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FirefoxWeb browser · Mozilla

CVE-2021-38505

MEDIUM · 6.5 CVSS v3.1 Published 2021-12-08
Fix available
A fix is available. Upgrade to 91.3.0 / 94.0 or later.
See remediation →
72/100
Remediation priority · Elevated
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
Microsoft introduced a new feature in Windows 10 known as Cloud Clipboard which, if enabled, will record data copied to the clipboard to the cloud, and make it available on other computers in certain scenarios. Applications that wish to prevent copied data from being recorded in Cloud History must use specific clipboard formats; and Firefox before versions 94 and ESR 91.3 did not implement them. This could have caused sensitive data to be recorded to a user's Microsoft account. *This bug only affects Firefox for Windows 10+ with Cloud Clipboard enabled. Other operating systems are unaffected.*. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.

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

Firefox and Thunderbird for Windows 10+ did not implement specific clipboard formats required to opt out of Microsoft Cloud Clipboard recording. When users copied sensitive data in these applications with Cloud Clipboard enabled, that data could be automatically uploaded to the user's Microsoft cloud account.

MitigationUpgrade to Firefox 94+, Thunderbird 91.3+, or Firefox ESR 91.3+; alternatively, disable Cloud Clipboard in Windows 10 Settings as a workaround.

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:< 94.0
Firefox EsrWeb browser
Affected:< 91.3.0
ThunderbirdApplication
Affected:< 91.3.0

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
Required
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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. Identify the installed Mozilla product
    Open the application and navigate to Help > About Firefox or Help > About Thunderbird to display the version number, or check Programs and Features in Windows Control Panel for the installed version.
    Affected if The product is Mozilla Firefox or Mozilla Thunderbird on Windows 10 or later.
  2. Compare the installed version to the affected ranges
    Note the version displayed in the About dialog. Compare it to the vulnerable versions: Firefox < 94.0, Firefox ESR < 91.3.0, Thunderbird < 91.3.0.
    Affected if The installed version is Firefox below 94.0, Firefox ESR below 91.3.0, or Thunderbird below 91.3.0.
  3. Verify Windows Cloud Clipboard is enabled
    Open Windows Settings > System > Clipboard, or press Win+R and run "ms-settings:clipboard" to access the Cloud Clipboard settings. Check if the toggle for "Cloud Clipboard" is turned On.
    Affected if Microsoft Cloud Clipboard is enabled on the Windows 10 or later system.
  4. Confirm clipboard usage with sensitive data
    Observe whether users copy sensitive information (such as passwords, financial data, or personal identifiers) from Firefox or Thunderbird while Cloud Clipboard remains enabled.
    Affected if Sensitive data is copied from the affected application while Cloud Clipboard is active.

A user is affected if they run an unpatched Firefox, Firefox ESR, or Thunderbird version (below 91.3.0 or 94.0 respectively) on Windows 10 or later with Cloud Clipboard enabled and copy sensitive data.

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 91.3.0 / 94.0 or later
Fixed in 91.3.094.0
Interim mitigation

Upgrade to Firefox 94+, Thunderbird 91.3+, or Firefox ESR 91.3+; alternatively, disable Cloud Clipboard in Windows 10 Settings as a workaround.

Recommended fix High confidence

Firefox 94.0, Firefox ESR 91.3.0, or Thunderbird 91.3.0

  1. Identify which product is installed: Firefox, Firefox ESR, or Thunderbird
  2. For Firefox users: Upgrade to Firefox 94.0 or later
  3. For Firefox ESR users: Upgrade to Firefox ESR 91.3.0 or later
  4. For Thunderbird users: Upgrade to Thunderbird 91.3.0 or later
  5. Verify the upgrade was successful by checking the application's version number
Caveat Standard upgrade risks apply - ensure extensions/add-ons are compatible with the new version

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,420
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,272.

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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-2021-38505 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