# CVE-2026-0818: CSS-based exfiltration of the content from partially encrypted emails when allowing remote content

> Attackers can exfiltrate decrypted email content via CSS injection if remote content loading is enabled, affecting Thunderbird users.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/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:** 28 Jan 2026
- **Updated:** 13 Apr 2026
- **Also known as:** DEBIAN-CVE-2026-0818, DLA-4466-1, DSA-6118-1, GHSA-r27j-r277-j56h, MGASA-2026-0036, SUSE-SU-2026:0388-1, UBUNTU-CVE-2026-0818, openSUSE-SU-2026:10128-1

## What is CVE-2026-0818?

CVE-2026-0818 is a medium-severity vulnerability in Mozilla Thunderbird, published on 28 Jan 2026. 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

When a user explicitly requested Thunderbird to decrypt an inline OpenPGP message that was embedded in a text section of an email that was formatted and styled with HTML and CSS, then the decrypted contents were rendered in a context in which the CSS styles from the outer messages were active. If the user had additionally allowed loading of the remote content referenced by the outer email message, and the email was crafted by the sender using a combination of CSS rules and fonts and animations, then it was possible to extract the secret contents of the email. This vulnerability was fixed in Thunderbird 147.0.1 and Thunderbird 140.7.1.

Weakness: CWE-352 (Cross-Site Request Forgery), CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor), CWE-116 (Improper Encoding or Escaping of Output)

## Which versions are affected by CVE-2026-0818?

It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `MozillaThunderbird` | SUSE:Linux Enterprise Module for Package Hub 15 SP7 | All versions | 140.7.1-150200.8.254.1 |
| `MozillaThunderbird` | SUSE:Linux Enterprise Workstation Extension 15 SP7 | All versions | 140.7.1-150200.8.254.1 |
| `MozillaThunderbird` | openSUSE:Leap 15.6 | All versions | 140.7.1-150200.8.254.1 |
| `MozillaThunderbird` | openSUSE:Tumbleweed | All versions | 140.7.1-1.1 |

Products: Mozilla Thunderbird.

## How do I fix CVE-2026-0818?

Upgrade to a fixed version.

## Is CVE-2026-0818 being exploited?

There is no known exploitation of CVE-2026-0818 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-2026-0818 and fix it if it is affected.

What it is: CVE-2026-0818 is a medium-severity vulnerability in Mozilla Thunderbird, published on 28 Jan 2026. 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-2026-0818 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-2026-0818/ (Markdown: https://insights.sbomify.com/CVE-2026-0818/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-0818
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-0818
- OSV: https://osv.dev/vulnerability/CVE-2026-0818

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-0818/
Summaries are generated and may contain mistakes; the official description above is the record.