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CVE-2025-3909— JavaScript Execution via Spoofed PDF Attachment and file:/// Link

Quick assessment

Affected
Mozilla Thunderbird
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mozilla Thunderbird是美国Mozilla基金会的一套从Mozilla Application Suite独立出来的电子邮件客户端软件。该软件支持IMAP、POP邮件协议以及HTML邮件格式。 Mozilla Thunderbird 128.10.1之前版本和138.0.1之前版本存在安全漏洞，该漏洞源于X-Mozilla-External-Attachment-URL标头处理不当可能导致JavaScript执行。

AI Predicted 9.8 Difficulty: Easy EPSS 0.44% · P36
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I. Basic Information for CVE-2025-3909

Vulnerability Information
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Vulnerability Title
JavaScript Execution via Spoofed PDF Attachment and file:/// Link
Source: CVE Program / CVE List V5
Vulnerability Description
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 was fixed in Thunderbird 128.10.1 and Thunderbird 138.0.1.
Source: CVE Program / CVE List V5
CVSS Information
N/A
Source: CVE Program / CVE List V5
Vulnerability Type
N/A
Source: CVE Program / CVE List V5
Vulnerability Title
Mozilla Thunderbird 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Mozilla Thunderbird是美国Mozilla基金会的一套从Mozilla Application Suite独立出来的电子邮件客户端软件。该软件支持IMAP、POP邮件协议以及HTML邮件格式。 Mozilla Thunderbird 128.10.1之前版本和138.0.1之前版本存在安全漏洞，该漏洞源于X-Mozilla-External-Attachment-URL标头处理不当可能导致JavaScript执行。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Mozilla Thunderbird 128.10.1 ~ 128.* -

II. Public POCs for CVE-2025-3909

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III. Intelligence Information for CVE-2025-3909

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Vendor Advisories for CVE-2025-3909 (2)

Same Patch Batch · Mozilla · 2025-05-14 · 3 CVEs total

CVE-2025-3875 Sender Spoofing via Malformed From Header in Thunderbird
CVE-2025-3932 Tracking Links in Attachments Bypassed Remote Content Blocking

IV. Related Vulnerabilities

Same product: Thunderbird
Same vendor: Mozilla

V. Comments for CVE-2025-3909

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