CVE-2025-56233 is a denial of service (DoS) vulnerability affecting the OpenIndiana operating system kernel, specifically SunOS 5.11. The vulnerability arises from the way the TCP stack processes packets with the RST (reset) or SYN (synchronize) flags set. According to RFC 5961, TCP implementations should strictly validate the sequence numbers of incoming RST or SYN packets, requiring them to exactly match the next expected sequence number to prevent malicious disruption of connections. However, OpenIndiana’s TCP implementation accepts a wide range of sequence numbers within the current receive window rather than requiring an exact match. This leniency violates the RFC and allows an attacker to send multiple TCP packets with RST or SYN flags and random sequence numbers that fall within the acceptable window. Because these packets are accepted, the attacker can forcibly interrupt or reset legitimate TCP connections, causing service interruptions and effectively a denial of service. The vulnerability does not require authentication or user interaction, making it exploitable remotely by an unauthenticated attacker who can send crafted TCP packets to a target system running the vulnerable kernel. No known exploits are currently reported in the wild, and no patches or fixes have been linked yet. The affected versions are not explicitly specified beyond the kernel version SunOS 5.11 used in OpenIndiana. This vulnerability targets the fundamental TCP/IP stack, which is critical for network communications, meaning that exploitation could disrupt any network service relying on TCP connections on affected systems.

For European organizations, the impact of this vulnerability could be significant, especially for those relying on OpenIndiana or similar Solaris-derived systems in their infrastructure. The denial of service caused by connection resets can disrupt critical network services such as web servers, database connections, and internal communication channels. This could lead to downtime, loss of productivity, and potential cascading failures in multi-tier applications. Organizations in sectors like finance, telecommunications, and government that require high availability and robust network communications could face operational risks. Additionally, the vulnerability could be exploited as part of a larger attack campaign to degrade network reliability or as a vector to distract from other simultaneous attacks. Since the vulnerability can be triggered remotely without authentication, it increases the attack surface for external threat actors. However, the lack of known exploits and the specialized nature of OpenIndiana deployments may limit the immediate widespread impact. Still, organizations using this OS should consider the risk of targeted attacks or opportunistic scanning by attackers seeking to cause disruption.

To mitigate this vulnerability, organizations should first identify any systems running OpenIndiana with the SunOS 5.11 kernel. Network segmentation should be employed to isolate vulnerable systems from untrusted networks, limiting exposure to external attackers. Implementing strict ingress and egress filtering on firewalls to block unsolicited TCP RST and SYN packets from untrusted sources can reduce attack vectors. Monitoring network traffic for unusual patterns of TCP resets or SYN floods can help detect exploitation attempts early. Until a patch or update is available, consider deploying intrusion prevention systems (IPS) with custom signatures to detect and block malformed TCP packets targeting this vulnerability. Organizations should engage with OpenIndiana maintainers or vendors to obtain patches or updates as soon as they become available. Additionally, consider fallback or alternative operating systems for critical services if patching is delayed. Regular backups and incident response plans should be updated to handle potential service disruptions caused by this vulnerability.