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.
Published: 2026-03-24
Score: 7.4 High
EPSS: < 1% Very Low
KEV: No
Impact: Memory corruption that can leak data or crash the client
Action: Apply Patch
AI Analysis

Impact

Thunderbird’s IMAP parser fails to verify the length of incoming strings. A malicious mail server can send a string with a negative length, causing the parser to read beyond the allocated buffer. This out‑of‑bounds read may expose sensitive memory contents or trigger a crash, resulting in loss of data confidentiality or availability.

Affected Systems

Mozilla Thunderbird clients older than version 149, including the extended‑support release builds prior to 140.9, are affected. Both the standard Thunderbird and the ESR releases are impacted.

Risk and Exploitability

The vulnerability carries a CVSS score of 7.4, indicating high severity, but the EPSS probability is below 1 %, and it is not listed in CISA’s KEV catalog, suggesting a low likelihood of active exploitation. Attackers must control a mail server or compromise the network connection to a target client, then send specially crafted IMAP messages containing negative length fields. Once processed, the out‑of‑bounds read could disclose internal memory or crash the client.

Generated by OpenCVE AI on March 27, 2026 at 19:39 UTC.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

  • Update Thunderbird to version 149 or later.
  • Update Thunderbird ESR to version 140.9 or later.
  • Verify that your system is running the latest patched release from Mozilla.

Generated by OpenCVE AI on March 27, 2026 at 19:39 UTC.

Tracking

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Advisories
Source ID Title
Debian DLA Debian DLA DLA-4511-1 thunderbird security update
Debian DSA Debian DSA DSA-6179-1 thunderbird security update
History

Mon, 13 Apr 2026 14:30:00 +0000

Type Values Removed Values Added
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 affects Thunderbird < 149 and Thunderbird < 140.9. 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.

Fri, 27 Mar 2026 18:15:00 +0000

Type Values Removed Values Added
CPEs cpe:2.3:a:mozilla:thunderbird:*:*:*:*:-:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:esr:*:*:*

Thu, 26 Mar 2026 00:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-130
References
Metrics threat_severity

None

 threat_severity

Important

Wed, 25 Mar 2026 17:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-126
Metrics cvssV3_1

{'score': 7.4, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H'}

ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Wed, 25 Mar 2026 12:00:00 +0000

Type Values Removed Values Added
First Time appeared Mozilla
Mozilla thunderbird
Vendors & Products Mozilla
Mozilla thunderbird

Tue, 24 Mar 2026 20:30:00 +0000

Type Values Removed Values Added
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 affects Thunderbird < 149 and Thunderbird < 140.9.
Title Out of bounds read in IMAP parsing
References

Subscriptions

Mozilla Thunderbird
cve-icon MITRE

Status: PUBLISHED

Assigner: mozilla

Published:

Updated: 2026-04-13T13:51:25.535Z

Reserved: 2026-03-18T10:03:43.909Z

Link: CVE-2026-4371

cve-icon Vulnrichment

Updated: 2026-03-25T16:24:43.854Z

cve-icon NVD

Status : Modified

Published: 2026-03-24T21:16:29.583

Modified: 2026-04-13T15:17:36.130

Link: CVE-2026-4371

cve-icon Redhat

Severity : Important

Publid Date: 2026-03-24T20:27:15Z

Links: CVE-2026-4371 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-03-27T20:26:36Z

Weaknesses