# CVE-2026-14899: Off-by-one out of bounds read in MIME header parser for forwarding

> An off-by-one error in Thunderbird's MIME header parser allows a single byte out-of-bounds read, potentially crashing the application when viewing all headers.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Jul 2026
- **Updated:** 27 Jul 2026
- **Also known as:** DEBIAN-CVE-2026-14899, GHSA-6667-m7mf-jcr7, UBUNTU-CVE-2026-14899

## What is CVE-2026-14899?

CVE-2026-14899 is a high-severity memory corruption vulnerability in Mozilla Thunderbird, published on 22 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The code to parse MIME headers for display when forwarding a message (if the setting to view all headers was enabled) had an off-by-one error, allowing a single byte to be read from the memory after the buffer for the headers, and potentially crashing Thunderbird. This vulnerability was fixed in Thunderbird 153 and Thunderbird 140.13.

Weakness: CWE-193 (Off-by-one Error)

## Which versions are affected by CVE-2026-14899?

It affects 1 product.


Products: Mozilla Thunderbird.

## How do I fix CVE-2026-14899?

Upgrade to a fixed version.

## Is CVE-2026-14899 being exploited?

There is no known exploitation of CVE-2026-14899 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-2026-14899 and fix it if it is affected.

What it is: CVE-2026-14899 is a high-severity memory corruption vulnerability in Mozilla Thunderbird, published on 22 Jul 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-2026-14899 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-2026-14899/ (Markdown: https://insights.sbomify.com/CVE-2026-14899/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-14899
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-14899
- OSV: https://osv.dev/vulnerability/CVE-2026-14899

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-14899/
Summaries are generated and may contain mistakes; the official description above is the record.