critical · 9.8 No patch yet

H2O affected by a deserialization vulnerability

CVE-2025-6544 · GHSA-5w3j-gwgh-4rfv · PYSEC-2026-349 · published 22 Sept 2025

How serious is it

  • Severity: critical, CVSS score 9.8 out of 10.
  • No exploit prediction (EPSS) score yet.
  • Not on the CISA list of exploited vulnerabilities.
  • No public exploit in Exploit-DB.

Is your app affected by CVE-2025-6544?

Paste this prompt into Claude Code, Cursor or any AI coding tool: it runs a free Shippable scan of your project and fixes what it finds. Built with Lovable, Bolt or v0? Scan your live app. Or upload your package file.

Who is affected

  • ai.h2o:h2o-core (Maven) <= 3.46.0.7
  • h2o (PyPI) <= 3.46.0.7

How to check

  1. Look for ai.h2o:h2o-core in your lockfile and compare its version with the range above.
  2. Run pip show h2o in your project to see the installed version, including copies pulled in by other packages.
  3. Or paste your lockfile into the checker: it lists every package with a known vulnerability, this one included.
  4. Or run npx shippable-dev scan localhost:3000 in your project folder. It reads your lockfile and flags this vulnerability along with every other known one.

How to fix

Copies a prompt for Claude Code, Cursor or Codex: it checks your lockfiles for this and upgrades it.
  1. No fixed version of ai.h2o:h2o-core yet. Replace or remove it, or apply the workaround in the advisory below.
  2. No fixed version of h2o yet. Replace or remove it, or apply the workaround in the advisory below.
  3. If a vulnerable version was live, check your logs and rotate the secrets that server could read.

The advisory

A deserialization vulnerability exists in h2oai/h2o-3 versions <= 3.46.0.7, 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.

Check your whole app

Dependencies are only half of it. Free and read-only: before you push, run npx shippable-dev scan localhost:3000 in your project folder, no account needed. For your live app, you prove it's yours with a one-line tag first.

References

Merged from GitHub Advisory Database, OSV. Last updated 28 Oct 2026. All vulnerabilities · RSS · JSON

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