`CVE-2025-6544` h2o

Affected software	h2o
Severity	9.8 Critical CVSS 3.0
Weakness	Deserialization of untrusted data (CWE-502)
Published	21 Sep 2025
Updated	5 Oct 2026

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.

Exploitation status

Proof of concept only

A proof of concept exists, but no reviewed exploit catalogue lists this CVE.

CISA KEV not listed
VulnCheck KEV not listed
CISA Vulnrichment exploitation: poc CISA recorded that an exploit has been demonstrated. Automatable: yes, technical impact: total.
Indexed PoC none indexed
EPSS 0.95% chance in 30 days

Waratek protection

Protected by Waratek

Waratek has a rule that blocks attacks on this vulnerability.

Protection full Deserialization, Process
Action available security rule
Review automated Inferred
Record active

Decided by	A vulnerability class already blocked by an ARMR security rule
Finding	Protected by Waratek's ARMR deserialization security rule, which blocks this class of attack at the JVM level without a rule written for this CVE specifically. The CVE's weakness is CWE-502: Deserialization of Untrusted Data.
Candidate fix	https://github.com/h2oai/h2o-3/commit/0298ee348f5c73673b7b542158081e79605f5f25 : a commit: the change itself

Decided by

A vulnerability class already blocked by an ARMR security rule

Finding

Protected by Waratek's ARMR deserialization security rule, which blocks this class of attack at the JVM level without a rule written for this CVE specifically. The CVE's weakness is CWE-502: Deserialization of Untrusted Data.

Candidate fix

https://github.com/h2oai/h2o-3/commit/0298ee348f5c73673b7b542158081e79605f5f25 : a commit: the change itself

How to mitigate CVE-2025-6544

Apply the vendor's fix for h2o. Until you can, Waratek RASP blocks exploitation with its deserialization and process rules. It runs inside the Java application, with no code changes.

Waratek RASP Runtime protection for Java applications

Affected software and releases

Maven packages

Affected and fixed versions, from OSV.dev

Package Introduced Fixed Last affected

ai.h2o:h2o-core 0 none 3.46.0.7

Package	Introduced	Fixed	Last affected
`ai.h2o:h2o-core`	0	none	3.46.0.7

Products and versions

2 products, 2 version entries

Source	Vendor	Product	Scheme	Affected versions
osv	ai.h2o	h2o-core	generic	≤ 3.46.0.7
nvd	h2o	h2o	generic	≥ 3.0.0.2 and ≤ 3.46.0.8

Source

Vendor

Product

Scheme

Affected versions

osv

ai.h2o

h2o-core

generic

≤ 3.46.0.7

nvd

h2o

generic

≥ 3.0.0.2 and ≤ 3.46.0.8

References

URL	Tags
https://github.com/h2oai/h2o-3/commit/0298ee348f5c73673b7b542158081e79605f5f25	commit
https://github.com/h2oai/h2o-3	PACKAGE
https://github.com/pypa/advisory-database/tree/main/vulns/h2o/PYSEC-2026-349.yaml	WEB
https://huntr.com/bounties/53f35a0f-d644-4f82-93aa-89fe7e0aed40	WEB
https://pypi.org/project/h2o	WEB

URL

Timeline

Published	2025-09-21	By the CVE Program.
NVD record modified	2026-10-05	NVD's own last-modified date for this record.

Published

2025-09-21

By the CVE Program.

NVD record modified

2026-10-05

NVD's own last-modified date for this record.

How VRT decided (17 of 24 rules ran)

17 of 24 rules ran. The rest didn't run because an earlier rule had already settled the CVE.

# Rule Outcome Status Detail

identity: Does this identifier describe a vulnerability at all? the first rule that matches decides

1 rejected no match CVE Program withdrawal of this identifier

scope: Is it a vulnerability Waratek protects? the first rule that matches decides

2 armr-patch-file no match An ARMR patch file on disk for this CVE

3 armr-secure-rule-file no match An ARMR security-rule template on disk for this CVE

4 manual-classification no match A researcher's recorded decision for this specific CVE

5 manual classification no match Manual classification of this CVE in the legacy dataset

6 non-java-upstream no match A root cause in non-Java code: an OS kernel, a C library, or a bundled runtime

7 oracle-component no match The defect being in a component of a tracked product the agent does not reach: JavaFX, the Java SE installer, the JInitiator browser plug-in, the native WebLogic proxy plug-in, or a native part of Oracle Application Server or Collaboration Suite (HTTP Server, Web Cache, Internet Directory, mail and calendar servers). Oracle gives these no CPE of their own and names them only in the description.

8 java-client-side no match Oracle's own risk-matrix note saying the CVE is reached only through untrusted code in the Java sandbox (applets, Web Start) or only through a client installation of Java, so a server running its own trusted code is not exposed

9 osv-maven matched In scope Maven packages: ai.h2o:h2o-core

10 cna-maven-package not reached An affected Maven artifact named by the CNA itself

11 known-cpe-product not reached An NVD CPE naming a product ARMR tracks

12 cna-cpe-product not reached A CPE the CNA published naming a tracked product. Oracle publishes these on advisory day, months before NVD.

13 oracle-advisory-family not reached A tracked product family named in Oracle's own risk matrix. CPE cannot carry a suite name, only a module.

14 cna-named-product not reached A tracked product named in words by the CVE's own CNA, on a record with no CPE yet. Apache and Eclipse publish these on advisory day, before NVD adds CPE data.

15 non-deployable-product not reached The only affected product being a sealed appliance image, where Java runs but no -javaagent can be attached

16 unmatched-product not reached No tracked product matched above, and none present at all

disposition: What do we do about one that is? every rule runs

17 patch-hint matched Mitigation candidate References include candidate fix URL (commit tier): https://github.com/h2oai/h2o-3/commit/0298ee348f5c73673b7b542158081e79605f5f25

18 open-source-maven skipped The affected package being open source on Maven, so the fix can be read

19 poc-derivable skipped An indexed exploit clearing the evidence bar to reverse-engineer, where there is no source to read

20 no-exploit skipped Nothing to build a rule from yet: no published exploit, no CISA record of exploitation and no source-level change to read

21 secure-rule-match decided it matched Protected by Waratek CWE-502 (Deserialization of Untrusted Data) is mitigable by an ARMR deserialization security rule.

22 exploited-no-fix-or-poc skipped Somebody exploiting this CVE (a CISA or VulnCheck KEV listing, or a CISA verdict of active exploitation), with no fix, source code or PoC to build a rule from

23 below-action-threshold skipped Medium or Low severity, no CISA or VulnCheck KEV listing, no PoC clearing the evidence bar, no CISA verdict, no Oracle advisory

24 no-exploit-published skipped Exploit code for this candidate, published in a reviewed catalogue or a GitHub repository with 5 or more stars, or in use per CISA or VulnCheck; not only a CISA verdict, a forecast, or nothing at all

Products the scope rules checked

The products this CVE's record names, and whether VRT tracks each one. The scope rules decided on these.

Vendor Product Tracked CPE Source

h2o h2o not tracked cpe:2.3:a:h2o:h2o:*:*:*:*:*:*:*:*

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