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Mozilla CVE-2025-3909

HIGH
Product UI does not Warn User of Unsafe Actions (CWE-356)
2025-05-14 security@mozilla.org
Information Disclosure Mozilla
8.1
CVSS 3.1 · NVD
View

Vulnerability Assessment

Weakness
CWE
CWE-356
OWASP Top 10:2025
A06:2021 - Vulnerable and Outdated Components
ASVS 5.0
V3 - Web Frontend Security Product UI inconsistency
Impact

CVSS score of 8.1 (HIGH). Attack characteristics: network attack vector, low complexity, no privileges required, user interaction required. EPSS exploitation probability: 0.42%.
Exploit ScenarioDerived

This vulnerability can be exploited remotely over the network. Successful exploitation could compromise the high-severity affected system.
This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
Patch available 0.4% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 28, 2026

Thunderbird's handling of the X-Mozilla-External-Attachment-URL header can be exploited to execute JavaScript in the file:/// context. Rated high severity (CVSS 8.1), this vulnerability is remotely exploitable, no authentication required, low attack complexity. No vendor patch available.

RemediationAI

No vendor patch is available at time of analysis. Monitor vendor advisories for updates. Apply vendor patches when available. Implement network segmentation and monitoring as interim mitigations.

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Technical ContextAI · show details

This vulnerability is classified under CWE-356. 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. Affected products include: Mozilla Thunderbird.

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  • Exploit and PoC references, threat intelligence from 22 sources
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Severity by source

NVD PRIMARY 8.1 HIGH
AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
SUSE HIGH
Red Hat 6.5 HIGH

Primary rating from NVD.

CVSS VectorNVD

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

Lifecycle Timeline

2
Patch released
Apr 06, 2026 - 02:30 nvd
Patch available
CVE Published
May 14, 2025 - 17:15 nvd
HIGH 8.1

DescriptionCVE.org

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.

Executive ViewAI

Why It Matters

CVE-2025-3909 presents notable security risk rated CVSS 8.1. Successful exploitation could significantly impact the confidentiality, integrity, or availability of affected systems. Organizations should apply vendor updates when available and consider compensating controls in the interim.

Business Impact: High
Compensating Controls

Apply vendor patch. If immediate patching is not feasible: (1) Implement network-level access controls to limit exposure; (2) Enable enhanced monitoring and alerting; (3) Apply principle of least privilege.

Threat Intelligence Overview

EPSS Score
0.4%
Top 38% exploitable
Data Quality
4 sources
Confidence: Medium
NVD CVE.org EPSS Vendor

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.4%
Patch available Yes
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Scheduled
Exploitation· derived
None
System Exposure· derived
Open
Automatable· derived
No
Human Impact
High

Priority Score

40
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +40
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.

Affected Products (CPE)CVE.org

cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*

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Same weakness CWE-356 – Product UI does not Warn User of Unsafe Actions

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Same technique Information Disclosure

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CVE-2025-68613 HIGH POC
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CVE-2015-1701 HIGH POC
7.8
CVE-2017-0147 HIGH POC
7.5

Vendor StatusVendor

SUSE

Severity: High
Product Status
SUSE Liberty Linux 8 Fixed
SUSE Liberty Linux 9 Fixed
SUSE Linux Enterprise Desktop 15 SP6 SUSE Linux Enterprise Server 15 SP6 SUSE Linux Enterprise Server for SAP Applications 15 SP6 SUSE Linux Enterprise Workstation Extension 15 SP6 Fixed
SUSE Linux Enterprise Desktop 15 SP7 SUSE Linux Enterprise Server 15 SP7 SUSE Linux Enterprise Server for SAP Applications 15 SP7 SUSE Linux Enterprise Workstation Extension 15 SP7 Fixed
SUSE Linux Enterprise Module for Package Hub 15 SP6 Fixed

References

Advisories & patches
ADVISORY https://access.redhat.com/errata/RHSA-2025:8196 ADVISORY https://access.redhat.com/errata/RHSA-2025:8203 ADVISORY https://access.redhat.com/errata/RHSA-2025:8324 ADVISORY https://access.redhat.com/errata/RHSA-2025:8325 ADVISORY https://access.redhat.com/errata/RHSA-2025:8326 ADVISORY https://access.redhat.com/errata/RHSA-2025:8391 ADVISORY https://access.redhat.com/errata/RHSA-2025:8507 ADVISORY https://access.redhat.com/errata/RHSA-2025:8594 ADVISORY https://access.redhat.com/errata/RHSA-2025:8756 ADVISORY https://access.redhat.com/errata/RHSA-2025:8784 ADVISORY https://www.mozilla.org/security/advisories/mfsa2025-34/ ADVISORY https://www.mozilla.org/security/advisories/mfsa2025-35/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2025:01660/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2025:15131/ REF https://bugzilla.mozilla.org/show_bug.cgi?id=1958376 REF https://lists.debian.org/debian-lts-announce/2025/05/msg00022.html

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