dbcve.org
H2oApplication

CVE-2025-6544

CRITICAL · 9.8 CVSS v3.0 Published 2025-09-21
Fix available
A fix is available. Upgrade to after 3.46.0.8 or later.
See remediation →
100/100
Remediation priority · Urgent
Public exploit Remotely reachable No privileges Zero-click Patch available

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
A deserialization vulnerability exists in h2oai/h2o-3 versions <= 3.46.0.8, allowing attackers to read arbitrary system files and execute arbitrary code. The vulnerability arises from improper handling of JDBC connection parameters, which can be exploited by bypassing regular expression checks and using double URL encoding. This issue impacts all users of the affected versions.

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

A deserialization vulnerability in H2O-3 versions <= 3.46.0.8 allows attackers to read arbitrary system files and execute arbitrary code through the JDBC connection parameter handling. The flaw stems from improper input validation that can be bypassed using double URL encoding to circumvent regex checks.

MitigationUpgrade H2O-3 to a version beyond 3.46.0.8, or implement strict input validation that rejects double URL encoding patterns in JDBC connection parameters.

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
H2oApplication
Affected:>= 3.0.0.2, <= 3.46.0.8

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 H2O-3 version
    Locate and inspect the H2O-3 version information in your deployment. This is typically visible in the H2O Flow web UI startup banner, in the H2O jar file name, or via the /3/Version REST API endpoint (GET http://<host>:54321/3/Version). Compare your version against the affected range: 3.0.0.2 through 3.46.0.8 inclusive.
    Affected if Your installed H2O-3 version falls within 3.0.0.2 to 3.46.0.8.
  2. Confirm JDBC interface exposure
    Determine if your H2O-3 instance is configured to accept JDBC connections. Check whether the H2O JDBC driver port (default 54321 or custom-configured) is exposed to network traffic or accessible by untrusted users or applications.
    Affected if The H2O JDBC interface is network-accessible or usable by untrusted actors who can supply connection parameters.
  3. Review JDBC connection parameter handling
    Examine how your applications or users connect to H2O via JDBC. Identify whether connection parameters passed to H2O are derived from user input or external sources without sanitization.
    Affected if User-supplied or externally-sourced JDBC connection parameters can be passed to H2O without validation.
  4. Inspect for double URL encoding patterns
    Review HTTP access logs or JDBC connection request logs for patterns indicating double URL encoded input (e.g., %252F, %252E, or similar double-encoded sequences in connection parameter values).
    Affected if Double URL encoded sequences appear in JDBC connection parameter data, indicating potential exploitation attempts.

You are affected if your H2O-3 version is between 3.0.0.2 and 3.46.0.8 inclusive AND the JDBC interface is accessible to untrusted users who can supply connection parameters.

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 3.46.0.8
Vendor patch github.com →
Interim mitigation

Upgrade H2O-3 to a version beyond 3.46.0.8, or implement strict input validation that rejects double URL encoding patterns in JDBC connection parameters.

Recommended fix Moderate confidence

H2O-3 version 3.46.1.0 or later

  1. Identify the current H2O-3 version in use by checking the JAR file name or running `h2o.init()` and noting the version printed in the console output
  2. Stop all H2O instances and any running jobs to ensure a safe upgrade process
  3. Download the fixed H2O-3 version (version 3.46.1.0 or later) from the official H2O GitHub releases page: https://github.com/h2oai/h2o-3/releases
  4. Replace the existing h2o.jar with the new version
  5. Restart the H2O cluster with the same configuration parameters previously used
  6. Verify the upgrade by checking that the version number matches the expected fixed version
  7. Test that JDBC connections work correctly with the new version to ensure the vulnerability is remediated
Caveat Review release notes for version 3.46.1.0+ to check for any API or functionality changes that may affect existing workflows

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

Fix this in H2o Scoped from the published advisory
  • Consultation4.0 h
  • Implementation14.0 h
  • Testing6.0 h
  • Review / QA4.0 h
28.0 hours of engineering $4,940
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $7,904.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2025-6544 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2025-6544 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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