dbcve.org
FedoraOperating system · Fedoraproject

CVE-2016-7943

CRITICAL · 9.8 CVSS v3.0 Published 2016-12-13
Fix available
A fix is available. Upgrade to after 1.6.3 or later.
See remediation →
100/100
Remediation priority · Urgent
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
The XListFonts function in X.org libX11 before 1.6.4 might allow remote X servers to gain privileges via vectors involving length fields, which trigger out-of-bounds write operations.

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 XListFonts function in libX11 before version 1.6.4 contains a vulnerability where malicious length fields in font name data can trigger out-of-bounds write operations. This allows a remote X server (or an attacker impersonating one) to write data beyond allocated buffer boundaries, potentially achieving code execution with elevated privileges on the client system.

MitigationUpdate libX11 to version 1.6.4 or later. If immediate patching is not feasible, restrict X11 communication to trusted X servers only and disable font serving from untrusted sources.

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
FedoraOperating system
Affected:= 25
Libx11Application
Affected:<= 1.6.3

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
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/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 libX11 version
    Run 'rpm -q libX11' on Fedora systems, or check the library file version with 'ldd --version' or look at the package manager output. On other Linux distributions, use the native package manager (dpkg, yum, etc.) to query the libX11 package version.
    Affected if The installed version is 1.6.3 or earlier, or the version cannot be determined (indicating an older installation).
  2. Verify X11 font handling is in use
    Check if any X11 applications that call XListFonts are running or configured. Look for processes using libX11 by running 'lsof | grep libX11' or checking active X client connections with 'xlsclients'. The vulnerability triggers when the client requests font lists from an X server.
    Affected if X11 applications that query font information from X servers are in use, particularly when connecting to network-accessible X servers.
  3. Check for untrusted X server connections
    Review the X11 environment variable DISPLAY to identify the connected X server. Examine /etc/X11/xorg.conf or run 'xset q' to see the current X server configuration. Determine if connections to untrusted or network-accessible X servers are possible.
    Affected if The client system connects to X servers that are not explicitly trusted, or X11 forwarding is enabled to untrusted remote systems.

A system is affected if it runs a libX11 version <= 1.6.3 and utilizes X11 applications that communicate with X servers (particularly untrusted or remote ones) where malicious font name length data could be received.

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 a release after 1.6.3
Interim mitigation

Update libX11 to version 1.6.4 or later. If immediate patching is not feasible, restrict X11 communication to trusted X servers only and disable font serving from untrusted sources.

Recommended fix High confidence

libX11 version 1.6.4 or later (Fedora 25 should receive updates via 'dnf update' or 'dnf update --security')

  1. Check current libX11 version using: rpm -qa | grep libX11
  2. Update libX11 package to version 1.6.4 or later using: dnf update libX11
  3. Alternatively, apply all pending security updates: dnf update --security
  4. Restart any running X servers or applications that use libX11 to ensure the patched library is loaded

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

Fix this in Fedora Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA3.0 h
21.0 hours of engineering $3,680
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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-2016-7943 in production — separate from our analysis above.

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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