# CVE-2025-3909: JavaScript Execution via Spoofed PDF Attachment and file:/// Link

> Attackers can execute JavaScript in the file:/// context by spoofing PDF attachments, potentially compromising user systems.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 14 May 2025
- **Updated:** 13 Apr 2026
- **Also known as:** DEBIAN-CVE-2025-3909, GHSA-h6cg-6m9j-xj9g, UBUNTU-CVE-2025-3909

## What is CVE-2025-3909?

CVE-2025-3909 is a high-severity cross-site scripting vulnerability in Mozilla Thunderbird, published on 14 May 2025. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Thunderbird's handling of the X-Mozilla-External-Attachment-URL header can be exploited to execute JavaScript in the file:/// context. By crafting a nested email attachment (message/rfc822) and setting its content type to application/pdf, Thunderbird may incorrectly render it as HTML when opened, allowing the embedded JavaScript to run without requiring a file download. This behavior relies on Thunderbird auto-saving the attachment to /tmp and linking to it via the file:/// protocol, potentially enabling JavaScript execution as part of the HTML. This vulnerability was fixed in Thunderbird 128.10.1 and Thunderbird 138.0.1.

Weakness: CWE-356 (Missing User Interaction for Potentially Unsafe Action)

## Which versions are affected by CVE-2025-3909?

It affects 1 product.


Products: Mozilla Thunderbird.

## How do I fix CVE-2025-3909?

Upgrade to a fixed version.

## Is CVE-2025-3909 being exploited?

There is no known exploitation of CVE-2025-3909 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-2025-3909 and fix it if it is affected.

What it is: CVE-2025-3909 is a high-severity cross-site scripting vulnerability in Mozilla Thunderbird, published on 14 May 2025. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2025-3909 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-2025-3909/ (Markdown: https://insights.sbomify.com/CVE-2025-3909/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-3909
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-3909
- OSV: https://osv.dev/vulnerability/CVE-2025-3909

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-3909/
Summaries are generated and may contain mistakes; the official description above is the record.