CVE-2025-63721 is a critical vulnerability affecting HummerRisk software versions through 1.5.0, caused by the inclusion of a vulnerable version of the Snakeyaml library. Snakeyaml is a YAML parser and emitter for Java, and this vulnerability relates to unsafe deserialization of YAML input, classified under CWE-502. Attackers with normal user privileges can send malicious payloads to the /rule/add API endpoint, which processes YAML data without proper validation or sanitization. This allows attackers to craft malicious YAML content that, when deserialized by the vulnerable Snakeyaml component, leads to remote code execution (RCE) on the server hosting HummerRisk. The attack requires no user interaction and no elevated privileges beyond normal user access, making exploitation straightforward if the API is reachable. The CVSS v3.1 base score of 9.8 reflects the high impact on confidentiality, integrity, and availability, as successful exploitation allows full server takeover. No patches or fixes are currently linked, increasing the urgency for defensive measures. Although no known exploits have been observed in the wild, the vulnerability's nature and ease of exploitation make it a critical threat. The vulnerability's root cause is unsafe deserialization, a common and dangerous flaw where untrusted input is deserialized without sufficient validation, enabling attackers to execute arbitrary code or commands on the target system.

For European organizations, this vulnerability poses a severe risk, especially for those using HummerRisk in security, risk management, or compliance environments. Successful exploitation can lead to complete compromise of affected servers, exposing sensitive data, disrupting operations, and enabling lateral movement within networks. Critical infrastructure or financial institutions relying on HummerRisk could face significant operational and reputational damage. The vulnerability's ease of exploitation without elevated privileges or user interaction increases the likelihood of attacks. Additionally, the lack of available patches means organizations must rely on compensating controls, increasing operational complexity. The potential for attackers to gain persistent access or deploy ransomware or espionage tools further elevates the threat level. European data protection regulations such as GDPR could also impose heavy penalties if breaches occur due to this vulnerability.

1. Immediately restrict network access to the /rule/add API endpoint by implementing strict firewall rules or network segmentation to limit exposure only to trusted users or systems. 2. Implement strong authentication and authorization controls around the API to ensure only fully trusted users can access it. 3. Monitor logs and network traffic for unusual or suspicious activity targeting the /rule/add endpoint, including unexpected YAML payloads or anomalous API calls. 4. Employ runtime application self-protection (RASP) or web application firewalls (WAF) with custom rules to detect and block malicious deserialization attempts. 5. Engage with HummerRisk vendors or developers to obtain patches or updates addressing the vulnerable Snakeyaml component as soon as they become available. 6. If patching is not immediately possible, consider disabling or limiting the functionality of the /rule/add API temporarily. 7. Conduct thorough security assessments and penetration testing focused on deserialization vulnerabilities within your environment. 8. Educate development and operations teams about the risks of unsafe deserialization and secure coding practices to prevent similar issues in the future.