public.

  3. CVE publishedCVE Program

    The CVE record was published.

  4. ADP timelineredhat-SADP

    Reported to Red Hat.

  5. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
cvssV3_1other:ssvc
redhat-SADPfirefox: thunderbird: Other issue in the JavaScript Engine component
other:Red Hat severity ratingcvssV3_1
  • 2026-05-12T14:01:57.656Z: Reported to Red Hat.
  • 2026-05-12T12:36:15.548Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
MozillaFirefox115.36, 140.11, 150.0.3Listed
MozillaThunderbird140.11Listed
Weakness

CWE details

CWE-119CWE-20CWE-475CWE-79

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.