CVE-2025-63653 is a vulnerability identified in the Monkey HTTP server, an open-source lightweight web server often used in embedded systems and resource-constrained environments. The flaw exists in the mk_vhost_fdt_close function within the mk_server/mk_vhost.c source file, where an out-of-bounds read occurs. This type of vulnerability (CWE-125) arises when the software reads data outside the bounds of allocated memory, potentially leading to undefined behavior such as crashes. In this case, an attacker can exploit the vulnerability by sending a crafted HTTP request to the server, triggering the out-of-bounds read and causing the server process to crash, resulting in a Denial of Service (DoS). The CVSS 3.1 base score is 7.5, reflecting high severity due to the network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), and impact limited to availability (A:H) without affecting confidentiality or integrity. The vulnerability was reserved in late 2025 and published in early 2026. No patches or known exploits are currently available, indicating that mitigation relies on detection and containment until fixes are released. The vulnerability affects all versions of Monkey HTTP server prior to patching, though specific affected versions are not listed. Given Monkey's usage in embedded and IoT devices, this vulnerability could impact a range of systems that rely on it for web serving capabilities.

For European organizations, the primary impact of CVE-2025-63653 is the potential for Denial of Service attacks against systems running the Monkey HTTP server. This can disrupt web services, internal applications, or embedded device management interfaces, leading to operational downtime and service unavailability. Critical infrastructure sectors such as telecommunications, manufacturing, and energy that may use embedded devices with Monkey server could experience interruptions. Although the vulnerability does not compromise data confidentiality or integrity, the loss of availability can affect business continuity and service level agreements. The ease of exploitation without authentication increases the risk of widespread scanning and attack attempts. Organizations with automated or remote management relying on Monkey HTTP server could face cascading failures if the server crashes repeatedly. The lack of current patches means organizations must rely on network-level protections and monitoring to mitigate risk. Overall, the threat could lead to degraded service performance and increased operational costs in affected European entities.

1. Monitor network traffic for unusual or malformed HTTP requests targeting Monkey HTTP servers, using intrusion detection systems (IDS) or web application firewalls (WAF) with custom rules. 2. Isolate devices running Monkey HTTP server from direct internet exposure where possible, placing them behind secure gateways or VPNs. 3. Implement rate limiting and connection throttling to reduce the impact of crafted request floods. 4. Maintain an inventory of all devices and applications using Monkey HTTP server to prioritize patching once updates become available. 5. Engage with vendors or open-source maintainers to obtain patches or mitigations promptly. 6. Employ robust logging and alerting to detect repeated crashes or service restarts indicative of exploitation attempts. 7. Consider deploying fallback or redundant services to maintain availability during potential DoS events. 8. Harden embedded devices by disabling unnecessary services and restricting access to management interfaces. 9. Conduct regular security assessments and penetration testing focusing on HTTP server components. 10. Educate IT and security teams about this specific vulnerability to improve incident response readiness.