CVE-2025-21566
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 · uneditedVulnerability in the MySQL Server product of Oracle MySQL (component: Server: Optimizer). Supported versions that are affected are 9.1.0 and prior. Easily exploitable vulnerability allows low privileged attacker with network access via multiple protocols to compromise MySQL Server. Successful attacks of this vulnerability can result in unauthorized ability to cause a hang or frequently repeatable crash (complete DOS) of MySQL Server. CVSS 3.1 Base Score 6.5 (Availability impacts). CVSS Vector: (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).
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 confidenceA denial-of-service vulnerability in the MySQL Server Optimizer component allows low-privileged users with network access to cause MySQL Server to hang or experience repeatable crashes. The vulnerability affects MySQL versions 9.1.0 and prior.
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>= 9.0.0, <= 9.1.0CVSS 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
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/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 checksWork through these to decide whether this CVE applies to you.
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Determine installed MySQL versionRun `mysql --version` from command line, or connect to MySQL and execute `SELECT VERSION();`Affected if Version falls within 9.0.0 through 9.1.0 (inclusive)
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Verify network listener statusCheck if MySQL is configured to accept remote connections by reviewing the bind-address parameter in the my.cnf or my.ini configuration file, or by using netstat or ss to confirm MySQL is listening on a non-loopback interfaceAffected if MySQL accepts connections on network interfaces reachable by remote hosts (not bound to 127.0.0.1 only)
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Confirm existence of network-accessible user accountsQuery the mysql.user table with `SELECT user, host FROM mysql.user WHERE host NOT IN ('localhost','127.0.0.1');` to identify accounts that can connect from remote hostsAffected if Any user accounts exist with network host access (non-localhost)
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Check for low-privilege accounts with network accessReview the privileges of network-accessible accounts using `SHOW GRANTS FOR 'username'@'host';` for each remote-access user identifiedAffected if Low-privileged users (non-admin, non-root equivalent) have network access to the MySQL server
Environment is affected if MySQL version is between 9.0.0 and 9.1.0 and the server is accessible over the network to low-privileged users, allowing them to trigger the Optimizer component vulnerability
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 dataApply Oracle's security patches for this vulnerability by upgrading to a patched MySQL version. As an interim measure, restrict network access to MySQL to trusted users only.
- Consultation2.0 h
- Implementation2.0 h
- Testing6.0 h
- Review / QA2.0 h
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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 sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-21566 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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- Version or environment caveats, and links to real fixes
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