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FirefoxWeb browser · Mozilla

CVE-2019-9801

MEDIUM · 5.3 CVSS v3.0 Published 2019-04-26
Fix available
A fix is available. Upgrade to 60.6 / 66.0 or later.
See remediation →
62/100
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
Firefox will accept any registered Program ID as an external protocol handler and offer to launch this local application when given a matching URL on Windows operating systems. This should only happen if the program has specifically registered itself as a "URL Handler" in the Windows registry. *Note: This issue only affects Windows operating systems. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.

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

On Windows, Firefox and Thunderbird will accept any registered Program ID as an external protocol handler and launch the corresponding local application when given a matching URL. This occurs because the software improperly validates protocol handler registrations in the Windows registry, allowing any application that has registered as a 'URL Handler' to be invoked without proper restrictions.

MitigationUpdate vulnerable installations to Firefox 66+, Firefox ESR 60.6+, or Thunderbird 60.6+. Organizations should deploy these updates through their standard patch management processes.

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:< 66.0
Firefox EsrWeb browser
Affected:< 60.6
ThunderbirdApplication
Affected:< 60.6

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
None
Integrity
Low
Availability
None

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/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. Confirm Windows operating system
    Verify the system is running Windows (this vulnerability only affects Windows environments). Check system properties or run 'winver' command.
    Affected if The system is NOT running Windows - this vulnerability only affects Windows.
  2. Check Firefox installation and version
    Locate Firefox installation - typically at C:\Program Files\Mozilla Firefox\firefox.exe or C:\Program Files (x86)\Mozilla Firefox\firefox.exe. Right-click the executable, select Properties, and check the 'File version' under the Details tab. Alternatively, open Firefox and navigate to Help > About Firefox.
    Affected if Firefox version is less than 66.0 or Firefox ESR version is less than 60.6.
  3. Check Thunderbird installation and version
    Locate Thunderbird installation - typically at C:\Program Files\Mozilla Thunderbird\thunderbird.exe or C:\Program Files (x86)\Mozilla Thunderbird\thunderbird.exe. Right-click the executable, select Properties, and check the 'File version' under the Details tab. Alternatively, open Thunderbird and navigate to Help > About Thunderbird.
    Affected if Thunderbird version is less than 60.6.
  4. Query installed Mozilla products via registry
    Open Registry Editor and navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\Mozilla Firefox (for Firefox) or HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\Mozilla Thunderbird (for Thunderbird). Check the 'Version' value in the 'Main' subkey.
    Affected if The installed version displayed is lower than 66.0 (Firefox), 60.6 (Firefox ESR), or 60.6 (Thunderbird).

The environment is affected if any Firefox version below 66.0, Firefox ESR below 60.6, or Thunderbird below 60.6 is installed on Windows.

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.

From vendor data
Upgrade available Upgrade to 60.6 / 66.0 or later
Fixed in 60.666.0
Interim mitigation

Update vulnerable installations to Firefox 66+, Firefox ESR 60.6+, or Thunderbird 60.6+. Organizations should deploy these updates through their standard patch management processes.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

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-2019-9801 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