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CVE Record

CVE-2026-84143: Internally found bugs fixed in Thunderbird 155, Thunderbird ESR 153.2 and Thunderbird ESR 140.15

Internally found bugs present in Thunderbird 154, Thunderbird ESR 153.1 and Thunderbird ESR 140.14. Some of these bugs showed evidence of memory corruption or another security-relevant defect and we presume that with enough effort some of these could have been exploited. This vulnerability was fixed in Firefox 155, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, 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

Mozilla fixed a group of internally discovered Thunderbird defects, including memory corruption and other security-relevant issues that might permit compromise with sufficient attacker effort. The bundle also reports corresponding Firefox fixes. Although the supplied 9.8 score is critical, the public evidence does not establish exploitation in the wild.

Executive priority

Treat this as an urgent patching item because the supplied severity is critical and Mozilla observed memory-corruption evidence. Accelerate Thunderbird updates, verify Firefox patch levels, and document exceptions. The absence of known active exploitation lowers immediate incident certainty, but it does not justify delaying remediation on high-value systems.

Technical view

The CVE aggregates multiple internally found defects rather than documenting one specific attack path. Mozilla reports evidence of memory corruption or other security-relevant behavior and presumes some defects could be exploitable. Fixes are identified in Thunderbird 155, ESR 153.2, ESR 140.15 and corresponding Firefox releases. Public details are insufficient to confirm exact impact per underlying bug.

Likely exposure

Thunderbird 154, ESR 153.1, and ESR 140.14 are explicitly described as containing the bugs. Organizations using those releases should treat endpoints as exposed. Firefox also received fixes, but the bundle does not clearly identify its corresponding vulnerable versions. Exposure depends on deployed versions and whether affected application functionality processes attacker-controlled content.

Exploitation context

No active exploitation is established: the bundle marks KEV false and provides no cited evidence of attacks in the wild. Mozilla describes potential exploitability based on memory corruption and related defects, not a confirmed exploit. The supplied CVSS vector indicates remote, low-complexity, unauthenticated exploitation without user interaction, but underlying bug-specific prerequisites remain publicly unclear.

Researcher notes

This is an umbrella CVE covering bugs of differing Mozilla severities, so the aggregate 9.8 score may not describe every underlying defect. Public material does not establish a single root cause, reachable component, proof of concept, or confirmed impact chain. CWE-200 is supplied, but Mozilla’s description emphasizes memory corruption and other security-relevant defects; further bug-level evidence is needed.

Mitigation direction

  • Update Thunderbird to 155, ESR 153.2, ESR 140.15, or a later supported release.
  • Update Firefox to 155, ESR 153.2, ESR 140.15, or a later supported release.
  • Prioritize externally exposed and high-value endpoints running the explicitly vulnerable Thunderbird versions.
  • Consult Mozilla advisories for platform-specific deployment guidance and any subsequently published mitigations.

Validation and detection

  • Inventory installed Thunderbird and Firefox versions across managed endpoints.
  • Confirm Thunderbird 154, ESR 153.1, and ESR 140.14 are absent after remediation.
  • Verify installations report the applicable fixed or later supported release.
  • Review endpoint telemetry for unusual crashes or process behavior, without treating crashes alone as proof of exploitation.
  • Recheck Mozilla advisories for clarified affected versions, prerequisites, or exploitation evidence.
Prepared
Confidence
high
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 · medium confidence lookup

CWE-200: Information exposure and cloud metadata lookup

Information exposure and SSRF weaknesses can make discovery, cloud metadata, and credential material review relevant. 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-2026-84143 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
10Source 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-2026-84143Attack 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
MozillaFirefox140.15, 153.2, 155Listed
MozillaThunderbird140.15, 153.2, 155Listed
Weakness

CWE details

CWE-200

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

CWE-200 · source CWE mapping

Exposure of Sensitive Information to an Unauthorized Actor

Exposure of Sensitive Information to an Unauthorized Actor represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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