How severe is CVE-2025-6544?

CVE-2025-6544 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2025-6544 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (53rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-6544?

CVE-2025-6544 affects H2o. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-6544?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2025-6544 have an EU (EUVD) identifier?

Yes. CVE-2025-6544 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-30380.

When was CVE-2025-6544 published?

CVE-2025-6544 was published on 2025-09-21 and last updated on 2026-06-17.

CVE-2025-6544

CVE-2025-6544 is a critical-severity vulnerability in H2o with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-502.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
EPSS exploit prediction: 1% (53rd percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-30380
Weakness: CWE-502
Affected product: H2o
Published: 2025-09-21
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2025-6544?: 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.
How severe is CVE-2025-6544?: CVE-2025-6544 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2025-6544 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (53rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-6544?: CVE-2025-6544 affects H2o. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-6544?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2025-6544 have an EU (EUVD) identifier?: Yes. CVE-2025-6544 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-30380.
When was CVE-2025-6544 published?: CVE-2025-6544 was published on 2025-09-21 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:h2o:h2o:*:*:*:*:*:*:*:*

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