FirefoxWeb browser · Mozilla
CVE-2020-26974
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 flex-basis was used on a table wrapper, a StyleGenericFlexBasis object could have been incorrectly cast to the wrong type. This resulted in a heap user-after-free, memory corruption, and a potentially exploitable crash. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.
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 confidenceA type confusion vulnerability in Firefox's style system allows incorrect casting of a StyleGenericFlexBasis object when flex-basis CSS property is used on a table wrapper. This causes heap user-after-free and memory corruption, leading to potentially exploitable crashes.
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
< 84.0
Firefox EsrWeb browser
< 78.6.0< 78.6.0CVSS 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 installed Mozilla productCheck if Firefox, Firefox ESR, or Thunderbird is installed by looking for the application in /Applications (macOS), Start Menu (Windows), or package manager (Linux). Common paths: /usr/bin/firefox, /usr/bin/thunderbird, or check about:support in the browser.Affected if Any of these three products is installed
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Determine Firefox versionOpen Firefox, go to Menu > Help > About Firefox, or enter 'about:support' in the address bar. The version number is displayed at the top.Affected if Version is less than 84.0 (for example, 83.x or earlier)
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Determine Firefox ESR versionOpen Firefox ESR, go to Menu > Help > About Firefox ESR, or enter 'about:support' in the address bar. The version displayed will show 'ESR' in the version string.Affected if Version is less than 78.6.0 (for example, 78.5.x or earlier ESR release)
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Determine Thunderbird versionOpen Thunderbird, go to Menu > Help > About Thunderbird, or enter 'about:support' in the address bar. The version number is displayed at the top.Affected if Version is less than 78.6.0 (for example, 78.5.x or earlier)
The environment is affected if Firefox (any channel) below 84.0, Firefox ESR below 78.6.0, or Thunderbird below 78.6.0 is installed and the browser is used to render web content with the flex-basis CSS property on table elements.
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.
From vendor data78.6.084.0
Upgrade affected products to Firefox 84+, Thunderbird 78.6+, or Firefox ESR 78.6+ to patch the type confusion and memory safety issues.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 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 $1,664.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-26974 — 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-2020-26974 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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