Raw vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:HCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
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 Junk Data (T1001.001); ranked at the 50th 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
- 🇪🇺 ENISA EUVD: EUVD-2026-15023
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)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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V3.5.5V4.2.1V4.2.2
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation (static analysis, fuzzing, bounds checking tests) directly finds buffer over-read flaws.
Input validation enforces correct length-to-data consistency checks on parsed messages before processing.
Engineering principles such as memory-safe design and bounds-checked abstractions structurally stop introduction of out-of-bounds reads.
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.
Vulnerability identification processes can discover buffer over-read flaws via scanning or review.
Patching or replacing vulnerable software removes known instances of buffer over-read bugs.
Secure development practices directly prevent introduction of buffer over-read weaknesses.
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.