CVE-2025-6544: A deserialization vulnerability exists in h2oai/h2o-3 versions <= 3.46.0.8, allowing attackers to read arbitra

Q: How do I fix CVE-2025-6544?

h2o: upgrade to newer than 3.46.0.8. Exact commands for each OS are on this page.

Q: Is CVE-2025-6544 being exploited?

CVE-2025-6544 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2025-6544 affect?

CVE-2025-6544 affects h2o. Severity is critical (CVSS 9.8). 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 c

Q: How urgent is CVE-2025-6544?

Priority 62/100. Critical severity; Public exploit reference; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action

This week

CRITICAL 9.8REMOTEUPGRADE

CVE-2025-6544 is a critical-severity flaw (CVSS 9.8) affecting h2o. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Critical severity
Public exploit reference
CISA SSVC: automatable with total technical impact
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.9% (top 40%)
CISA exploited list	No
Weakness	CWE-502
Published	2025-09-21
Last modified	2026-10-05

Publisher (CNA): huntr.dev · Vendor: h2o

Last updated 2026-10-05 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2025-09-21 09:15 UTCPublished CVE record published
2026-10-05 14:10 UTCUpdated Record last modified

h2o h2o UPGRADE

Affected: >= 3.0.0.2 and <= 3.46.0.8

Fixed in: newer than 3.46.0.8

Safe to downgrade to: a release before 3.0.0.2

1. Check installed version

apt-cache policy h2o || rpm -q h2o || h2o --version

2. UPGRADE to newer than 3.46.0.8 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade h2o

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade h2o

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.0.0.2 (Debian / Ubuntu)

apt-cache madison h2o sudo apt install h2o=<VERSION_BEFORE_3.0.0.2> sudo apt-mark hold h2o

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list h2o sudo dnf downgrade h2o-<VERSION_BEFORE_3.0.0.2> sudo dnf versionlock add h2o

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Exact fixes by distribution

Maven · ai.h2o:h2o-core · NO FIX YET · vulnerable: <= 3.46.0.7 · source

Python (pip) · h2o · NO FIX YET · vulnerable: <= 3.46.0.7 · source

Fedora 46 · wordpress · FIXED · 7.1.4-1.fc46 · FEDORA-2026-907394725e · source

sudo dnf upgrade --advisory=FEDORA-2026-907394725e

Fedora ELN · rhc · FIXED · 0.3.15-1.eln159 · FEDORA-2026-1cd9f35750 · source

sudo dnf upgrade --advisory=FEDORA-2026-1cd9f35750

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2025-6544?

h2o: upgrade to newer than 3.46.0.8. Exact commands for each OS are on this page.

Is CVE-2025-6544 being exploited?

CVE-2025-6544 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2025-6544 affect?

CVE-2025-6544 affects h2o. Severity is critical (CVSS 9.8). 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 c

How urgent is CVE-2025-6544?

Priority 62/100. Critical severity; Public exploit reference; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-3960 CRITICAL 9.8 UPGRADE A critical remote code execution vulnerability exists in the unauthenticated REST API endpoint /99/ImportSQLTa
CVE-2026-8751 HIGH 7.3 UPGRADE A security flaw has been discovered in h2oai h2o-3 up to 7402
CVE-2026-8750 MEDIUM 5.3 UPGRADE A vulnerability was identified in h2oai h2o-3 up to 7402
CVE-2026-8752 MEDIUM 5.3 UPGRADE A weakness has been identified in h2oai h2o-3 up to 7402
CVE-2025-10768 MEDIUM 6.3 UPGRADE A flaw has been found in h2oai h2o-3 up to 3.46.08

References

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