CVE-2025-56233 identifies a denial of service vulnerability in the Openindiana operating system kernel, specifically SunOS 5.11. The vulnerability stems from the kernel's TCP stack implementation, which accepts TCP packets with RST or SYN flags set if their sequence numbers fall within a broad receive window rather than requiring an exact match to the next expected sequence number. This behavior violates RFC 5961, which mandates stricter sequence number validation to prevent off-path attackers from injecting malicious TCP packets. By exploiting this flaw, an attacker can send numerous TCP RST or SYN packets with random sequence numbers that fall within the acceptable window, causing legitimate TCP connections to be reset or interrupted. This results in a denial of service condition affecting network communications. The vulnerability requires no authentication or user interaction and can be exploited remotely over the network. The CVSS v3.1 score is 7.5 (high), reflecting the network attack vector, low attack complexity, no privileges required, no user interaction, and a high impact on availability. Although no known exploits are currently reported in the wild and no patches have been released, the vulnerability poses a significant risk to systems running Openindiana, especially those providing critical network services. The underlying weakness relates to CWE-400 (Uncontrolled Resource Consumption), as the attack can overwhelm system resources by forcing repeated connection resets. The lack of patch availability necessitates interim mitigations such as network filtering and anomaly detection to prevent exploitation.

For European organizations, this vulnerability can cause significant disruption to network services relying on Openindiana or SunOS 5.11 kernels. Denial of service attacks exploiting this flaw can interrupt critical TCP connections, impacting availability of applications, services, and infrastructure components. Organizations in sectors such as telecommunications, finance, government, and energy that may use Openindiana in legacy or specialized environments could face outages or degraded service quality. The attack can be launched remotely without authentication, increasing the risk of widespread disruption. Additionally, the denial of service could be leveraged as part of multi-stage attacks to distract or degrade defenses. The lack of patches means organizations must rely on network-level controls and monitoring to mitigate risk, which may be challenging in complex or high-throughput environments. Overall, the vulnerability threatens operational continuity and could lead to financial losses, reputational damage, and regulatory scrutiny under European data protection and operational resilience frameworks.

1. Implement network-level filtering to detect and block anomalous TCP RST and SYN packets with suspicious sequence numbers, using intrusion detection/prevention systems (IDS/IPS) capable of deep packet inspection. 2. Deploy rate limiting on incoming TCP RST and SYN packets to reduce the impact of flooding attacks targeting the vulnerability. 3. Monitor network traffic for unusual patterns of TCP resets or connection interruptions indicative of exploitation attempts. 4. Isolate or segment systems running Openindiana to limit exposure to untrusted networks and reduce attack surface. 5. Engage with Openindiana maintainers or community to track patch releases and apply updates promptly once available. 6. Consider deploying alternative or updated TCP/IP stack implementations if feasible to mitigate the vulnerability. 7. Conduct regular incident response drills simulating denial of service scenarios to improve readiness. 8. Document and enforce strict network access controls and firewall rules to restrict unnecessary inbound TCP traffic to vulnerable systems. 9. Collaborate with upstream ISPs and network providers to implement upstream filtering or blackholing of attack traffic if under active exploitation. These targeted measures go beyond generic advice by focusing on TCP sequence number anomaly detection and network segmentation specific to this vulnerability.