# CVE-2026-57963: Chat UI manipulation by injection

> Attackers can inject malicious HTML and CSS into chat messages to manipulate the UI and facilitate phishing attacks in Thunderbird.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/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 Jul 2026
- **Updated:** 1 Jul 2026
- **Also known as:** DEBIAN-CVE-2026-57963, GHSA-396p-jm6v-v2p3, UBUNTU-CVE-2026-57963

## What is CVE-2026-57963?

CVE-2026-57963 is a medium-severity cross-site scripting vulnerability in Mozilla Thunderbird, published on 1 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An attacker who can send HTML chat messages (via Matrix or XMPP) can inject arbitrary styled content, phishing links, and CSS that manipulates the chat UI. This vulnerability was fixed in Thunderbird 152.0.1 and Thunderbird 140.12.1.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-57963?

It affects 1 product.


Products: Mozilla Thunderbird.

## How do I fix CVE-2026-57963?

Upgrade to a fixed version.

## Is CVE-2026-57963 being exploited?

There is no known exploitation of CVE-2026-57963 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-2026-57963 and fix it if it is affected.

What it is: CVE-2026-57963 is a medium-severity cross-site scripting vulnerability in Mozilla Thunderbird, published on 1 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-2026-57963 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-2026-57963/ (Markdown: https://insights.sbomify.com/CVE-2026-57963/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-57963
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-57963
- OSV: https://osv.dev/vulnerability/CVE-2026-57963

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-57963/
Summaries are generated and may contain mistakes; the official description above is the record.