CVE-2025-5399 is a high-severity vulnerability affecting libcurl versions 8.13.0 and 8.14.0, specifically within its WebSocket implementation. The flaw arises from a programming error that causes libcurl to enter an infinite busy-loop when interacting with a malicious WebSocket server that sends a specially crafted packet. This infinite loop has an unreachable exit condition, meaning the affected libcurl instance cannot recover or break out of the loop without forcibly terminating the thread or process. The vulnerability is classified under CWE-835 (Loop with Unreachable Exit Condition), indicating a logic flaw that leads to denial of service (DoS) conditions. Exploitation requires no authentication or user interaction and can be triggered remotely by any attacker controlling a malicious WebSocket server. The CVSS v3.1 score of 7.5 reflects a network attack vector with low complexity, no privileges required, and no user interaction, resulting in high impact on availability but no impact on confidentiality or integrity. While no known exploits are currently in the wild, the vulnerability poses a significant risk to applications and services that embed libcurl for WebSocket communication, as they can be rendered unresponsive or crash due to resource exhaustion caused by the infinite loop.

For European organizations, the impact of this vulnerability can be substantial, especially for those relying on libcurl for WebSocket communications in critical infrastructure, financial services, telecommunications, and cloud services. The infinite loop can cause denial of service by consuming CPU resources indefinitely, potentially leading to application crashes or degraded service availability. This can disrupt business operations, cause downtime, and impact customer trust. Since libcurl is widely used in various open-source and commercial software stacks, the vulnerability could affect a broad range of applications, including web clients, IoT devices, and backend services. The lack of authentication or user interaction requirements means attackers can exploit this vulnerability remotely and at scale, increasing the risk of widespread service disruption. European organizations with strict uptime and availability requirements, such as those in banking and healthcare sectors, may face regulatory and compliance challenges if services are interrupted due to this vulnerability.

To mitigate this vulnerability, European organizations should: 1) Immediately update libcurl to a patched version once available, as no patch links are currently provided but are expected soon given the public disclosure. 2) Implement network-level controls to restrict or monitor WebSocket connections to untrusted or unknown servers, reducing exposure to malicious payloads. 3) Employ application-level timeouts and watchdog mechanisms to detect and recover from unresponsive states caused by infinite loops, allowing graceful termination or restart of affected processes. 4) Conduct thorough testing of applications using libcurl for WebSocket functionality to identify potential hangs or resource exhaustion scenarios. 5) Consider deploying runtime protection tools that can detect abnormal CPU usage patterns indicative of infinite loops and trigger alerts or automated remediation. 6) Maintain an inventory of software and devices using libcurl to prioritize patching and risk assessment. 7) Engage with vendors and open-source communities to track patch releases and advisories related to this vulnerability.