CVE-2025-63651 is a use-after-free vulnerability identified in the mk_string_char_search function within the mk_core/mk_string.c source file of the Monkey HTTP server, specifically in commit f37e984. Use-after-free (CWE-416) vulnerabilities occur when a program continues to use memory after it has been freed, leading to undefined behavior such as crashes or potential code execution. In this case, the vulnerability allows an unauthenticated remote attacker to cause a denial of service by sending a crafted HTTP request that triggers the use-after-free condition. The vulnerability affects the availability of the server by causing it to crash or become unresponsive, but does not impact confidentiality or integrity. The CVSS 3.1 base score is 7.5 (high), reflecting network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), no impact on confidentiality or integrity (C:N/I:N), and high impact on availability (A:H). No patches or fixes have been published yet, and no known exploits are currently in the wild. The vulnerability highlights the importance of memory safety in server software and the risks posed by malformed inputs in network-facing services.

For European organizations, the primary impact of CVE-2025-63651 is the potential for denial of service attacks against servers running the Monkey HTTP server. This could disrupt web services, internal applications, or APIs relying on Monkey, leading to operational downtime and potential loss of business continuity. While the vulnerability does not expose sensitive data or allow unauthorized data modification, the availability impact can affect customer trust, regulatory compliance (e.g., GDPR mandates on service availability), and incident response costs. Critical infrastructure or public sector services using Monkey could face increased risk of targeted disruption. The lack of authentication or user interaction requirements means attackers can exploit this vulnerability remotely and at scale, increasing the threat level. Organizations with internet-facing Monkey servers are particularly vulnerable, and the absence of patches necessitates interim protective measures.

1. Monitor official Monkey HTTP server channels and security advisories for patches or updates addressing CVE-2025-63651 and apply them promptly once available. 2. Implement network-level protections such as web application firewalls (WAFs) or intrusion prevention systems (IPS) to detect and block malformed HTTP requests targeting the mk_string_char_search function. 3. Restrict external access to Monkey HTTP servers by using network segmentation, VPNs, or IP whitelisting to limit exposure to untrusted networks. 4. Employ rate limiting and anomaly detection to identify and mitigate suspicious traffic patterns that could indicate exploitation attempts. 5. Conduct internal code audits and fuzz testing on the Monkey server if feasible to identify similar memory safety issues proactively. 6. Prepare incident response plans for potential DoS events, including failover and redundancy strategies to maintain service availability. 7. Consider migrating critical services to alternative, actively maintained HTTP servers with robust security track records if Monkey usage is not essential.