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FirefoxWeb browser · Mozilla

CVE-2025-4086

MEDIUM · 6.5 CVSS v3.1 Published 2025-04-29
Fix available
A fix is available. Upgrade to 138.0 or later.
See remediation →
74/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
A specially crafted filename containing a large number of encoded newline characters could obscure the file's extension when displayed in the download dialog. *This bug only affects Thunderbird for Android. Other versions of Thunderbird are unaffected.*. This vulnerability was fixed in Firefox 138 and Thunderbird 138.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Thunderbird for Android has a vulnerability where specially crafted filenames containing encoded newline characters can obscure the file extension in the download dialog. This UI display issue could trick users into opening files they believe are safe (e.g., an executable appearing as a text file). The vulnerability stems from improper handling of encoded newline characters during filename rendering.

MitigationUpgrade Thunderbird for Android to version 138 or later, as this version contains the fix for proper rendering of filenames with encoded newline characters.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
FirefoxWeb browser
Affected:< 138.0
ThunderbirdApplication
Affected:< 138.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check Firefox for Android version
    Open Firefox for Android, go to Settings > About Firefox. The version number is displayed on this screen.
    Affected if The version shown is below 138.0 (e.g., 137.x or earlier)
  2. Check Thunderbird for Android version
    Open Thunderbird for Android, go to Settings > About Thunderbird. The version number is displayed on this screen.
    Affected if The version shown is below 138.0 (e.g., 137.x or earlier)
  3. Verify download dialog behavior with test filename
    If running an affected version, attempt to trigger a download with a filename containing encoded newline characters (\n or \r\n encoded within the filename string) and observe how the download dialog displays the file extension.
    Affected if The file extension displayed in the download dialog does not match the actual file extension, or the filename appears to be truncated or broken across lines

A user is affected if they are running Firefox for Android or Thunderbird for Android with a version number lower than 138.0 and they receive a download with a specially crafted filename containing encoded newline characters.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 138.0 or later
Fixed in 138.0
Interim mitigation

Upgrade Thunderbird for Android to version 138 or later, as this version contains the fix for proper rendering of filenames with encoded newline characters.

Recommended fix High confidence

Firefox 138.0 and Thunderbird 138.0

  1. 1. Check your current Firefox or Thunderbird version by navigating to Help > About (or Menu > Help > About on Android).
  2. 2. If the version is below 138.0, download Firefox 138.0 or Thunderbird 138.0 from the official Mozilla download page (www.mozilla.org).
  3. 3. Install the updated version. On desktop, the automatic update should trigger; on Android, install the downloaded APK or update via Google Play Store.

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.

Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2025-4086 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data