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FirefoxWeb browser · Mozilla

CVE-2020-15656

HIGH · 8.8 CVSS v3.1 Published 2020-08-10
Fix available
A fix is available. Upgrade to 78.1 / 79.0 or later.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable No privileges

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
JIT optimizations involving the Javascript arguments object could confuse later optimizations. This risk was already mitigated by various precautions in the code, resulting in this bug rated at only moderate severity. This vulnerability affects Firefox ESR < 78.1, Firefox < 79, and Thunderbird < 78.1.

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

JIT compiler optimizations in Firefox and Thunderbird's JavaScript engine could create confusion with later optimizations involving the arguments object, potentially leading to unexpected behavior or security issues despite existing mitigations.

MitigationUpdate affected installations to Firefox ESR 78.1+, Firefox 79+, or Thunderbird 78.1+ to obtain the patched version containing the fix.

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:< 79.0
Firefox EsrWeb browser
Affected:< 78.1
Ubuntu LinuxOperating system
Affected:= 16.04= 18.04= 20.04
ThunderbirdApplication
Affected:< 78.1
LeapOperating system
Affected:= 15.2

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
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

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. Identify installed Firefox version
    Run 'firefox --version' or check About Firefox in the application menu
    Affected if Version is below 79.0 (or below 78.1 for ESR releases)
  2. Identify installed Thunderbird version
    Run 'thunderbird --version' or check About Thunderbird in the application menu
    Affected if Version is below 78.1
  3. Check if Ubuntu system has vulnerable Firefox package
    Run 'dpkg -l | grep firefox' or 'apt list --installed | grep firefox' to list installed Firefox packages
    Affected if Firefox package version indicates an unpatched build (check against 79.0+ or 78.1+ for ESR)
  4. Check if OpenSUSE system has vulnerable Firefox package
    Run 'rpm -qa | grep firefox' to list installed Firefox packages
    Affected if Firefox package version indicates an unpatched build (check against 79.0+ or 78.1+ for ESR)

Your environment is affected if Firefox (any edition) below 79.0 or below 78.1 ESR, or Thunderbird below 78.1, is installed and being used.

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 78.1 / 79.0 or later
Fixed in 78.179.0
Interim mitigation

Update affected installations to Firefox ESR 78.1+, Firefox 79+, or Thunderbird 78.1+ to obtain the patched version containing the fix.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,424.

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-2020-15656 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