dbcve.org
FreeBSDOperating system

CVE-2016-6559

CRITICAL · 9.8 CVSS v3.0 Published 2018-07-13
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
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
Improper bounds checking of the obuf variable in the link_ntoa() function in linkaddr.c of the BSD libc library may allow an attacker to read or write from memory. The full impact and severity depends on the method of exploit and how the library is used by applications. According to analysis by FreeBSD developers, it is very unlikely that applications exist that utilize link_ntoa() in an exploitable manner, and the CERT/CC is not aware of any proof of concept. A blog post describes the functionality of link_ntoa() and points out that none of the base utilities use this function in an exploitable manner. For more information, please see FreeBSD Security Advisory SA-16:37.

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

Improper bounds checking of the obuf variable in the link_ntoa() function in linkaddr.c of BSD libc allows memory read/write. However, FreeBSD developers analysis indicates it is very unlikely applications use this function in an exploitable manner, and no proof of concept exists.

MitigationApply the FreeBSD Security Advisory SA-16:37 patch to BSD libc. If unable to patch, audit any custom applications using link_ntoa() for potential untrusted input paths.

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
FreeBSDOperating system
Affected:= 9.3= 10.1= 10.2= 10.3= 11.0

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
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. Check FreeBSD version
    Run 'uname -a' or 'freebsd-version -u' to identify the installed FreeBSD release
    Affected if The version matches 9.3, 10.1, 10.2, 10.3, or 11.0
  2. Identify usage of link_ntoa function
    Search application source code and compiled binaries for calls to link_ntoa using 'grep -r link_ntoa /path/to/code' or 'nm -g libc.so.7 | grep link_ntoa'
    Affected if Any custom or third-party application invokes link_ntoa
  3. Audit input sources to link_ntoa
    Review code paths that pass data to link_ntoa and determine if the input originates from untrusted sources such as network packets, user input, or external files
    Affected if link_ntoa receives input that could be controlled by an untrusted party
  4. Verify patch status
    Check if the FreeBSD Security Advisory SA-16:37 patch has been applied by examining the linkaddr.c source or libc version; compare file timestamps against the patch release date
    Affected if The patch has not been applied and the system is running an affected version

A system is affected if it runs FreeBSD 9.3, 10.1, 10.2, 10.3, or 11.0, has unpatched BSD libc, and runs applications that use link_ntoa with potentially untrusted input.

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
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Apply the FreeBSD Security Advisory SA-16:37 patch to BSD libc. If unable to patch, audit any custom applications using link_ntoa() for potential untrusted input paths.

Recommended fix Moderate confidence

FreeBSD 10.4+ or 11.x (depending on support lifecycle); FreeBSD 9.4 for 9.x branch

  1. 1. Identify the current FreeBSD version using 'uname -a' or 'freebsd-version'
  2. 2. For FreeBSD 9.x systems: Upgrade to FreeBSD 9.4 or migrate to a supported release (10.4 or later)
  3. 3. For FreeBSD 10.x systems: Upgrade to FreeBSD 10.4 or later (11.x recommended if 10.x EOL)
  4. 4. Backup all critical data before performing the upgrade
  5. 5. Review the FreeBSD Handbook upgrading documentation at https://www.freebsd.org/doc/handbook/updating-upgrading.html
  6. 6. For production systems, test the upgrade in a non-production environment first
  7. 7. Apply the upgrade using 'freebsd-update fetch' and 'freebsd-update install' or by rebuilding from source
Caveat Upgrading across major FreeBSD versions (e.g., 9.x to 10.x or 10.x to 11.x) may introduce compatibility changes with third-party software and require rebuild of ports/packages

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

Fix this in FreeBSD Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,060
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Scan for this in your stack

Free · runs locally
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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-2016-6559 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