FirefoxWeb browser · Mozilla
CVE-2021-43535
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 · uneditedA use-after-free could have occured when an HTTP2 session object was released on a different thread, leading to memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 93, 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 confidenceA use-after-free vulnerability in Mozilla Firefox and Thunderbird where an HTTP2 session object is released on a different thread than where it was used, leading to memory corruption and a potentially exploitable crash. The race condition occurs during HTTP2 session object lifecycle management across threads.
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
< 93.0
Firefox EsrWeb browser
< 91.3.0
Debian LinuxOperating system
= 9.0= 10.0= 11.0< 91.3.0CVSS 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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Check installed Firefox versionOpen Firefox, go to Menu > Help > About Firefox, or run: firefox --versionAffected if Version is below 93.0 (or below 91.3.0 for ESR)
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Check installed Thunderbird versionOpen Thunderbird, go to Menu > Help > About Thunderbird, or run: thunderbird --versionAffected if Version is below 91.3.0
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Check HTTP/2 protocol support is enabledIn Firefox address bar, navigate to about:config and search for network.http.spdy.enabled.http2 - verify it is set to trueAffected if HTTP/2 is enabled (true) and the browser version is vulnerable
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Check for HTTP/2 related crash logsOn Linux, check ~/.mozilla/firefox/crashes/ or ~/.thunderbird/crashes/ for recent crash reports mentioning HTTP2 or HTTP/2Affected if Crash logs exist and reference HTTP/2 session handling in vulnerable version ranges
A user is affected if they run Firefox below 93.0 (or ESR below 91.3.0) or Thunderbird below 91.3.0 with HTTP/2 enabled.
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 · scoped91.3.093.0
Upgrade to Firefox 93+, Thunderbird 91.3+, or Firefox ESR 91.3+ to patch this vulnerability. Prioritize systems with external network exposure.
Firefox 93.0+, Firefox ESR 91.3.0+, Thunderbird 91.3.0+
- Upgrade Firefox to version 93.0 or later
- Upgrade Firefox ESR to version 91.3.0 or later
- Upgrade Thunderbird to version 91.3.0 or later
- For Debian systems, apply the latest security updates via 'apt update && apt upgrade' or install the updated packages from Debian security advisories
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation4.0 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 $2,560.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-43535 — 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-2021-43535 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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