CVE-2025-59089 is a vulnerability in the kdcproxy component of Red Hat Enterprise Linux 10 that results from improper resource allocation controls during TCP response handling from a Kerberos Key Distribution Center (KDC). The kdcproxy service acts as a proxy for Kerberos authentication requests, forwarding them to a KDC server. The vulnerability is triggered when an attacker causes kdcproxy to connect to a malicious, attacker-controlled KDC server, for example, through server-side request forgery (SSRF). The core issue is that kdcproxy does not enforce limits on the size of the TCP response it receives. Specifically, during the reception of the KDC response, kdcproxy repeatedly copies the entire buffered stream into a new buffer on each recv() call, even if the transfer is incomplete. This behavior causes excessive memory allocation and CPU consumption. Furthermore, kdcproxy continues to accept incoming response chunks as long as the total received data length does not exactly match the length indicated in the response header, even if individual chunks or the total buffer exceed the maximum allowed size for a Kerberos message. This allows an attacker to send unbounded amounts of data until the connection times out (approximately 12 seconds), leading to exhaustion of server memory and CPU resources. When multiple such requests are made concurrently, the accept queue can overflow, resulting in denial of service to legitimate clients. The vulnerability does not impact confidentiality or integrity but severely affects availability. Exploitation does not require authentication or user interaction but has a high complexity due to the need to manipulate kdcproxy’s connection targets. The CVSS v3.1 base score is 5.9, reflecting these factors. No known exploits are currently in the wild, and no patches are listed yet, indicating the need for proactive mitigation. This vulnerability is particularly relevant for environments relying on Kerberos authentication via kdcproxy on Red Hat Enterprise Linux 10 systems.

The primary impact of CVE-2025-59089 is denial of service (DoS) against systems running Red Hat Enterprise Linux 10 with the kdcproxy service enabled. For European organizations, this can disrupt authentication services that rely on Kerberos, potentially causing widespread service outages in enterprise networks, government agencies, and critical infrastructure sectors such as finance, telecommunications, and public administration. The exhaustion of memory and CPU resources can degrade system performance or cause crashes, leading to operational downtime. Multiple concurrent exploitation attempts can saturate network queues, preventing legitimate authentication requests from being processed, which could halt access to critical applications and services. Although the vulnerability does not compromise data confidentiality or integrity, the availability impact can have cascading effects on business continuity and security monitoring. Organizations with large-scale deployments of Red Hat Enterprise Linux 10 in sensitive environments are at higher risk. The absence of known exploits in the wild currently reduces immediate threat but does not eliminate the risk of future exploitation. The medium CVSS score reflects moderate urgency but should not lead to complacency given the potential for service disruption.

To mitigate CVE-2025-59089, European organizations should implement the following specific measures: 1) Monitor and restrict kdcproxy’s network connections to only trusted and verified KDC servers, preventing attacker-controlled servers from being contacted. This can be enforced via firewall rules, network segmentation, or application-level allowlists. 2) Implement rate limiting and connection throttling on kdcproxy to prevent resource exhaustion from excessive or malformed responses. 3) Monitor system resource usage closely on hosts running kdcproxy for unusual spikes in memory or CPU consumption that may indicate exploitation attempts. 4) Apply any patches or updates from Red Hat promptly once available, as they will likely address the resource allocation flaws. 5) Harden server-side request forgery (SSRF) protections in applications that may trigger kdcproxy connections to untrusted KDCs, reducing the attack surface. 6) Consider deploying intrusion detection or prevention systems (IDS/IPS) tuned to detect anomalous Kerberos traffic patterns indicative of this attack. 7) Conduct regular security audits and penetration tests focusing on Kerberos infrastructure to identify potential misconfigurations or vulnerabilities. These targeted actions go beyond generic advice by focusing on controlling kdcproxy’s network interactions and resource usage, which are the root causes of this vulnerability.