Cyber Resilience

CVE-2026-3889

Mozilla Thunderbird ≤ 140.9.0

Published
24 March 2026
Modified
13 April 2026
Patch / advisory
View advisory
CVSS Score v3.1 6.5
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
EPSS Score 0.0020 10th percentile
Risk Priority 48 floored blend · peak EPSS

Summary

CVE-2026-3889 is a medium-severity User Interface (UI) Misrepresentation of Critical Information (CWE-451) vulnerability in Mozilla Thunderbird. Its CVSS base score is 6.5 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Phishing (T1566); ranked at the 10th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

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-2026-3889 is a spoofing vulnerability (CWE-451) affecting Mozilla Thunderbird email client. It enables user interface misrepresentation, allowing attackers to spoof critical information displayed to users. The issue was addressed in Thunderbird version 149 and Thunderbird Extended Support Release (ESR) 140.9, as detailed in Mozilla's security advisories.

Attackers can exploit this vulnerability remotely over the network with low complexity and no required privileges, but it necessitates user interaction, such as clicking a malicious link or opening a crafted email. Successful exploitation results in high integrity impact (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N, score 6.5), potentially tricking users into performing unintended actions, such as divulging sensitive information or executing malicious content, without compromising confidentiality or availability.

Mozilla's security advisories (MFSA 2026-23 and MFSA 2026-24) and the associated Bugzilla entry (bug 2020723) confirm the fix in the specified Thunderbird versions, recommending users update immediately to mitigate the risk. No workarounds are mentioned beyond applying the patches.

OWASP Top 10 for Web (2025)

A06:2025 Insecure Design

EU & UK References

Vulnerability Data

Spoofing issue in Thunderbird. This vulnerability was fixed in Thunderbird 149 and Thunderbird 140.9.

CWE(s)
CWE-451

Related Threats

MITRE ATT&CK Enterprise Techniques

T1566 Phishing Initial Access
Adversaries may send phishing messages to gain access to victim systems.
attack.mitre.org →
T1566.002 Spearphishing Link Initial Access
Adversaries may send spearphishing emails with a malicious link in an attempt to gain access to victim systems.
attack.mitre.org →
T1566.003 Spearphishing via Service Initial Access
Adversaries may send spearphishing messages via third-party services in an attempt to gain access to victim systems.
attack.mitre.org →
T1598 Phishing for Information Reconnaissance
Adversaries may send phishing messages to elicit sensitive information that can be used during targeting.
attack.mitre.org →
T1598.003 Spearphishing Link Reconnaissance
Adversaries may send spearphishing messages with a malicious link to elicit sensitive information that can be used during targeting.
attack.mitre.org →
T1534 Internal Spearphishing Lateral Movement
After they already have access to accounts or systems within the environment, adversaries may use internal spearphishing to gain access to additional information or compromise other users within the same organization.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2025-5986Same product: Mozilla Thunderbird
CVE-2025-3523Same product: Mozilla Thunderbird
CVE-2025-8043Same product: Mozilla Thunderbird
CVE-2026-8964Same product: Mozilla Thunderbird
CVE-2025-4086Same product: Mozilla Thunderbird
CVE-2026-16403Same product: Mozilla Thunderbird
CVE-2024-6610Same product: Mozilla Thunderbird
CVE-2024-7529Same product: Mozilla Thunderbird
CVE-2025-3522Same product: Mozilla Thunderbird
CVE-2026-57963Same product: Mozilla Thunderbird

Affected Assets

mozilla
thunderbird
≤ 140.9.0 · ≤ 149.0

Mitigating Controls

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 SDLC practices directly address design and implementation of accurate, non-spoofable UI elements.

PR.AT-01 partial match
prevents

User awareness training helps people recognize and avoid harm from UI misrepresentation such as phishing, but does not prevent the flaw itself.

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 can detect UI misrepresentation vulnerabilities before deployment.

A.6.3 Information security awareness, education and training partial match
prevents

Security awareness training can teach users to recognize UI misrepresentation and phishing attempts.

A.8.23 Web filtering partial match
mitigates

Web filtering can block known phishing sites that exploit UI misrepresentation.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle includes UI/UX security requirements that can prevent misrepresentation of critical information.

A.8.26 Application security requirements partial match
prevents

Application security requirements can mandate proper display and validation of critical information in the UI.

A.8.28 Secure coding partial match
prevents

Secure coding practices can prevent UI flaws that obscure or spoof critical information.

References