CVE-2025-6544: H2o H2o Deserialization RCE Vulnerability

CVE-2025-6544 Overview

A critical insecure deserialization vulnerability has been identified in h2oai/h2o-3, an open-source machine learning platform. The vulnerability exists in versions <= 3.46.0.8 and allows attackers to read arbitrary system files and execute arbitrary code on affected systems. The flaw arises from improper handling of JDBC connection parameters, which can be exploited by bypassing regular expression checks through double URL encoding techniques.

Critical Impact

This vulnerability enables remote attackers to achieve arbitrary file read and remote code execution through JDBC parameter manipulation, potentially leading to complete system compromise.

Affected Products

• h2oai/h2o-3 versions <= 3.46.0.8
• H2O Open Source Machine Learning Platform
• All deployments utilizing vulnerable JDBC connection handling

Discovery Timeline

• 2025-09-21 - CVE-2025-6544 published to NVD
• 2025-10-08 - Last updated in NVD database

Technical Details for CVE-2025-6544

Vulnerability Analysis

This insecure deserialization vulnerability (CWE-502) stems from inadequate input validation in the JDBC connection parameter handling within H2O-3. The vulnerability allows attackers to bypass security controls that were implemented using regular expression checks. By leveraging double URL encoding, malicious actors can circumvent these filters and inject malicious serialized objects through JDBC connection strings.

The exploitation of this vulnerability can result in two primary attack outcomes: arbitrary file system read access and remote code execution. The network-accessible nature of the vulnerability, combined with no authentication requirements and no user interaction needed, makes this a particularly dangerous flaw that can be exploited remotely without any prerequisites.

Root Cause

The root cause lies in the improper sanitization and validation of JDBC connection parameters. The application relies on regular expression patterns to filter potentially malicious input, but the implementation fails to account for double URL encoding. When user-supplied data passes through the JDBC parameter handling code, attackers can encode malicious payloads twice, effectively bypassing the regex-based security checks. Once the encoded payload reaches the deserialization point, it gets decoded and processed, allowing the injection of arbitrary serialized objects.

Attack Vector

The attack is conducted over the network, targeting the JDBC connection handling functionality in H2O-3. An attacker crafts a malicious JDBC connection string with double URL-encoded payload designed to bypass the regular expression validation. The attack flow involves:

1. The attacker identifies an exposed H2O-3 instance accepting JDBC connections
2. A malicious serialized payload is crafted and double URL-encoded
3. The encoded payload is submitted through JDBC connection parameters
4. The regex-based security filters fail to detect the encoded malicious content
5. Upon processing, the payload is decoded and deserialized
6. The deserialized object executes arbitrary code or reads system files

The vulnerability requires no authentication and no user interaction, making it exploitable by any network-adjacent attacker with access to the H2O-3 service.

Detection Methods for CVE-2025-6544

Indicators of Compromise

• Unusual JDBC connection attempts containing double URL-encoded characters (e.g., %25 sequences)
• Unexpected file system access patterns from the H2O-3 process
• Anomalous process spawning or command execution originating from the H2O service
• Log entries showing malformed or suspicious JDBC connection strings

Detection Strategies

• Monitor network traffic for JDBC connections containing encoded payload patterns, particularly sequences with %25 which indicate double encoding
• Implement application-level logging to capture all JDBC connection parameter values before and after decoding
• Deploy endpoint detection and response (EDR) solutions to monitor for suspicious process behavior from H2O-3 services
• Configure file integrity monitoring on sensitive system files that could be targeted for exfiltration

Monitoring Recommendations

• Enable verbose logging for JDBC connection handling in H2O-3 deployments
• Implement network segmentation to limit exposure of H2O-3 instances to trusted networks only
• Deploy SentinelOne agents on systems running H2O-3 to detect and respond to exploitation attempts
• Establish baseline behavior for H2O-3 processes and alert on deviations such as unexpected child processes or network connections

How to Mitigate CVE-2025-6544

Immediate Actions Required

• Upgrade h2oai/h2o-3 to a patched version above 3.46.0.8 immediately
• If immediate patching is not possible, restrict network access to H2O-3 instances to trusted IP addresses only
• Review logs for any indicators of prior exploitation attempts
• Implement network-level filtering to block suspicious JDBC connection attempts

Patch Information

H2O.ai has released a security patch addressing this vulnerability. The fix is available in the GitHub commit 0298ee3. Organizations should upgrade to a version containing this fix as the primary remediation action.

Additional details about the vulnerability disclosure can be found in the Huntr bounty report.

Workarounds

• Restrict network access to H2O-3 services using firewall rules or network segmentation
• Implement a web application firewall (WAF) rule to detect and block double URL-encoded payloads in JDBC parameters
• Disable or restrict JDBC functionality if not required for your deployment
• Run H2O-3 with minimal system privileges to limit the impact of successful exploitation

# Example: Restrict H2O-3 network access using iptables
# Allow connections only from trusted network segment
iptables -A INPUT -p tcp --dport 54321 -s 10.0.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 54321 -j DROP