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FirefoxWeb browser · Mozilla

CVE-2024-3855

MEDIUM · 6.5 CVSS v3.1 Published 2024-04-16
Fix available
A fix is available. Upgrade to 125.0 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
In certain cases the JIT incorrectly optimized MSubstr operations, which led to out-of-bounds reads. This vulnerability affects Firefox < 125.

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 · moderate confidence

A Just-In-Time (JIT) compiler flaw in Mozilla's SpiderMonkey engine incorrectly optimized MSubstr (substring) operations, causing the JavaScript engine to read memory beyond allocated buffer boundaries. This out-of-bounds read could expose sensitive memory contents to malicious web content.

MitigationUpgrade Firefox to version 125 or later. Organizations should ensure automatic browser updates are enabled or deploy the patched version through their endpoint management systems.

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:< 125.0

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
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/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 checks

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

  1. Confirm Firefox is installed
    Check for Firefox installation by looking in standard locations: Windows: C:\Program Files\Mozilla Firefox\firefox.exe, macOS: /Applications/Firefox.app, Linux: /usr/bin/firefox or check Start Menu/Application menu
    Affected if Firefox browser is present on the system
  2. Retrieve installed Firefox version
    Open Firefox and navigate to about:support, or run 'firefox --version' from command line, or right-click Firefox shortcut and select Properties to view file version
    Affected if Version number is lower than 125.0 (for example 124.x, 123.x, etc.)
  3. Verify JIT compiler is enabled
    Type 'about:config' in Firefox address bar, press Enter, accept warning, then search for 'javascript.options.jit' and check if the value is set to true
    Affected if javascript.options.jit is set to true (enabled) - the vulnerability only applies when JIT compilation is active
  4. Check SpiderMonkey JIT configuration
    In about:config, search for 'jit' related options such as 'jit.chrome' and 'jit.content' to confirm JIT is active for both browser chrome and web content
    Affected if JIT options are enabled (jit.chrome and/or jit.content are true)

System is affected if Firefox version is below 125.0 and JavaScript JIT compilation is enabled in the browser settings.

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 125.0 or later
Fixed in 125.0
Interim mitigation

Upgrade Firefox to version 125 or later. Organizations should ensure automatic browser updates are enabled or deploy the patched version through their endpoint management systems.

Recommended fix High confidence

Firefox 125.0

  1. 1. Back up any important data (bookmarks, passwords, etc.) from your current Firefox profile
  2. 2. Open Firefox and navigate to the menu (three horizontal lines in the top right)
  3. 3. Click 'Help' > 'About Firefox'
  4. 4. The about dialog will check for updates and show the current version
  5. 5. If updates are available, click 'Update to latest version' or wait for the automatic download to complete
  6. 6. Restart Firefox when prompted
  7. 7. Verify the version by going to Menu > Help > About Firefox - it should show version 125.0 or later

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Firefox Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,424.

Scan for this in your stack

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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-2024-3855 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