CVE-2025-4086Public exploitNetwork attack vector

Download-type spoofing via post-sanitization filename decoding

Published Apr 29, 2025 · Updated Apr 13, 2026

UI spoofing in Mozilla Firefox and Thunderbird before 138 allows remote attackers to disguise an Android APK as a document. The Android download path sanitizes Content-Disposition filenames before percent-decoding them, allowing repeated encoded newlines to push the true extension out of the visible dialog. The effect is limited to Android download prompts and misrepresents the file type without changing the downloaded file.

CVSS severity6.5
Medium
EPSS probability0.28%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

UI spoofing in Mozilla Firefox and Thunderbird before 138 allows remote attackers to disguise an Android APK as a document. The Android download path sanitizes Content-Disposition filenames before percent-decoding them, allowing repeated encoded newlines to push the true extension out of the visible dialog. The effect is limited to Android download prompts and misrepresents the file type without changing the downloaded file.

The record

CVE
CVE-2025-4086
Published
Apr 29, 2025
Updated
Apr 13, 2026
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-451, T1036
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 29, 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

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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

6.5CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/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.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-451User Interface (UI) Misrepresentation of Critical Information
T1036Masquerading

Public exploit references

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

Technologies

Your stack

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Your stack

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