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FirefoxWeb browser · Mozilla

CVE-2020-6813

MEDIUM · 5.3 CVSS v3.1 Published 2020-03-25
Fix available
A fix is available. Upgrade to 74.0 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
When protecting CSS blocks with the nonce feature of Content Security Policy, the @import statement in the CSS block could allow an attacker to inject arbitrary styles, bypassing the intent of the Content Security Policy. This vulnerability affects Firefox < 74.

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

When using nonce-based Content Security Policy to protect CSS blocks, the @import statement within those protected CSS blocks can be leveraged to load external stylesheets. This allows an attacker who can control some portion of the CSS to inject arbitrary styles by bypassing the nonce protection mechanism, effectively defeating the CSP's security intent.

MitigationUpgrade Firefox to version 74 or later, which contains the fix for this CSP nonce bypass 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:< 74.0

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
None
Integrity
Low
Availability
None

CVSS:3.1/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. Check installed Firefox version
    Navigate to Firefox menu > Help > About Firefox, or enter 'about:support' in the address bar
    Affected if The version displayed is less than 74.0 (e.g., 73.x, 72.x, etc.)
  2. Compare version against affected range
    Review the version number shown in the About dialog - it follows the format major.minor.patch (e.g., 73.0.1)
    Affected if The major version is 73 or lower, or the full version string is lower than 74.0 when compared numerically
  3. Verify CSP nonce protection in use
    Inspect web application headers via browser DevTools (F12) > Network tab, or check server configuration for 'Content-Security-Policy' headers containing 'nonce-' directives
    Affected if The site or application uses 'unsafe-inline' style-src with nonce-based CSP, and the Firefox version is below 74.0

A user is affected if running any Firefox version below 74.0, with heightened relevance when the browser loads content protected by nonce-based Content Security Policy.

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 74.0 or later
Fixed in 74.0
Interim mitigation

Upgrade Firefox to version 74 or later, which contains the fix for this CSP nonce bypass vulnerability.

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-2020-6813 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