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FirefoxWeb browser · Mozilla

CVE-2018-12395

HIGH · 7.5 CVSS v3.0 Published 2019-02-28
Fix available
A fix is available. Upgrade to 60.3 / 63.0 or later.
See remediation →
84/100
Remediation priority · High
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
By rewriting the Host: request headers using the webRequest API, a WebExtension can bypass domain restrictions through domain fronting. This would allow access to domains that share a host that are otherwise restricted. This vulnerability affects Firefox ESR < 60.3 and Firefox < 63.

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

A WebExtension using the webRequest API can rewrite Host request headers, enabling domain fronting attacks that bypass domain restrictions and allow access to otherwise restricted domains sharing a host.

MitigationUpgrade to Firefox ESR 60.3 or later, or Firefox 63 or later, to remediate this vulnerability.

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:< 63.0
Firefox EsrWeb browser
Affected:< 60.3
Debian LinuxOperating system
Affected:= 8.0= 9.0
Ubuntu LinuxOperating system
Affected:= 14.04= 16.04= 18.04= 18.10
Enterprise Linux DesktopOperating system
Affected:= 6.0= 7.0
Enterprise Linux ServerOperating system
Affected:= 6.0= 7.0
Enterprise Linux Server EusOperating system
Affected:= 7.5
Enterprise Linux WorkstationOperating system
Affected:= 6.0= 7.0

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
High
Integrity
None
Availability
None

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/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 installed Firefox version
    In Firefox, navigate to Help > About Firefox (or go to about:support). The version number is displayed at the top. For ESR, the version will include 'ESR' in the name (e.g., '60.2 ESR').
    Affected if The version is below 63.0 for standard Firefox, or below 60.3 for Firefox ESR.
  2. Verify WebExtension support is enabled
    In Firefox address bar, type about:config and press Enter. Search for the preference 'extensions.webextensions.enabled'. Check if it is set to true.
    Affected if The preference is set to true (extensions are enabled).
  3. Inspect installed WebExtensions using webRequest
    Navigate to about:addons, click the gear icon, and select 'Debug Add-ons'. Review any installed extensions that may use the webRequest API. Alternatively, manually inspect extensions in the profile folder (typically at ~/.mozilla/firefox/*.default/extensions/) for manifest.json files containing 'webRequest' in permissions.
    Affected if Any WebExtension with 'webRequest' permission is installed and enabled.
  4. Confirm domain restriction bypass capability
    This is a design flaw in the webRequest API. If a WebExtension with 'webRequest' and 'host' permissions is installed, it can potentially rewrite Host headers. No specific config to check - the vulnerability exists if conditions in steps 1-3 are all true.
    Affected if A vulnerable Firefox version (step 1) is used with an enabled WebExtension that has webRequest permissions (steps 2-3).

A user is affected if they are running Firefox below version 63.0 (or ESR below 60.3) with one or more WebExtensions that utilize the webRequest API and have host permissions enabled.

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 60.3 / 63.0 or later
Fixed in 60.363.0
Interim mitigation

Upgrade to Firefox ESR 60.3 or later, or Firefox 63 or later, to remediate this vulnerability.

Fix this in Firefox Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing4.0 h
  • Review / QA2.0 h
18.0 hours of engineering $3,200
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An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $5,120.

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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-2018-12395 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