# CVE-2026-126135: SMTP and POP3 response injection when using STARTTLS


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 10 Nov 2026
- **Updated:** 10 Nov 2026
- **Also known as:** DEBIAN-CVE-2026-126135, GHSA-p47m-32pf-24wg, UBUNTU-CVE-2026-126135

## What is CVE-2026-126135?

CVE-2026-126135 is a vulnerability in Mozilla Thunderbird, published on 10 Nov 2026. 

### Official description

If an attacker was able to perform a man-in-the-middle attack on an SMTP or POP3 server connection using STARTTLS, the attacker could inject server responses before the TLS handshake completed. Thunderbird would process these responses after the handshake, as if they had been received from the server over the encrypted connection. This could be used to make Thunderbird report a different outcome for sending or fetching mail, and to show attacker-controlled text in error messages. The encrypted connection itself was not compromised, and credentials were not exposed. Connections using SSL/TLS instead of STARTTLS were not affected. This vulnerability was fixed in Thunderbird 160 and Thunderbird 153.7.

## Which versions are affected by CVE-2026-126135?


## How do I fix CVE-2026-126135?

Upgrade to a fixed version.

## Is CVE-2026-126135 being exploited?

There is no known exploitation of CVE-2026-126135 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126135 and fix it if it is affected.

What it is: CVE-2026-126135 is a vulnerability in Mozilla Thunderbird, published on 10 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-126135 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-2026-126135/ (Markdown: https://insights.sbomify.com/CVE-2026-126135/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126135
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126135
- OSV: https://osv.dev/vulnerability/CVE-2026-126135

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126135/
Summaries are generated and may contain mistakes; the official description above is the record.