CVE-2026-4371

HIGH

7.4

CVSS v3

Description

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.

CWE-126Buffer Over-read CWE-130

How to fix

Remediation Available

thunderbirdDebian

Fixed in:1:140.9.0esr-1~deb11u1CVE-2026-4371

Fixed in:1:140.9.0esr-1~deb12u1CVE-2026-4371

Fixed in:1:140.9.0esr-1~deb13u1CVE-2026-4371

Fixed in:1:140.9.0esr-1CVE-2026-4371

thunderbirdRocky

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el10_0RHSA-2026:8315

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el8_2RHSA-2026:8285

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_4RHSA-2026:8850

Fixed in:0:140.9.0-1.el8_8RHSA-2026:8288

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

thunderbirdRed Hat / RHEL

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el10_0RHSA-2026:8315

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_2RHSA-2026:8285

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_8RHSA-2026:8288

Fixed in:0:140.9.0-1.el8_4RHSA-2026:8850

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

thunderbird-debuginfoRocky

Fixed in:0:140.9.0-1.el10_0RHSA-2026:8315

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el10_0RHSA-2026:8315

Fixed in:0:140.9.0-1.el8_8RHSA-2026:8288

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_8RHSA-2026:8288

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_2RHSA-2026:8285

Fixed in:0:140.9.0-1.el8_4RHSA-2026:8850

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

thunderbird-debuginfoRed Hat / RHEL

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el10_0RHSA-2026:8315

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el10_0RHSA-2026:8315

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_8RHSA-2026:8288

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_2RHSA-2026:8285

Fixed in:0:140.9.0-1.el8_8RHSA-2026:8288

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_4RHSA-2026:8850

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

thunderbird-debugsourceRocky

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el10_0RHSA-2026:8315

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_2RHSA-2026:8285

Fixed in:0:140.9.0-1.el8_8RHSA-2026:8288

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_4RHSA-2026:8850

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

thunderbird-debugsourceRed Hat / RHEL

Fixed in:0:140.9.0-1.el10_0RHSA-2026:8315

Fixed in:0:140.9.0-1.el10_1RHSA-2026:6342

Fixed in:0:140.9.0-1.el8_2RHSA-2026:8285

Fixed in:0:140.9.0-1.el8_10RHSA-2026:6917

Fixed in:0:140.9.0-1.el8_8RHSA-2026:8288

Fixed in:0:140.9.0-1.el8_6RHSA-2026:8289

Fixed in:0:140.9.0-1.el8_4RHSA-2026:8850

Fixed in:0:140.9.0-1.el9_7RHSA-2026:6188

Fixed in:0:140.9.0-1.el9_2RHSA-2026:8287

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_4RHSA-2026:8290

Fixed in:0:140.9.0-1.el9_6RHSA-2026:8284

Fixed in:0:140.9.0-1.el9_0RHSA-2026:8286

Mozilla Thunderbird (en-US)Windows application

Affected:149.0Fixed in:149.0Mozilla

Affected:140.9.0Fixed in:140.9.0Mozilla

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityHigh

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityHigh

IntegrityNone

AvailabilityHigh

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

Exploit Intelligence

0.36%probability of exploitation in 30 days

29thpercentile

Low risk: more likely to be exploited than 29% of all known CVEs.

References

Vendor Advisory2

Other references16

Related Vulnerabilities

Other CWE-126 (Buffer Over-read) vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2023-49285	High	8.6	89%	-	Fix
CVE-2017-7668	High	7.5	57%	-	Fix
CVE-2017-7679	Critical	9.8	39%	-	Fix
CVE-2025-21277	High	7.5	38%	-	Fix
CVE-2024-38071	High	7.5	36%	-	Fix
CVE-2009-2495	Medium	6.5	34%	-	-

Common questions

How do I fix CVE-2026-4371?

Published advisories record a fix for 8 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.

Is CVE-2026-4371 being actively exploited?

Not that we know of. CVE-2026-4371 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 0.36% is the estimated probability that it will be exploited in the next 30 days. That is higher than 29% of all scored CVEs.

How severe is CVE-2026-4371?

CVE-2026-4371 has a CVSS v3 base score of 7.4, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2026-4371 affect?

Published advisories record a fix for thunderbird (Debian), thunderbird (Rocky), thunderbird (Red Hat / RHEL), thunderbird-debuginfo (Rocky), and 4 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-07-15.