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ThunderbirdApplication · Mozilla

CVE-2026-4371

HIGH · 7.4 CVSS v3.1 Published 2026-03-24
Fix available
A fix is available. Upgrade to 140.9.0 / 149.0 or later.
See remediation →
83/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A buffer over-read vulnerability in Thunderbird's mail parser allows a malicious mail server to send malformed strings with negative lengths, causing the parser to read memory outside the buffer bounds. This can lead to application crashes or sensitive data leakage.

MitigationUpgrade Thunderbird to version 149 or 140.9 or later to obtain the patched version with proper bounds checking for string length validation in the mail parser.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
ThunderbirdApplication
Affected:< 140.9.0< 149.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
High

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check if Thunderbird is installed
    Locate Thunderbird executable: On Windows check C:\Program Files\Mozilla Thunderbird\thunderbird.exe or C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe. On macOS check /Applications/Thunderbird.app. On Linux check /usr/bin/thunderbird or /opt/thunderbird/thunderbird.
    Affected if Thunderbird is not found on the system, the user is not affected by this vulnerability in Thunderbird.
  2. Determine installed Thunderbird version
    Open Thunderbird and go to Help > About Thunderbird, or run thunderbird --version from command line. The version number displayed (e.g., 140.8.0, 149.0.1) is the installed version.
    Affected if Unable to determine version means detection cannot be completed.
  3. Compare version against affected ranges
    Compare the installed version number to the affected ranges: versions below 140.9.0 OR versions below 149.0 (but above 140.9.0) are vulnerable. For example, 140.8.0 is affected; 140.9.0 and 149.0 are patched.
    Affected if Installed version is below 140.9.0 (such as 140.8.x, 128.x, 115.x) OR installed version is 140.9.x through 148.x, meaning the version is below 149.0 but at or above 140.9.0.
  4. Identify mail account configuration (optional context)
    Check Thunderbird Account Settings to see configured mail accounts. This vulnerability is triggered when the mail parser processes malformed strings from a malicious mail server.
    Affected if This step provides context: users with configured mail accounts that receive mail from external servers are at potential risk if the version is vulnerable.

User is affected if Thunderbird is installed and the installed version is below 140.9.0 or below 149.0 (versions 140.9.0 through 148.x are also vulnerable).

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 140.9.0 / 149.0 or later
Fixed in 140.9.0149.0
Interim mitigation

Upgrade Thunderbird to version 149 or 140.9 or later to obtain the patched version with proper bounds checking for string length validation in the mail parser.

Recommended fix High confidence

Thunderbird 140.9 or Thunderbird 149

  1. 1. Check current Thunderbird version by clicking Help > About Thunderbird
  2. 2. Download Thunderbird 140.9 (for continued use of the 140.x ESR branch) or Thunderbird 149 (latest release) from the official Mozilla Thunderbird website
  3. 3. Close Thunderbird completely before upgrading
  4. 4. Run the installer for the downloaded version and follow the prompts to upgrade
Caveat Standard Thunderbird upgrade may require re-enabling some extensions after the update; ensure backups of mail profiles exist before upgrading

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Thunderbird Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
13.0 hours of engineering $2,280
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,648.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-4371 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data