CVE-2025-3909Network attack vector

JavaScript execution via misrendered nested attachment

Published May 14, 2025 · Updated Apr 13, 2026

Improper attachment rendering in Mozilla Thunderbird ESR before 128.10.1 and Thunderbird before 138.0.1 allows remote attackers to run JavaScript. Thunderbird auto-saves a nested message/rfc822 attachment to /tmp, then renders attacker-labeled application/pdf content as HTML through an X-Mozilla-External-Attachment-URL file link. Opening the spoofed attachment is required; execution remains in the file:/// context, with no published availability impact.

CVSS severity8.1
High
EPSS probability0.44%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Improper attachment rendering in Mozilla Thunderbird ESR before 128.10.1 and Thunderbird before 138.0.1 allows remote attackers to run JavaScript. Thunderbird auto-saves a nested message/rfc822 attachment to /tmp, then renders attacker-labeled application/pdf content as HTML through an X-Mozilla-External-Attachment-URL file link. Opening the spoofed attachment is required; execution remains in the file:/// context, with no published availability impact.

The record

CVE
CVE-2025-3909
Published
May 14, 2025
Updated
Apr 13, 2026
Vendor
Unknown vendor
Product
Unknown product
Classifications
T1204.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 14, 2025CVE publishedPublication date reported by the CVE source.
  2. Apr 13, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

8.1UbuntuCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1204.002User Execution: Malicious File

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo