FirefoxWeb browser · Mozilla
CVE-2020-26961
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 DNS over HTTPS is in use, it intentionally filters RFC1918 and related IP ranges from the responses as these do not make sense coming from a DoH resolver. However when an IPv4 address was mapped through IPv6, these addresses were erroneously let through, leading to a potential DNS Rebinding attack. 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 confidenceWhen DNS over HTTPS (DoH) is enabled, Firefox and Thunderbird are supposed to filter out RFC1918 private IP addresses (like 192.168.x.x, 10.x.x.x, 172.16.x.x) from responses since these addresses shouldn't come from a public DoH resolver. However, a logic error allowed IPv4 addresses that were mapped through IPv6 format (e.g., ::ffff:192.168.1.1) to bypass this filter. This enables DNS Rebinding attacks where an attacker can use DoH to deliver private IP addresses to the browser, potentially bypassing network isolation or access controls.
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
- None
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
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 if DNS over HTTPS (DoH) is enabledIn Firefox, open about:config and search for 'network.trr.mode'. If set to 2 or 3, DoH is enabled. Alternatively, check Settings > Privacy & Security and look for 'Enable DNS over HTTPS' under the Network Settings section.Affected if DoH is enabled (network.trr.mode is 2 or 3, or the DNS over HTTPS option is checked in settings)
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Check installed Firefox versionOpen a new tab and navigate to about:support, or go to Help > About Firefox. The version number is displayed at the top.Affected if Version is less than 83.0 (e.g., 82.x, 81.x, etc.)
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Check installed Firefox ESR versionOpen a new tab and navigate to about:support, or go to Help > About Firefox. Look for the ESR designation and version number.Affected if Version is less than 78.5 (e.g., 78.4.x or earlier)
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Check installed Thunderbird versionGo to Help > About Thunderbird. The version number is displayed in the window that opens.Affected if Version is less than 78.5 (e.g., 78.4.x or earlier)
User is affected only if DoH is enabled AND the installed version of Firefox (non-ESR < 83.0), Firefox ESR (< 78.5), or Thunderbird (< 78.5) falls within the vulnerable range.
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 or later, Firefox ESR 78.5 or later, or Thunderbird 78.5 or later. These versions contain the fix that properly filters IPv6-mapped IPv4 addresses in DoH responses.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation4.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-26961 — 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-2020-26961 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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- Version or environment caveats, and links to real fixes
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