CVE-2025-3909

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 < 128.10.1 and Thunderbird < 138.0.1.

References

Affected packages

Git /

Affected ranges

Database specific

source

"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-3909.json"

unresolved_ranges

[
    {
        "events": [
            {
                "introduced": "0"
            },
            {
                "fixed": "128.10.1"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "129.0"
            },
            {
                "fixed": "138.0.1"
            }
        ]
    }
]