# H2O affected by a deserialization vulnerability

Advisory record · `GHSA-5w3j-gwgh-4rfv`

- **Identifiers:** `GHSA-5w3j-gwgh-4rfv` · `CVE-2025-6544` · `PYSEC-2026-349`
- **State:** Confirmed
- **Severity:** CVSS 9.8 Critical
- **Critical:** Yes: CVSS 9+
- **Weakness (CWE):** [CWE-502](https://cwe.mitre.org/data/definitions/502.html)
- **Tags:** [Insecure deserialization](https://agentprime.io/advisories/tag/deserialization.md)
- **Published:** 22 Sept 2025
- **Updated:** 27 Oct 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 22 Sept 2025, 18:30 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Critical

Critical, because:

- CVSS score 9.8: 9.0 or more.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected packages

- `ai.h2o:h2o-core` (Maven): affected `<=3.46.0.7`; no fixed version yet
- `h2o` (PyPI): affected `<=3.46.0.7`; no fixed version yet

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> The H2O package for Maven (ai.h2o:h2o-core) and PyPI (h2o) is vulnerable in versions up to and including 3.46.0.7, due to a deserialization flaw in how JDBC connection parameters are handled. The source says an attacker can bypass regular expression checks using double URL encoding, which can allow reading arbitrary system files and executing arbitrary code. No fixed version is listed, so the source gives no upgrade path; check the advisory for any mitigation guidance.

Written 31 Oct 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 9.8 Critical, CVSS 3.0: `CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`, from CNA (@huntr_ai)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0095, the 61st percentile, on 13 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:07 UTC: First seen by agentprime
- 31 Oct 2026, 14:07 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-5w3j-gwgh-4rfv)

## Sources

- [GitHub Advisory Database: GHSA-5w3j-gwgh-4rfv](https://github.com/advisories/GHSA-5w3j-gwgh-4rfv)
- [OSV: GHSA-5w3j-gwgh-4rfv](https://osv.dev/vulnerability/GHSA-5w3j-gwgh-4rfv)
- <https://nvd.nist.gov/vuln/detail/CVE-2025-6544>
- <https://github.com/h2oai/h2o-3/commit/0298ee348f5c73673b7b542158081e79605f5f25>
- <https://github.com/h2oai/h2o-3>
- <https://github.com/pypa/advisory-database/tree/main/vulns/h2o/PYSEC-2026-349.yaml>
- <https://huntr.com/bounties/53f35a0f-d644-4f82-93aa-89fe7e0aed40>
- <https://pypi.org/project/h2o>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-5w3j-gwgh-4rfv>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-5w3j-gwgh-4rfv/>. Everything for agents: <https://agentprime.io/llms.txt>.