FirefoxWeb browser · Mozilla
CVE-2020-26959
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 · uneditedDuring browser shutdown, reference decrementing could have occured on a previously freed object, resulting in a use-after-free, memory corruption, and a potentially exploitable crash. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
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 use-after-free vulnerability in Mozilla Firefox and Thunderbird where reference decrementing occurs on a previously freed object during browser shutdown, leading to memory corruption and 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
< 83.0
Firefox EsrWeb browser
< 78.5< 78.5CVSS 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 for presence of Firefox or Thunderbird on the system. Look for executable files or installed applications named 'Firefox' or 'Thunderbird' in typical installation directories (e.g., C:\Program Files\Mozilla Firefox or /Applications/Firefox on macOS/Linux).Affected if Either Firefox or Thunderbird is installed on the system
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Determine Firefox versionOpen Firefox, click the menu button (three horizontal lines), select Help, then About Firefox. The version number is displayed in the window that opens.Affected if Firefox version is less than 83.0
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Determine Firefox ESR versionOpen Firefox ESR, click the menu button, select Help, then About Firefox. The version displayed will show 'ESR' alongside the version number.Affected if Firefox ESR version is less than 78.5
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Determine Thunderbird versionOpen Thunderbird, click the menu button (three horizontal lines or Application menu), select Help, then About Thunderbird. The version number is displayed in the window that opens.Affected if Thunderbird version is less than 78.5
The environment is affected if Mozilla Firefox (any channel) below version 83.0, Firefox ESR below version 78.5, or Thunderbird below version 78.5 is installed and the browser Thunderbird is closed or shutting down, triggering the use-after-free condition.
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 data78.583.0
Upgrade to Firefox 83+, Firefox ESR 78.5+, or Thunderbird 78.5+ to obtain the patched version.
Fix this in Firefox
Scoped from the published advisory
- Consultation1.0 h
- Implementation1.0 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 $1,376.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-26959 — 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-2020-26959 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
- No spam, self-promotion, credentials, or personal data