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CVE Record

CVE-2026-84121: Sandbox escape due to use-after-free in the DOM: Security component

Sandbox escape due to use-after-free in the DOM: Security component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

CriticalCVSS 9.6Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

A memory-safety flaw in Mozilla’s DOM security code could let a malicious webpage or message escape the application sandbox after user interaction. Successful exploitation could compromise data, alter systems, or disrupt availability. Mozilla has released fixed Firefox and Thunderbird versions.

Executive priority

Treat as an urgent endpoint patching priority because sandbox escape can turn malicious content into broader system compromise. Accelerate updates for internet-facing users and high-value workstations, while verifying completion through endpoint inventory rather than relying solely on update policy.

Technical view

CVE-2026-84121 is a CWE-416 use-after-free in the DOM Security component that enables sandbox escape. CVSS 3.1 is 9.6: network-reachable, low complexity, no privileges, user interaction required, changed scope, and potentially high confidentiality, integrity, and availability impact.

Likely exposure

Exposure exists where Firefox or Thunderbird remains below the applicable fixed release. The bundle identifies fixes in Firefox 155 and ESR 115.40, 140.15, and 153.2; Thunderbird 155, 140.15, and 153.2. Exact vulnerable starting versions are not provided.

Exploitation context

The CVSS vector indicates remote delivery without authentication but requires user interaction. The supplied record is not in KEV and provides no evidence of active exploitation or a public exploit. Absence from KEV does not establish that exploitation is impossible.

Researcher notes

The source bundle labels the listed releases as affected versions, while its description explicitly says they contain fixes. They should therefore be treated as remediation thresholds, not vulnerable releases. The exact use-after-free trigger, vulnerable version floor, platform limitations, and exploit maturity are not established by the supplied evidence.

Mitigation direction

  • Update Firefox to 155 or the applicable fixed ESR release, or later.
  • Update Thunderbird to 155 or the applicable fixed ESR release, or later.
  • Prioritize systems handling untrusted webpages, HTML email, or externally supplied content.
  • Check Mozilla advisories for platform-specific guidance and any subsequently revised affected-version details.

Validation and detection

  • Inventory installed Firefox and Thunderbird versions across managed endpoints.
  • Confirm each installation meets or exceeds its applicable fixed release.
  • Verify update policies cover Firefox and Thunderbird ESR channels in use.
  • Review deployment telemetry for failed, deferred, or unmanaged browser and mail-client updates.
Prepared
Confidence
high
Sources
10

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-416: 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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description · low confidence lookup

Privilege behavior lookup

The CVE wording references privilege impact, so privilege escalation and authorization behavior review may help. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

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cve · low confidence lookup

CVE-2026-84121 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
9.6 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/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
9Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical 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
9.6CVSS 3.1CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H2.86CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

9.6Critical
CVSS 3.1 vector shape for CVE-2026-84121Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/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
MozillaFirefox115.40, 140.15, 153.2, 155Listed
MozillaThunderbird140.15, 153.2, 155Listed
Weakness

CWE details

CWE-416

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

CWE-416 · source CWE mapping

Use After Free

Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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