CVE-2026-0818: CSS-based exfiltration of the content from partially encrypted emails when allowing remote content

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-01-28 · updated 2026-06-17
Weakness: CWE-116

Description

When a user explicitly requested Thunderbird to decrypt an inline OpenPGP message that was embedded in a text section of an email that was formatted and styled with HTML and CSS, then the decrypted contents were rendered in a context in which the CSS styles from the outer messages were active. If the user had additionally allowed loading of the remote content referenced by the outer email message, and the email was crafted by the sender using a combination of CSS rules and fonts and animations, then it was possible to extract the secret contents of the email. This vulnerability was fixed in Thunderbird 147.0.1 and Thunderbird 140.7.1.

Affected products

Mozilla Thunderbird: before 140.7.1; before 147.0.1 — fixed in 140.7.1, 147.0.1

How to fix

Upgrade to a fixed version: Mozilla Thunderbird 140.7.1, 147.0.1. Check the vendor advisory for mitigations.

References

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