# CVE-2026-126133: Calendar server credentials could be sent to an HTTP proxy


- **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-2026-126133, GHSA-c6f8-6299-g8jq, UBUNTU-CVE-2026-126133

## What is CVE-2026-126133?

CVE-2026-126133 is a vulnerability in Mozilla Thunderbird, published on 10 Nov 2026. 

### Official description

When a CalDAV or ICS calendar was accessed through an HTTP proxy that requested authentication, Thunderbird could answer the proxy with the calendar server's username and password. This affected both setting up a new calendar and syncing existing calendars, where a saved password could be sent without any user interaction. A malicious or compromised proxy could use this to obtain the calendar credentials, even for calendars served over HTTPS. This vulnerability was fixed in Thunderbird 160 and Thunderbird 153.7.

## Which versions are affected by CVE-2026-126133?


## How do I fix CVE-2026-126133?

Upgrade to a fixed version.

## Is CVE-2026-126133 being exploited?

There is no known exploitation of CVE-2026-126133 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126133 and fix it if it is affected.

What it is: CVE-2026-126133 is a vulnerability in Mozilla Thunderbird, published on 10 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-126133 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-2026-126133/ (Markdown: https://insights.sbomify.com/CVE-2026-126133/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126133
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126133
- OSV: https://osv.dev/vulnerability/CVE-2026-126133

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126133/
Summaries are generated and may contain mistakes; the official description above is the record.