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FirefoxWeb browser · Mozilla

CVE-2021-38507

MEDIUM · 6.5 CVSS v3.1 Published 2021-12-08
Fix available
A fix is available. Upgrade to 91.3.0 / 94.0 or later.
See remediation →
72/100
Remediation priority · Elevated
Remotely reachable No privileges

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 · unedited
The Opportunistic Encryption feature of HTTP2 (RFC 8164) allows a connection to be transparently upgraded to TLS while retaining the visual properties of an HTTP connection, including being same-origin with unencrypted connections on port 80. However, if a second encrypted port on the same IP address (e.g. port 8443) did not opt-in to opportunistic encryption; a network attacker could forward a connection from the browser to port 443 to port 8443, causing the browser to treat the content of port 8443 as same-origin with HTTP. This was resolved by disabling the Opportunistic Encryption feature, which had low usage. This vulnerability affects Firefox < 94, 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 confidence

The Opportunistic Encryption feature in HTTP2 (RFC 8164) allows transparent TLS upgrade while maintaining same-origin properties with unencrypted port 80 connections. An attacker on the same network can forward traffic from port 443 to a second encrypted port (e.g., 8443) that does not opt-in to opportunistic encryption, causing the browser to incorrectly treat the port 8443 content as same-origin with HTTP, enabling potential data theft or manipulation.

MitigationUpdate affected products to Firefox 94+, Thunderbird 91.3+, or Firefox ESR 91.3+ which disable the Opportunistic Encryption feature. Alternatively, if updates are not possible, consider network-level controls to prevent unauthorized traffic forwarding to internal encrypted services.

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
Affected:< 94.0
Firefox EsrWeb browser
Affected:< 91.3.0
Debian LinuxOperating system
Affected:= 9.0= 10.0= 11.0
ThunderbirdApplication
Affected:< 91.3.0

CVSS 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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Firefox version
    Open Firefox, go to Help > About Firefox, or run 'firefox --version' from command line
    Affected if Version is below 94.0 (or below 91.3.0 for ESR)
  2. Identify installed Thunderbird version
    Open Thunderbird, go to Help > About Thunderbird, or run 'thunderbird --version' from command line
    Affected if Version is below 91.3.0
  3. Check for Opportunistic Encryption configuration
    In Firefox address bar, type 'about:config' and press Enter. Search for settings related to HTTP/2 opportunistic encryption or HTTP upgrade. Look for any boolean preference that enables this feature.
    Affected if Any HTTP/2 opportunistic encryption or upgrade-related preference is set to true (enabled)
  4. Verify browser is used in environment with unencrypted HTTP
    Check if the browser routinely connects to HTTP (port 80) sites that could be exploited for same-origin confusion
    Affected if Browser regularly accesses unencrypted HTTP sites alongside encrypted services on alternate ports

A user is affected if they run Firefox below 94.0 (or ESR/Thunderbird below 91.3.0) and the Opportunistic Encryption feature is enabled in the browser configuration.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 91.3.0 / 94.0 or later
Fixed in 91.3.094.0
Interim mitigation

Update affected products to Firefox 94+, Thunderbird 91.3+, or Firefox ESR 91.3+ which disable the Opportunistic Encryption feature. Alternatively, if updates are not possible, consider network-level controls to prevent unauthorized traffic forwarding to internal encrypted services.

Recommended fix High confidence

Firefox 94.0, Firefox ESR 91.3.0, or Thunderbird 91.3.0

  1. Upgrade Firefox to version 94.0 or later
  2. Upgrade Firefox ESR to version 91.3.0 or later
  3. Upgrade Thunderbird to version 91.3.0 or later

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
9.0 hours of engineering $1,600
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,560.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-38507 in production — separate from our analysis above.

No notes yet

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What this is

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.

What belongs here
  • 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