# CVE-2026-4371: Out of bounds read in IMAP parsing

> A malicious mail server can trigger an out-of-bounds read in Thunderbird, potentially crashing the application or leaking sensitive data.

- **Severity:** High 7.4 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Mar 2026
- **Updated:** 15 Jul 2026
- **Also known as:** DEBIAN-CVE-2026-4371, GHSA-4p49-pghr-968w, UBUNTU-CVE-2026-4371

## What is CVE-2026-4371?

CVE-2026-4371 is a high-severity memory corruption vulnerability in Mozilla Thunderbird, published on 24 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### 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.

Weakness: CWE-130 (Improper Handling of Length Parameter Inconsistency), CWE-126 (Buffer Over-read)

## Which versions are affected by CVE-2026-4371?

It affects 1 product.


Products: Mozilla Thunderbird.

## How do I fix CVE-2026-4371?

Upgrade to a fixed version.

## Is CVE-2026-4371 being exploited?

There is no known exploitation of CVE-2026-4371 in the wild and no public exploit code.

## Other vulnerabilities in Mozilla Thunderbird

- [CVE-2026-92238](https://insights.sbomify.com/CVE-2026-92238/): Ambiguous parsing of mail headers (critical 9.8)
- [CVE-2026-84637](https://insights.sbomify.com/CVE-2026-84637/): Calendar invitation attachments could launch local executables (critical 9.8)
- [CVE-2021-43529](https://insights.sbomify.com/CVE-2021-43529/): Memory corruption in Thunderbird (critical 9.8)
- [CVE-2019-11703](https://insights.sbomify.com/CVE-2019-11703/): Memory corruption in Thunderbird (critical 9.8)
- [CVE-2019-11704](https://insights.sbomify.com/CVE-2019-11704/): Memory corruption in Thunderbird (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-4371 and fix it if it is affected.

What it is: CVE-2026-4371 is a high-severity memory corruption vulnerability in Mozilla Thunderbird, published on 24 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-4371 in the wild and no public exploit code.

Affected product: Mozilla Thunderbird. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-4371/ (Markdown: https://insights.sbomify.com/CVE-2026-4371/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-4371
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-4371
- OSV: https://osv.dev/vulnerability/CVE-2026-4371

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-4371/
Summaries are generated and may contain mistakes; the official description above is the record.