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FirefoxWeb browser · Mozilla

CVE-2019-9793

MEDIUM · 5.9 CVSS v3.0 Published 2019-04-26
Fix available
A fix is available. Upgrade to 60.6 / 66.0 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

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
A mechanism was discovered that removes some bounds checking for string, array, or typed array accesses if Spectre mitigations have been disabled. This vulnerability could allow an attacker to create an arbitrary value in compiled JavaScript, for which the range analysis will infer a fully controlled, incorrect range in circumstances where users have explicitly disabled Spectre mitigations. *Note: Spectre mitigations are currently enabled for all users by default settings.*. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.

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

A vulnerability in Firefox/Thunderbird's JavaScript engine removes certain bounds checking for string, array, or typed array accesses when Spectre mitigations are disabled. This allows attackers to create arbitrary values in compiled JavaScript with incorrectly inferred ranges, potentially enabling memory corruption or information disclosure attacks.

MitigationEnsure Spectre mitigations remain enabled (default setting) and update to Firefox 66+, Firefox ESR 60.6+, or Thunderbird 60.6+. Organizations should verify that browser configurations have not had mitigations manually disabled.

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:< 66.0
Firefox EsrWeb browser
Affected:< 60.6
ThunderbirdApplication
Affected:< 60.6

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
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.0/AV:N/AC:H/PR:N/UI:N/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, click the menu button (three horizontal lines), then Help > About Firefox. Note the version number displayed.
    Affected if The version is below 66.0 (or below 60.6 for ESR)
  2. Identify installed Thunderbird version
    Open Thunderbird, click the menu button, then Help > About Thunderbird. Note the version number displayed.
    Affected if The version is below 60.6
  3. Check Spectre mitigation setting in Firefox
    Type about:config in the address bar, press Enter, accept the warning. Search for the key named 'javascript.options.ion.baseline' or look for any config entries related to Spectre/variant1 mitigation.
    Affected if The mitigation is explicitly disabled (values set to false) rather than using default settings
  4. Check Spectre mitigation setting in Thunderbird
    Type about:config in the Thunderbird address bar, accept the warning. Search for configuration keys that control JIT compilation or Spectre-related mitigations.
    Affected if Mitigation settings have been manually disabled (non-default values)
  5. Verify browser is used to access untrusted content
    Confirm that the Firefox or Thunderbird installation is used for browsing web content or rendering HTML emails with JavaScript enabled.
    Affected if JavaScript execution is enabled and the browser processes untrusted web content

A user is affected if they run Firefox below 66.0 (or ESR below 60.6) or Thunderbird below 60.6 AND have manually disabled Spectre mitigations in about:config, as the default protected configuration is not vulnerable.

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.

From vendor data
Upgrade available Upgrade to 60.6 / 66.0 or later
Fixed in 60.666.0
Interim mitigation

Ensure Spectre mitigations remain enabled (default setting) and update to Firefox 66+, Firefox ESR 60.6+, or Thunderbird 60.6+. Organizations should verify that browser configurations have not had mitigations manually disabled.

Fix this in Firefox Scoped from the published advisory
  • Consultation4.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA2.0 h
11.0 hours of engineering $2,000
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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-2019-9793 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