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

Published Nov 10, 2026
·
Updated

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.

Affected Software

2 affected componentsFixes available
Mozilla Thunderbird<160
160
Mozilla Thunderbird<153.7
153.7

Remediation

Recommended actions to resolve this vulnerability, in priority order.

  1. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 160
  2. Upgrade

    Upgrade to a fixed release to a version that resolves this vulnerability.

    Fixed in 153.7
  3. Upgrade

    Upgrade Thunderbird to a version that resolves this vulnerability.

    Fixed in 160
  4. Upgrade

    Upgrade Thunderbird to a version that resolves this vulnerability.

    Fixed in 153.7

Event History

Nov 10, 2026
CVE Published
via Mozilla·12:00 AM
Data Sourced
via Mozilla·12:00 AM
DescriptionSeverityAffected Software
CVE Published
via MITRE·06:59 PM
Data Sourced
via MITRE·06:59 PM
Description
Data Sourced
via NVD·07:21 PM
Description

Parent advisories

This vulnerability appears in the following advisories.

Peer vulnerabilities

Found alongside the following vulnerabilities.

Frequently Asked Questions

1

Who is exposed to this issue?

Thunderbird users who access CalDAV or ICS calendars through an HTTP proxy that requests authentication are exposed. The proxy may obtain the calendar server credentials even when the calendar itself is served over HTTPS.

2

Can credentials be exposed without user interaction?

Yes. During synchronization of an existing calendar, Thunderbird could send a previously saved calendar password to the authenticating HTTP proxy without user interaction.

3

What attacker capability is required?

An attacker needs to operate or compromise an HTTP proxy used when Thunderbird accesses the calendar, and have that proxy request authentication.

4

What version fixes the issue?

The issue was fixed in Thunderbird 160.

Company

Resources

Contact

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