CVE-2009-0021
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 · uneditedNTP 4.2.4 before 4.2.4p5 and 4.2.5 before 4.2.5p150 does not properly check the return value from the OpenSSL EVP_VerifyFinal function, which allows remote attackers to bypass validation of the certificate chain via a malformed SSL/TLS signature for DSA and ECDSA keys, a similar vulnerability to CVE-2008-5077.
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 confidenceNTP versions 4.2.4 before 4.2.4p5 and 4.2.5 before 4.2.5p150 fail to properly check the return value from OpenSSL's EVP_VerifyFinal function when validating SSL/TLS certificate signatures. This allows remote attackers to bypass certificate chain validation for DSA and ECDSA keys by sending malformed signatures that should be rejected but are instead accepted due to missing error checking.
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<= 4.2.4p4= 4.2.0= 4.2.2= 4.2.4p1= 4.2.4p2= 4.2.4p3CVSS 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
- Authentication
- None
- Confidentiality
- Partial
- Integrity
- None
- Availability
- None
AV:N/AC:L/Au:N/C:P/I:N/A:N
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 checksWork through these to decide whether this CVE applies to you.
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Identify if NTP is installedRun 'ntpd --version' or check your system's package manager for the ntp packageAffected if NTP is not installed, this CVE does not apply
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Determine the installed NTP versionRun 'ntpd --version' or 'ntpdc -version' and note the full version string (e.g., 4.2.4p2)Affected if The version is 4.2.4p4 or earlier, or 4.2.5p149 or earlier, meaning it falls within the vulnerable range
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Check if NTP autokey authentication is enabledInspect the NTP configuration file (typically /etc/ntp.conf) for the 'crypto' directive and ensure 'enable auth' is presentAffected if The 'crypto' directive is present and uncommented, enabling public key authentication with certificate validation
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Verify DSA or ECDSA keys are in useCheck for NTP keys in /etc/ntp/keys or /usr/local/etc/ntp/keys, and look for .pub or .priv key files in the NTP keys directory; also check ntp.conf for 'server' entries using autokey ('keyid' with public key mode)Affected if DSA or ECDSA cryptographic keys are configured for NTP authentication, as the vulnerability specifically bypasses validation for these key types
You are affected if NTP is installed with a version in the vulnerable range (4.2.4p4 or earlier, or 4.2.5p149 or earlier) AND autokey authentication with DSA or ECDSA keys is enabled in the configuration.
Generated from the published advisory. Verify against your own configuration.
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.
From vendor dataUpgrade to NTP 4.2.4p5, 4.2.5p150, or later versions which correctly validate the return value from EVP_VerifyFinal before accepting certificate signatures.
- Consultation3.0 h
- Implementation2.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,072.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2009-0021 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sources- secunia.com
- secunia.com
- secunia.com
- secunia.com
- secunia.com
- www.vupen.com
- www.vupen.com
- lists.apple.com
- lists.opensuse.org
- lists.opensuse.org
- slackware.com
- support.apple.com
- www.ocert.org
- www.redhat.com
- www.securityfocus.com
- www.securitytracker.com
- www.us-cert.gov
- lists.ntp.org
- oval.cisecurity.org
- nvd.nist.gov
Practitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2009-0021 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- 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
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