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FirefoxWeb browser · Mozilla

CVE-2015-4520

MEDIUM · 6.4 CVSS v2.0 Published 2015-09-24
Fix available
A fix is available. Upgrade to after 40.0.3 or later.
See remediation →
73/100
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
Mozilla Firefox before 41.0 and Firefox ESR 38.x before 38.3 allow remote attackers to bypass CORS preflight protection mechanisms by leveraging (1) duplicate cache-key generation or (2) retrieval of a value from an incorrect HTTP Access-Control-* response header.

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

Mozilla Firefox before 41.0 and Firefox ESR 38.x before 38.3 contain a CORS preflight bypass vulnerability. Attackers can exploit either duplicate cache-key generation or improper handling of Access-Control-* response headers to bypass cross-origin restrictions without proper preflight validation.

MitigationUpgrade Firefox to version 41.0 or later, or Firefox ESR to version 38.3 or later, to receive the security patch.

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:= 38.0= 38.0.1= 38.0.5= 38.1.0= 38.1.1= 38.2.0= 38.2.1<= 40.0.3

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
Authentication
None
Confidentiality
Partial
Integrity
Partial
Availability
None

AV:N/AC:L/Au:N/C:P/I:P/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 version number
    Open Firefox and navigate to Help > About Firefox, or type 'about:support' in the address bar and press Enter. The version number is displayed at the top of the page.
    Affected if The version displayed is 38.0, 38.0.1, 38.0.5, 38.1.0, 38.1.1, 38.2.0, 38.2.1, or any version between 38.0 and 40.0.3 inclusive.
  2. Confirm Firefox ESR version if applicable
    If running Firefox ESR, the version format includes 'ESR' (e.g., 38.2.1esr). Check the About page to determine if the ESR version matches the affected versions.
    Affected if The ESR version is 38.0, 38.0.1, 38.0.5, 38.1.0, 38.1.1, 38.2.0, or 38.2.1 (any version before 38.3).
  3. Verify CORS functionality is in use
    Review your web applications or browser extensions that perform cross-origin requests. Check if the applications use XMLHttpRequest or fetch API with custom Access-Control-* headers that would trigger a CORS preflight.
    Affected if You have web applications or extensions that rely on CORS preflight requests for cross-origin communication while running an affected Firefox version.

You are affected if your installed Firefox is version 38.0 through 40.0.3, or Firefox ESR 38.0 through 38.2.1, and you use or host web applications that perform CORS preflight requests.

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 a release after 40.0.3
Interim mitigation

Upgrade Firefox to version 41.0 or later, or Firefox ESR to version 38.3 or later, to receive the security patch.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation0.5 h
  • Testing0.5 h
  • Review / QA0.5 h
2.5 hours of engineering $750
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,200.

Scan for this in your stack

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dbcve dependency scanner

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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-2015-4520 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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