Cyber Resilience

CVE-2026-4371

Memory Safety in Mozilla Thunderbird ≤ 140.9.0

Memory Safety
Published
24 March 2026
Modified
15 July 2026
Patch / advisory
View advisory
CVSS Score v3.1 7.4
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H
EPSS Score 0.0036 29th percentile
Risk Priority 55 floored blend · peak EPSS

Summary

CVE-2026-4371 is a high-severity Buffer Over-read (CWE-126) vulnerability in Mozilla Thunderbird. Its CVSS base score is 7.4 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique OS Credential Dumping (T1003); ranked at the 29th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.

Deeper analysis AI-assisted summary

Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.

CVE-2026-4371 is a buffer over-read vulnerability (CWE-126) in Thunderbird's mail parser, triggered by malformed strings with negative lengths sent from a malicious mail server. This causes the parser to read memory outside the allocated buffer. The issue affects Thunderbird versions prior to 149 and 140.9, with a CVSS v3.1 base score of 7.4 (AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H). It was published on 2026-03-24.

An attacker controlling a mail server or a compromised connection to a mail server that Thunderbird connects to can exploit this remotely with high attack complexity and no privileges required. Successful exploitation could cause the parser to malfunction, potentially leading to Thunderbird crashes (availability impact) or leakage of sensitive data from memory (confidentiality impact), though no integrity impact is possible.

Mozilla's security advisories (MFSA 2026-23 and MFSA 2026-24) and the associated Bugzilla entry (bug 2023493) confirm the fix in Thunderbird 149 and Thunderbird 140.9, recommending users update to these patched versions for mitigation. No workarounds are specified in the provided details.

EU & UK References

Vulnerability Data

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,…

more

potentially crashing Thunderbird or leaking sensitive data. This vulnerability was fixed in Thunderbird 149 and Thunderbird 140.9.

CWE(s)
CWE-126CWE-130

Related Threats

MITRE ATT&CK Enterprise Techniques

T1003 OS Credential Dumping Credential Access
Adversaries may attempt to dump credentials to obtain account login and credential material, normally in the form of a hash or a clear text password.
attack.mitre.org →
T1005 Data from Local System Collection
Adversaries may search local system sources, such as file systems, configuration files, local databases, virtual machine files, or process memory, to find files of interest and sensitive data prior to Exfiltration.
attack.mitre.org →
T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-14899Same product: Mozilla Thunderbird
CVE-2024-8394Same product: Mozilla Thunderbird
CVE-2025-5262Same product: Mozilla Thunderbird
CVE-2024-11159Same product: Mozilla Thunderbird
CVE-2025-2830Same product: Mozilla Thunderbird
CVE-2025-3932Same product: Mozilla Thunderbird
CVE-2026-57963Same product: Mozilla Thunderbird
CVE-2025-1015Same product: Mozilla Thunderbird
CVE-2026-0818Same product: Mozilla Thunderbird
CVE-2023-0616Same product: Mozilla Thunderbird

Affected Assets

mozilla
thunderbird
≤ 140.9.0 · ≤ 149.0

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V3.5.5
  • V4.2.1
  • V4.2.2

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation (static analysis, fuzzing, bounds checking tests) directly finds buffer over-read flaws.

SI-10 Information Input Validation good match

Input validation enforces correct length-to-data consistency checks on parsed messages before processing.

SA-8 Security and Privacy Engineering Principles partial match

Engineering principles such as memory-safe design and bounds-checked abstractions structurally stop introduction of out-of-bounds reads.

SC-39 Process Isolation partial match

Process isolation limits the blast radius of an over-read to the compromised domain.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure development practices directly prevent introduction of buffer over-read weaknesses.

ID.RA-01 partial match
prevents

Vulnerability identification processes can discover buffer over-read flaws via scanning or review.

PR.PS-02 partial match
prevents

Patching or replacing vulnerable software removes known instances of buffer over-read bugs.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing in development can detect buffer over-reads before release.

A.8.25 Secure development life cycle partial match
prevents

Secure SDLC mandates input validation and bounds checking that can prevent buffer over-reads.

A.8.26 Application security requirements partial match
prevents

Application security requirements can specify buffer-size and bounds-checking rules.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles include memory-safety and bounds-checking design choices.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly require bounds-checked buffer access, mitigating over-reads.

References