CVE-2026-4371

CVE-2026-4371 is a high-severity vulnerability in Mozilla Thunderbird with a CVSS 3.x base score of 7.4. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-126.

Key facts

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.

Frequently asked questions

What is CVE-2026-4371?
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.
How severe is CVE-2026-4371?
CVE-2026-4371 has a CVSS 3.x base score of 7.4, rated high severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability high.
Is CVE-2026-4371 being actively exploited?
It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (28th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-4371?
CVE-2026-4371 primarily affects Mozilla Thunderbird. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2026-4371?
Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-4371 have an EU (EUVD) identifier?
Yes. CVE-2026-4371 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-15023.
When was CVE-2026-4371 published?
CVE-2026-4371 was published on 2026-03-24 and last updated on 2026-06-30.

References

Affected products (2)

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