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.

The version of Thunderbird installed on the remote macOS or Mac OS X host is prior to 128.10.1. It is, therefore, affected by multiple vulnerabilities as referenced in the mfsa2025-34 advisory.

  - 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. (CVE-2025-3877)

  - 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     <spoofed@example.com> <legitimate@example.com>, Thunderbird treats spoofed@example.com as the actual     address. (CVE-2025-3875)

  - 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. (CVE-2025-3909)

  - 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-2025-3932)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The version of Thunderbird installed on the remote Windows host is prior to 128.10.1. It is, therefore, affected by multiple vulnerabilities as referenced in the mfsa2025-34 advisory.

  - 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. (CVE-2025-3877)

  - 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     <spoofed@example.com> <legitimate@example.com>, Thunderbird treats spoofed@example.com as the actual     address. (CVE-2025-3875)

  - 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. (CVE-2025-3909)

  - 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-2025-3932)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The version of Thunderbird installed on the remote macOS or Mac OS X host is prior to 138.0.1. It is, therefore, affected by multiple vulnerabilities as referenced in the mfsa2025-35 advisory.

  - 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. (CVE-2025-3877)

  - 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     <spoofed@example.com> <legitimate@example.com>, Thunderbird treats spoofed@example.com as the actual     address. (CVE-2025-3875)

  - 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. (CVE-2025-3909)

  - 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-2025-3932)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The version of Thunderbird installed on the remote Windows host is prior to 138.0.1. It is, therefore, affected by multiple vulnerabilities as referenced in the mfsa2025-35 advisory.

  - 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. (CVE-2025-3877)

  - 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     <spoofed@example.com> <legitimate@example.com>, Thunderbird treats spoofed@example.com as the actual     address. (CVE-2025-3875)

  - 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. (CVE-2025-3909)

  - 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-2025-3932)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

ID	Name	Product	Family	Severity
236405	Mozilla Thunderbird < 128.10.1	Nessus	MacOS X Local Security Checks	medium
236404	Mozilla Thunderbird < 128.10.1	Nessus	Windows	medium
236403	Mozilla Thunderbird < 138.0.1	Nessus	MacOS X Local Security Checks	medium
236402	Mozilla Thunderbird < 138.0.1	Nessus	Windows	medium

Detections

Analytics

CVE-2025-3909

medium

Tenable Plugins

medium

medium

medium

medium