FirefoxWeb browser · Mozilla
CVE-2020-26971
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 · uneditedCertain blit values provided by the user were not properly constrained leading to a heap buffer overflow on some video drivers. 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 · moderate confidenceA heap buffer overflow vulnerability exists in Firefox and Thunderbird's graphics rendering where blit values (image transfer parameters) provided by users are not properly validated or constrained. This allows specially crafted blit operations to overflow heap memory on systems with affected video drivers.
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 or Thunderbird is installed on the system. In Windows, look in Program Files for Mozilla Firefox or Mozilla Thunderbird. On Linux, check /usr/lib/firefox or /usr/lib/thunderbird directories, or use package manager queries like dpkg -l or rpm -qa.Affected if Neither Firefox nor Thunderbird is installed - the system is not affected by this CVE.
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Determine Firefox versionFor Firefox, type about:support in the address bar or go to Help > Troubleshooting Information. The version is displayed as 'Firefox 84.0' or similar. On command line, run 'firefox --version' or check the executable properties.Affected if Firefox version is less than 84.0 (for standard releases) or less than 78.6.0 (for ESR releases).
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Determine Thunderbird versionFor Thunderbird, type about:support in the address bar or go to Help > Troubleshooting Information. The version is displayed as 'Thunderbird 78.x' or similar. On command line, run 'thunderbird --version' or check the executable properties.Affected if Thunderbird version is less than 78.6.0.
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Confirm ESR vs standard release for FirefoxIn Firefox, go to Help > About Firefox. The window displays either 'Firefox ESR' or just 'Firefox' along with the version number. This distinction matters because ESR versions differ from standard releases.Affected if Running Firefox ESR version less than 78.6.0, or standard Firefox version less than 84.0.
A system is affected if Mozilla Firefox (standard) < 84.0, Firefox ESR < 78.6.0, or Thunderbird < 78.6.0 is installed and the application uses graphics rendering with blit operations on affected video drivers.
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
Update to Firefox 84, Thunderbird 78.6, Firefox ESR 78.6, or later versions which contain the proper bounds checking for blit values.
Fix this in Firefox
Scoped from the published advisory
- Consultation4.0 h
- Implementation16.0 h
- Testing8.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $8,960.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-26971 — 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-2020-26971 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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- 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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