CVE-2026-118177

A flaw was found in Thunderbird. The Mozilla Foundation's Security Advisory describes the following issue: When looking for calendars in a directory on a server, during account setup or when adding a new network calendar, Thunderbird accepted calendar locations pointing to other servers, local files or network shares. A malicious server, or a network attacker when the lookup used plain HTTP, could make Thunderbird subscribe to such a location. On Windows this could leak the user's NTLM credentials to an attacker-controlled host on every calendar refresh, and changes to the calendar could be written to the attacker's share or overwrite a local file.

Red Hat Product Security rates the severity of this flaw as determined by the Mozilla Foundation Security Advisory.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

--- CME Mitigation Guidance (2026-11-03) --- Disclaimer: The following mitigation solutions are provided as a courtesy, pending the application of an official patch. This content was generated utilizing AI tools in conjunction with data from https://cmetaxonomy.org. Customers are advised to independently evaluate and test these temporary mitigations within their own infrastructure.

The highest-value action is to reject calendar discovery URLs pointing to local file paths, network shares, or external servers during Thunderbird calendar setup: upgrade to Thunderbird 159 and Thunderbird 153.6 (or the fixed Thunderbird/Rhel10/Thunderbird Flatpak builds), which reject calendar discovery locations pointing to local files, network shares, or unauthorized remote servers, closing all 3 injection vectors (remote network share URL injection, local file path subscription injection, external server calendar redirection). Until patched, this is a concern on Thunderbird clients performing account setup or adding network calendars across all supported platforms, so Avoid configuring accounts or adding network calendars over unencrypted HTTP, and avoid connecting to untrusted calendar directory servers until the software is updated. As a compensating measure, Confine Thunderbird filesystem access boundaries to prevent the application from reading or overwriting arbitrary local files when subscribing to calendars and Enforce strict allowlist validation on calendar discovery URL schemes and endpoints, denying local file or UNC network locations by default. These controls limit blast radius and contain exploitation, but they do not fix the underlying calendar discovery path validation defect — apply the upstream patch to fully remediate.

Highest-impact CME: CME-1301 Path Traversal Prevention (Canonicalization) (high)