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FirefoxWeb browser · Mozilla

CVE-2023-6859

HIGH · 8.8 CVSS v3.1 Published 2023-12-19
Fix available
A fix is available. Upgrade to 115.6 / 121.0 or later.
See remediation →
95/100
Remediation priority · Urgent
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
A use-after-free condition affected TLS socket creation when under memory pressure. This vulnerability affects Firefox ESR < 115.6, Thunderbird < 115.6, and Firefox < 121.

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 use-after-free vulnerability in the TLS socket creation code that is triggered under memory pressure conditions, allowing potential arbitrary code execution or crash in affected Firefox and Thunderbird versions.

MitigationUpdate affected installations to Firefox 121+, Firefox ESR 115.6+, or Thunderbird 115.6+ to obtain the patched versions.

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:< 121.0
Firefox EsrWeb browser
Affected:< 115.6
Debian LinuxOperating system
Affected:= 10.0= 11.0= 12.0
ThunderbirdApplication
Affected:< 115.6

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
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 checks

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

  1. Identify installed Mozilla product
    Open the application and navigate to Help > About (Firefox or Thunderbird). On Linux, check the package with commands like 'dpkg -l | grep -i firefox' or 'rpm -qa | grep -i firefox'. On Windows, check Add/Remove Programs or the executable properties.
    Affected if The system has Firefox, Firefox ESR, or Thunderbird installed
  2. Determine installed version number
    Note the version shown in Help > About. Compare it to the affected ranges: Firefox < 121.0, Firefox ESR < 115.6, Thunderbird < 115.6. On Linux, use 'firefox --version', 'thunderbird --version', or check the package version.
    Affected if The installed version is lower than 121.0 for Firefox, lower than 115.6 for Firefox ESR, or lower than 115.6 for Thunderbird
  3. Verify TLS/SSL usage
    The vulnerability exists in TLS socket creation code. TLS is enabled by default for all secure web connections (HTTPS) and secure email connections (IMAPS, POP3S, SMTPS). Check browser history for HTTPS sites or check email account settings for encrypted connections.
    Affected if TLS/SSL is actively being used, which is the default for modern web browsing and secure email communication

You are affected if you have Firefox below 121.0, Firefox ESR below 115.6, or Thunderbird below 115.6, and you use TLS/SSL functionality (which includes most web browsing and secure email).

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 115.6 / 121.0 or later
Fixed in 115.6121.0
Interim mitigation

Update affected installations to Firefox 121+, Firefox ESR 115.6+, or Thunderbird 115.6+ to obtain the patched versions.

Recommended fix High confidence

Firefox 121.0, Firefox ESR 115.6, Thunderbird 115.6

  1. For Firefox: Upgrade to version 121.0 or later by visiting https://www.mozilla.org/firefox/ or using your system's software update mechanism
  2. For Firefox ESR: Upgrade to version 115.6 or later from https://www.mozilla.org/firefox/enterprise/
  3. For Thunderbird: Upgrade to version 115.6 or later from https://www.thunderbird.net/
  4. For Debian systems: Run 'apt update && apt upgrade' to install the updated packages from Debian security repositories
Caveat Standard version upgrade; ensure your system meets Firefox 121 or Thunderbird 115 minimum system requirements

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-2023-6859 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