CVE-2026-4371: A malicious mail server could send malformed strings with negative lengths, causing the parser to read memory

Q: How do I fix CVE-2026-4371?

thunderbird: upgrade to 140.9.0, 149.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-4371 being exploited?

CVE-2026-4371 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-4371 affect?

CVE-2026-4371 affects thunderbird. Severity is high (CVSS 7.4). 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 malfun

Q: How urgent is CVE-2026-4371?

Priority 38/100. High severity; Remotely exploitable, no login, no user action

Routine

HIGH 7.4REMOTEUPGRADE

CVE-2026-4371 is a high-severity flaw (CVSS 7.4) affecting thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.4 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 70%)
CISA exploited list	No
Weakness	CWE-126, CWE-130
Published	2026-03-24
Last modified	2026-07-15

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-07-15 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-24 21:16 UTCPublished CVE record published
2026-07-15 02:22 UTCUpdated Record last modified

mozilla thunderbird UPGRADE

Affected: < 140.9.0 · < 149.0

Fixed in: 140.9.0, 149.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy thunderbird || rpm -q thunderbird || thunderbird --version

2. UPGRADE to 140.9.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade thunderbird

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade thunderbird

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-4371?

thunderbird: upgrade to 140.9.0, 149.0. Exact commands for each OS are on this page.

Is CVE-2026-4371 being exploited?

CVE-2026-4371 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-4371 affect?

CVE-2026-4371 affects thunderbird. Severity is high (CVSS 7.4). 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 malfun

How urgent is CVE-2026-4371?

Priority 38/100. High severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Thunderbird vulnerabilities

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References

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