FirefoxWeb browser · Mozilla
CVE-2021-23995
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 · uneditedWhen Responsive Design Mode was enabled, it used references to objects that were previously freed. We presume that with enough effort this could have been exploited to run arbitrary code. This vulnerability affects Firefox ESR < 78.10, Thunderbird < 78.10, and Firefox < 88.
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 confidenceUse-after-free vulnerability in Firefox and Thunderbird's Responsive Design Mode (RDM). When RDM was enabled, the browser referenced objects that had already been freed from memory, potentially allowing arbitrary code execution via memory corruption.
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
< 88.0
Firefox EsrWeb browser
< 78.10< 78.10CVSS 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 checksWork through these to decide whether this CVE applies to you.
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Identify the installed Mozilla product and versionOpen the application and navigate to Help > About Firefox or Help > About Thunderbird. Alternatively, run 'firefox --version' or 'thunderbird --version' from command line.Affected if The version shown is Firefox < 88.0, Firefox ESR < 78.10, or Thunderbird < 78.10
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Confirm the product typeDetermine whether the installed product is standard Firefox, Firefox ESR, or Thunderbird by checking the About dialog or application name.Affected if The product is Firefox (any channel), Firefox ESR, or Thunderbird and the version falls below the thresholds in step 1
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Verify RDM feature accessibilityCheck if Responsive Design Mode can be accessed: in Firefox, press Ctrl+Shift+M or open Developer Tools (F12) and click the device toggle icon. In Thunderbird, check if Developer Tools RDM is available.Affected if RDM is present and accessible in the installed product (RDM is a standard developer feature in these versions)
You are affected if your installed Firefox version is below 88.0, or your Firefox ESR or Thunderbird version is below 78.10, and RDM feature is accessible in the application.
Generated from the published advisory. Verify against your own configuration.
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 · scoped78.1088.0
Upgrade to Firefox ESR 78.10 or later, Thunderbird 78.10 or later, or Firefox 88 or later to patch the use-after-free vulnerability.
Firefox 88.0, Firefox ESR 78.10, or Thunderbird 78.10 (or later)
- 1. Identify which Mozilla product is in use: Firefox, Firefox ESR, or Thunderbird
- 2. Check current version to confirm it is affected (Firefox < 88.0, Firefox ESR < 78.10, or Thunderbird < 78.10)
- 3. For Firefox: Upgrade to version 88.0 or later
- 4. For Firefox ESR: Upgrade to version 78.10 or later
- 5. For Thunderbird: Upgrade to version 78.10 or later
- 6. Restart the application after upgrading
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,984.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-23995 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-23995 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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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.
- 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
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