Content of other messages could be included when replying to a crafted message

Summary

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. This vulnerability was fixed in Thunderbird 159 and Thunderbird 153.6.

Remediation

1. Inventory every Mozilla Thunderbird deployment, including staging.
2. No fixed version appears in the public sources yet. Check the references below and the vendor directly.

References

• Referencehttps://bugzilla.mozilla.org/show_bug.cgi?id=2081720
• Referencehttps://www.mozilla.org/security/advisories/mfsa2026-121/
• Referencehttps://www.mozilla.org/security/advisories/mfsa2026-122/