# CVE-2026-84642: Allowed UNC hostnames for attachments interpreted as a regular expression

> Attackers can bypass attachment host restrictions by exploiting unescaped regex patterns in configuration, allowing unauthorized remote attachment retrieval.

- **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:** 1 Sep 2026
- **Updated:** 2 Sep 2026
- **Also known as:** GHSA-5392-q9gh-3cc8, UBUNTU-CVE-2026-84642

## What is CVE-2026-84642?

CVE-2026-84642 is a high-severity server-side request forgery vulnerability in Mozilla Thunderbird, published on 1 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression without escaping. For some possible valid hostnames, this could allow certain unintended hostnames to also match and serve remote attachments. This vulnerability was fixed in Thunderbird 155 and Thunderbird 153.2.

Weakness: CWE-1333 (Inefficient Regular Expression Complexity)

## Which versions are affected by CVE-2026-84642?

It affects 1 product.


Products: Mozilla Thunderbird.

## How do I fix CVE-2026-84642?

Upgrade to a fixed version.

## Is CVE-2026-84642 being exploited?

There is no known exploitation of CVE-2026-84642 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-2026-84642 and fix it if it is affected.

What it is: CVE-2026-84642 is a high-severity server-side request forgery vulnerability in Mozilla Thunderbird, published on 1 Sep 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-2026-84642 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-2026-84642/ (Markdown: https://insights.sbomify.com/CVE-2026-84642/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-84642
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-84642
- OSV: https://osv.dev/vulnerability/CVE-2026-84642

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-84642/
Summaries are generated and may contain mistakes; the official description above is the record.