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CVE Record

CVE-2026-84129: Site isolation issue in the DOM: Navigation component

Site isolation issue in the DOM: Navigation component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.

CriticalCVSS 9.8Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

A flaw in Mozilla’s site-isolation controls could weaken the boundary intended to keep content from different sites separated. The supplied CVSS assessment indicates potentially severe confidentiality, integrity, and availability impact without authentication or user interaction. Mozilla reports fixes in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.

Executive priority

Prioritize rapid enterprise-wide updating because the supplied rating is critical and the assessed impact is broad. Focus first on privileged users and endpoints accessing sensitive systems. Absence from KEV and lack of reported exploitation reduce evidence of immediate attack activity, but do not justify delaying the named security updates.

Technical view

CVE-2026-84129 is a DOM Navigation component site-isolation issue classified as CWE-346. It has CVSS 3.1 score 9.8 with network access, low complexity, no privileges, and no user interaction. The bundle does not explain the precise failure mechanism, prerequisites, affected version ranges, or whether each theoretical impact was demonstrated.

Likely exposure

Potential exposure includes managed or unmanaged Firefox, Firefox ESR, and Thunderbird installations not updated to their applicable fixed release. The supplied sources do not specify complete affected version ranges, platforms, configurations, or whether Thunderbird requires particular content-handling behavior. Inventory evidence is therefore necessary before determining organisational exposure.

Exploitation context

The supplied bundle provides no evidence of public exploitation, proof-of-concept availability, or attacks in the wild, and the CVE is not identified as KEV. The CVSS vector describes potentially remote, low-complexity exploitation without privileges or user interaction, but it does not establish that practical exploitation has occurred.

Researcher notes

Available detail is limited to a site-isolation failure in the DOM Navigation component and CWE-346 classification. The bundle does not disclose root cause, reachable states, process-boundary consequences, exploit reliability, or platform differences. Treat the CVSS properties as assessment claims rather than proof of a working exploit, and consult the linked Mozilla records for later technical disclosure.

Mitigation direction

  • Update Firefox to version 155 or a later vendor-supported release.
  • Update Firefox ESR to version 153.2 or a later applicable ESR release.
  • Update Thunderbird to version 155 or 153.2, according to the deployed branch.
  • Check Mozilla’s advisories for platform-specific deployment guidance and any subsequent revisions.
  • Accelerate updates on endpoints handling sensitive browsing, email, credentials, or administrative access.

Validation and detection

  • Inventory Firefox, Firefox ESR, and Thunderbird versions across all managed endpoints.
  • Confirm each installation meets or exceeds its applicable Mozilla fixed release.
  • Review deployment reports for failed, deferred, or user-blocked updates.
  • Check unmanaged-device and bring-your-own-device exposure where corporate data is accessible.
  • Recheck Mozilla advisories for clarified affected ranges or updated remediation guidance.
Prepared
Confidence
medium
Sources
7

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-346: 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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cve · low confidence lookup

CVE-2026-84129 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
Critical
CVSS
9.8 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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.

1CVSS vectors
3Timeline events
1ADP providers
6Source links

SSVC decision data

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

CVSS vector scores

1 official score

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
9.8CVSS 3.1CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H3.95.9CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

9.8Critical
CVSS 3.1 vector shape for CVE-2026-84129Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

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

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
cvssV3_1other:ssvc

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
MozillaFirefox153.2, 155Listed
MozillaThunderbird153.2, 155Listed
Weakness

CWE details

CWE-346

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

CWE-346 · source CWE mapping

Origin Validation Error

Origin Validation Error represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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