CVE-2026-21925 is a vulnerability identified in the Remote Method Invocation (RMI) component of Oracle Java SE and Oracle GraalVM products, including Oracle Java SE versions 8u471 through 25.0.1, GraalVM for JDK versions 17.0.17 and 21.0.9, and GraalVM Enterprise Edition 21.3.16. The flaw allows an unauthenticated attacker with network access to exploit multiple protocols to gain unauthorized access to certain data within these Java environments. Specifically, the attacker can perform unauthorized update, insert, or delete operations on some accessible data, as well as unauthorized read access to a subset of data. The vulnerability is considered difficult to exploit due to high attack complexity, meaning it requires specific conditions or knowledge to successfully carry out an attack. No privileges or user interaction are required, increasing the risk if the attacker can reach the vulnerable service over the network. The vulnerability is particularly relevant for Java deployments that run sandboxed Java Web Start applications or sandboxed Java applets which load untrusted code from the internet and rely on the Java sandbox for security. Exploitation can occur through APIs exposed by the RMI component, such as web services supplying data to these APIs. The CVSS 3.1 base score of 4.8 reflects a medium severity with limited confidentiality and integrity impacts and no availability impact. No public exploits or active exploitation have been reported at this time. The vulnerability underscores the ongoing risks associated with legacy Java technologies and the importance of securing Java-based services exposed to untrusted networks.

For European organizations, this vulnerability poses a risk primarily to environments running affected versions of Oracle Java SE and GraalVM, especially where Java Web Start applications or applets are used to load untrusted code. Successful exploitation could lead to unauthorized modification or disclosure of sensitive data, potentially impacting confidentiality and integrity of business-critical information. Sectors relying heavily on Java-based middleware, enterprise applications, or legacy systems—such as finance, manufacturing, telecommunications, and government—may face increased risk. The ability to exploit the vulnerability without authentication and user interaction means attackers could potentially leverage exposed network services to compromise systems remotely. However, the high attack complexity and lack of known exploits reduce immediate risk. Still, the presence of this vulnerability in widely deployed Java runtimes means that organizations with insufficient patching or outdated Java versions could be targeted in supply chain attacks or through compromised web services. Data integrity issues could disrupt business processes, while unauthorized data reads could lead to information leakage or compliance violations under GDPR. The impact is mitigated if organizations have phased out Java Web Start and applets or have strong network segmentation and monitoring in place.

European organizations should implement a multi-layered mitigation strategy beyond generic patching advice. First, promptly apply Oracle's security updates for all affected Java SE and GraalVM versions to remediate the vulnerability. Where immediate patching is not feasible, restrict network access to Java RMI services using firewalls or network segmentation to limit exposure to untrusted networks. Disable or remove legacy Java Web Start applications and sandboxed applets that load untrusted code, replacing them with modern, secure alternatives. Conduct an inventory of Java deployments to identify vulnerable versions and usage patterns, focusing on services exposed via multiple protocols. Implement strict input validation and API security controls on web services interfacing with the RMI component to reduce attack surface. Employ runtime application self-protection (RASP) or Java security managers to enforce stricter sandboxing policies. Monitor network traffic and logs for unusual RMI activity or unauthorized data access attempts. Finally, educate developers and system administrators about the risks of running untrusted Java code and encourage migration to supported, secure Java platforms.