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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 3 May 2022.
FirefoxWeb browser · Mozilla

CVE-2020-6820

HIGH · 8.1 CVSS v3.1 Published 2020-04-24
Fix available
A fix is available. Upgrade to 68.6.1 / 68.7.0 or later.
See remediation →
100/100
Remediation priority · Urgent
In the wild 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
Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.

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

A race condition in the ReadableStream implementation in Firefox and Thunderbird can lead to a use-after-free memory corruption flaw. This memory safety vulnerability allows an attacker to potentially execute arbitrary code by exploiting the race between stream operations and object destruction. Targeted attacks have been observed in the wild exploiting this flaw.

MitigationApply vendor-supplied updates: Thunderbird to version 68.7.0 or later, Firefox to version 74.0.1 or later, or Firefox ESR to version 68.6.1 or later. Alternatively, disable or restrict access to vulnerable browser applications until patching can be completed.

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:< 68.6.1< 74.0.1
ThunderbirdApplication
Affected:< 68.7.0

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
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/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
    Navigate to menu > Help > About Firefox, or run 'firefox --version' from command line, or check the application binary properties
    Affected if Version is below 74.0.1 or below 68.6.1 for ESR releases
  2. Identify installed Thunderbird version
    Navigate to menu > Help > About Thunderbird, or run 'thunderbird --version' from command line
    Affected if Version is below 68.7.0
  3. Confirm product is Firefox ESR if version starts with 68
    Check if the installed Firefox is the ESR (Extended Support Release) variant by examining the version string or package name
    Affected if Firefox ESR version is below 68.6.1 (ESR 68.x releases before 68.6.1)
  4. Note on ReadableStream usage
    This vulnerability exists in the built-in ReadableStream implementation used by web content; no user-configurable feature toggle exists - detection relies solely on version matching
    Affected if N/A - this is a built-in browser component, not a toggle

User is affected if Firefox (any channel) is below 74.0.1, Firefox ESR is below 68.6.1, or Thunderbird is below 68.7.0.

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.

dbcve · scoped
Upgrade available Upgrade to 68.6.1 / 68.7.0 / 74.0.1 or later
Fixed in 68.6.168.7.074.0.1
Interim mitigation

Apply vendor-supplied updates: Thunderbird to version 68.7.0 or later, Firefox to version 74.0.1 or later, or Firefox ESR to version 68.6.1 or later. Alternatively, disable or restrict access to vulnerable browser applications until patching can be completed.

Recommended fix High confidence

Firefox 74.0.1 / Firefox ESR 68.6.1 / Thunderbird 68.7.0

  1. Identify the installed Firefox, Firefox ESR, or Thunderbird version
  2. For Firefox: upgrade to version 74.0.1 or later
  3. For Firefox ESR: upgrade to version 68.6.1 or later
  4. For Thunderbird: upgrade to version 68.7.0 or later
  5. Restart the application after upgrading
  6. Verify the installed version matches or exceeds the fixed release
Caveat Minor version updates typically do not introduce breaking changes for end users; ensure system compatibility requirements are met

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Firefox Exploited in the wild — priority engagement
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,600
Get it fixed fast

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

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-2020-6820 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