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FirefoxWeb browser · Mozilla

CVE-2023-6856

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
The WebGL `DrawElementsInstanced` method was susceptible to a heap buffer overflow when used on systems with the Mesa VM driver. This issue could allow an attacker to perform remote code execution and sandbox escape. 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

The WebGL DrawElementsInstanced method contains a heap buffer overflow vulnerability when running on systems with the Mesa VM driver. This memory corruption in the graphics rendering path can be triggered remotely and allows attackers to escape browser sandboxing and achieve remote code execution.

MitigationUpgrade affected Firefox installations to version 121+ and Thunderbird/Firefox ESR to 115.6+ to obtain the patched version, or apply Mesa VM driver security updates if upgrading the browser is not immediately feasible.

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. Check Mozilla Firefox version
    Open Firefox, navigate to about:firefox, or run firefox --version from command line, or check package manager (dpkg -l firefox-esr or rpm -q firefox)
    Affected if Version is earlier than 121.0 (for Firefox) or earlier than 115.6 (for Firefox ESR)
  2. Check Mozilla Thunderbird version
    Open Thunderbird, navigate to Help > About Thunderbird, or run thunderbird --version from command line, or check package manager (dpkg -l thunderbird or rpm -q thunderbird)
    Affected if Version is earlier than 115.6
  3. Confirm WebGL is enabled in the browser
    In Firefox address bar, type about:config and search for webgl.disabled; verify the value is false (meaning WebGL is enabled)
    Affected if WebGL is enabled (webgl.disabled is false or not set to true)
  4. Verify Mesa VM driver is in use
    Check graphics driver information: on Linux run glxinfo | grep -i 'OpenGL renderer' or review browser console (navigator.gpu) for WebGL renderer string containing 'llvmpipe' or 'softpipe'
    Affected if The renderer shows Mesa with software virtualization (llvmpipe, softpipe, or swrast)
  5. Check Debian system version (if applicable)
    Run cat /etc/debian_version or lsb_release -a to identify Debian release
    Affected if Running Debian 10 (Buster), 11 (Bullseye), or 12 (Bookworm) with vulnerable Firefox/Thunderbird packages installed

The environment is affected if a vulnerable Firefox version (<121.0 or <115.6 ESR) or Thunderbird version (<115.6) is installed AND WebGL is enabled, particularly on systems using Mesa software rendering drivers.

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

Upgrade affected Firefox installations to version 121+ and Thunderbird/Firefox ESR to 115.6+ to obtain the patched version, or apply Mesa VM driver security updates if upgrading the browser is not immediately feasible.

Recommended fix High confidence

Firefox 121.0, Firefox ESR 115.6, Thunderbird 115.6

  1. Check current Firefox version by navigating to about:support in the browser
  2. Check current Thunderbird version by navigating to Help > About Thunderbird
  3. For Firefox: Download and install version 121.0 or later from mozilla.org
  4. For Firefox ESR: Download and install version 115.6 or later from mozilla.org
  5. For Thunderbird: Download and install version 115.6 or later from mozilla.org
  6. On Debian systems, run: apt update && apt upgrade (to receive security updates for Firefox/Thunderbird packages)
  7. Restart the browser/application after upgrade

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

Fix this in Firefox Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing3.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,210
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,936.

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-2023-6856 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