Thunderbird: Buffer Over-read
CVE-2026-4371 is a high-severity buffer over-read vulnerability in Thunderbird from Mozilla, with a CVSS 3.1 base score of 8.2 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. There is no confirmed exploitation so far. EPSS estimates a 0.38% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Check which version of Thunderbird you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on mozilla.org for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- LowSome restricted data can be read.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H. Scored by CNA.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 7.4 | High | CISA (ADP) |
| CVSS 3.1 | 8.2 | High | CNA |
Official 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.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Buffer Over-read
The product reads from a buffer using buffer access mechanisms such as indexes or pointers that reference memory locations after the targeted buffer.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Other references
- bugzilla.mozilla.org/show_bug.cgi?id=2023493Permissions Required
- access.redhat.com/errata/RHSA-2026:6188
- access.redhat.com/errata/RHSA-2026:6342
- access.redhat.com/errata/RHSA-2026:6917
- access.redhat.com/errata/RHSA-2026:8284
- access.redhat.com/errata/RHSA-2026:8285
- access.redhat.com/errata/RHSA-2026:8286
- access.redhat.com/errata/RHSA-2026:8287
- access.redhat.com/errata/RHSA-2026:8288
- access.redhat.com/errata/RHSA-2026:8289
- access.redhat.com/errata/RHSA-2026:8290
- access.redhat.com/errata/RHSA-2026:8315
- access.redhat.com/errata/RHSA-2026:8850
- access.redhat.com/security/cve/CVE-2026-4371
- bugzilla.redhat.com/show_bug.cgi?id=2451001
Other vulnerabilities in Thunderbird
- 8.8 High severity
- 8.8 High severity
- 8.8 High severity
- 8.8 High severity
- 8.8 High severity
- 8.8 High severity
Recent Buffer Over-read vulnerabilities
- 7.8 High severity
- 7.5 High severity
- 7.5 High severity
- 9.1 Critical severity
- 7.3 High severity
- 7.5 High severity
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