Analysis Summary

CVE-2025-3909 CVSS:6.5

Thunderbird's handling of the X-Mozilla-External-Attachment-URL header can be exploited to execute JavaScript in the file:/// context. By crafting a nested email attachment (message/rfc822) and setting its content type to application/pdf, Thunderbird may incorrectly render it as HTML when opened, allowing the embedded JavaScript to run without requiring a file download. This behavior relies on Thunderbird auto-saving the attachment to /tmp and linking to it via the file:/// protocol, potentially enabling JavaScript execution as part of the HTML. This vulnerability affects Thunderbird.

CVE-2025-3932 CVSS:6.5

It was possible to craft an email that showed a tracking link as an attachment. If the user attempted to open the attachment, Thunderbird automatically accessed the link. The configuration to block remote content did not prevent that. Thunderbird has been fixed to no longer allow access to web pages listed in the X-Mozilla-External-Attachment-URL header of an email.

CVE-2025-3875 CVSS:7.5

Thunderbird parses addresses in a way that can allow sender spoofing in case the server allows an invalid From address to be used. For example, if the From header contains an (invalid) value "Spoofed Name ", Thunderbird treats spoofed@example.com as the actual address.

CVE-2025-3877 CVSS:5.4

A crafted HTML email using mailbox:/// links can trigger automatic, unsolicited downloads of .pdf files to the user's desktop or home directory without prompting, even if auto-saving is disabled. This behavior can be abused to fill the disk with garbage data (e.g. using /dev/urandom on Linux) or to leak Windows credentials via SMB links when the email is viewed in HTML mode. While user interaction is required to download the .pdf file, visual obfuscation can conceal the download trigger. Viewing the email in HTML mode is enough to load external content.

Impact

• Security Bypass
• Information Disclosure

Indicators of Compromise

CVE

• CVE-2025-3909

• CVE-2025-3932

• CVE-2025-3875

• CVE-2025-3877

Affected Vendors

Affected Products

• Mozilla Thunderbird - 128.10.1
• Mozilla Thunderbird - 138.0.1

Remediation

Refer to Mozilla Security Advisory for patch, upgrade, or suggested workaround information.

Mozilla Security Advisory