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CVE Record

CVE-2026-4371: Out of bounds read in IMAP parsing

A malicious mail server could send malformed strings with negative lengths, causing the parser to read memory outside the buffer. If a mail server or connection to a mail server were compromised, an attacker could cause the parser to malfunction, potentially crashing Thunderbird or leaking sensitive data. This vulnerability was fixed in Thunderbird 149 and Thunderbird 140.9.

HighCVSS 8.2Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2026-4371 is a high-severity Thunderbird IMAP parsing flaw. A malicious or compromised mail server could send malformed data that makes Thunderbird read outside an expected memory buffer, potentially crashing the client or exposing sensitive memory.

Executive priority

Treat this as a prompt desktop-client patching item, not an emergency internet-edge incident. The main business risk is email client disruption and possible data exposure if users connect to malicious or compromised IMAP infrastructure.

Technical view

Mozilla describes an out-of-bounds read in Thunderbird IMAP parsing caused by malformed strings with negative lengths. The CVSS 3.1 score is 8.2, with network attack vector, no privileges, no user interaction, low confidentiality impact, and high availability impact. Mozilla states it was fixed in Thunderbird 149 and 140.9.

Likely exposure

Exposure is most likely where users run Thunderbird for IMAP mail, especially against untrusted, compromised, or intercepted mail server connections. The bundle does not identify other affected products beyond Mozilla Thunderbird.

Exploitation context

The provided record is not marked KEV, and the bundle does not cite active exploitation or public weaponization. Exploitation requires a malicious mail server or compromised connection path to deliver malformed IMAP parser input.

Researcher notes

The source bundle supports CWE-126 and CWE-130 classification. Evidence points to parser length handling around IMAP strings, but Bugzilla details may be restricted. Do not infer RCE, broader Mozilla product impact, or confirmed exploitation from the provided sources.

Mitigation direction

  • Upgrade Thunderbird to fixed versions 149, 140.9, or later vendor-supported builds.
  • Apply Mozilla or operating-system vendor security updates for Thunderbird packages.
  • Prioritize endpoints using Thunderbird with IMAP accounts.
  • Verify mail connections use trusted servers and protected transport.
  • Monitor vendor advisories for corrected package availability and backports.

Validation and detection

  • Inventory Thunderbird installations and versions across managed endpoints.
  • Confirm installed builds include Mozilla’s CVE-2026-4371 fix.
  • Identify Thunderbird profiles configured for IMAP accounts.
  • Review endpoint crash telemetry for Thunderbird failures during IMAP activity.
  • Check Red Hat or OS package advisory status where Thunderbird is distro-managed.
Prepared
Confidence
high
Sources
8

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-126: 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-130: 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
cve · low confidence lookup

CVE-2026-4371 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
8.2 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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
19Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical 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
8.2CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H3.94.2redhat-SADP
7.4CVSS 3.1HighCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H2.25.2CISA-ADP

Vulnerability scoring details

Base CVSS 3.1 score

8.2High
CVSS 3.1 vector shape for CVE-2026-4371Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/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-SADPthunderbird: Out of bounds read in IMAP parsing
other:Red Hat severity ratingcvssV3_1
  • 2026-03-24T21:01:59.309Z: Reported to Red Hat.
  • 2026-03-24T20:27:15.198Z: Made public.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
MozillaThunderbird140.9, 149Listed
Weakness

CWE details

CWE-126CWE-130

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

CWE-126 · source CWE mapping

Buffer Over-read

Buffer Over-read represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs
CWE-130 · source CWE mapping

Improper Handling of Length Parameter Inconsistency

Improper Handling of Length Parameter Inconsistency represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs