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Vulnerabilities

Vulnerability · CVE-2026-4371

A malicious mail server could send malformed strings with negative lengths, causing the parser to read memory outside the buffer.

Published Mar 24, 2026 · catalog updated Jul 15, 2026 · A malicious mail server

Check if you run it

No confirmed exploitation is on record for CVE-2026-4371. EPSS is 0.64%. Act only if a listed version of A malicious mail server is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (A malicious mail server).
  • No version range is stored. A customer cannot check an installed version against this record.
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
Severity
High7.4
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.64%
Remediation
Advisory link
Affected products
cpe mozilla/thunderbird

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe mozilla/thunderbirdversion unspecified

How to fix CVE-2026-4371

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2026-4371. EPSS is 0.64%. Act only if a listed version of A malicious mail server is installed.

Which versions are affected?

Affected version ranges are not on this record. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-126; CWE-130. EPSS 0.64%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. Mar 24, 2026 · NIST NVD

    Detected

    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.

  2. Jul 15, 2026 · NIST NVD

    Updated

    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.

Sources

  • CVE.orgVulnerability recordCVE-2026-4371

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2026-4371

    30-day exploitation probability 0.6%

    first.org
  • NIST NVDVulnerability recordCVE-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

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 18, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.