CVE-2026-118175

A flaw was found in Thunderbird. The Mozilla Foundation's Security Advisory describes the following issue: When replying to, forwarding, or editing a crafted email message as new, Thunderbird could embed parts of a different message stored in the same folder into the message being composed, without showing a blocked content warning. If the user sent the message, the content of the other message was disclosed to the recipients.

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 prevent the email composition buffer from pulling content from adjacent folder messages during reply, forward, or edit-as-new operations: upgrade to Thunderbird 159 and 153.6 (or the fixed Thunderbird/Rhel10/Thunderbird Flatpak builds), which prevents Thunderbird from embedding portions of unrelated messages stored in the same folder into the composed draft during reply, forward, or edit-as-new actions, closing all 3 injection vectors (replying to a crafted email message, forwarding a crafted email message, editing a crafted email message as new). Until patched, this is a concern on Thunderbird client installations where users process crafted emails stored in the same folder as other messages, so avoid replying to, forwarding, or editing untrusted messages kept in folders containing sensitive emails, and review composed message bodies before sending. These controls limit blast radius and contain exploitation, but they do not fix the underlying information disclosure defect — apply the upstream patch to fully remediate.