CVE-2025-63658 identifies a stack overflow vulnerability in the mk_http_index_lookup function within the Monkey HTTP server's source code (mk_server/mk_http.c). The flaw arises when the server processes a crafted HTTP request that triggers a buffer overflow on the stack, leading to memory corruption. This corruption results in a Denial of Service (DoS) condition as the server crashes or becomes unresponsive. The vulnerability is remotely exploitable without any authentication or user interaction, making it accessible to any attacker capable of sending network traffic to the affected server. The CVSS v3.1 score of 7.5 (High) reflects the ease of exploitation (low attack complexity), no privileges required, and the impact limited to availability (no confidentiality or integrity loss). The vulnerability is classified under CWE-121 (Stack-based Buffer Overflow), a common and dangerous class of memory corruption bugs. Although no patches or known exploits are currently documented, the vulnerability poses a significant risk to deployments of Monkey HTTP server, especially in embedded or lightweight web server contexts where Monkey is popular. The absence of affected version details suggests that users should verify their version against the vulnerable commit (f37e984) and monitor for vendor patches. The vulnerability's exploitation vector is network-based, emphasizing the need for perimeter defenses and input validation improvements in the server code.

For European organizations, the primary impact of CVE-2025-63658 is service disruption due to Denial of Service attacks against systems running the Monkey HTTP server. This can affect embedded devices, IoT gateways, or lightweight web services that rely on Monkey for HTTP handling. Critical infrastructure sectors such as manufacturing, energy, and telecommunications that utilize embedded web servers for device management could experience outages or degraded service availability. The disruption could lead to operational downtime, loss of productivity, and potential cascading effects if the affected devices are part of larger control systems. Since the vulnerability does not compromise confidentiality or integrity, data breaches are unlikely; however, the availability impact alone can be severe in time-sensitive or safety-critical environments. The lack of authentication requirement means attackers can exploit this vulnerability remotely, increasing the attack surface. European organizations with limited patch management capabilities for embedded systems may face prolonged exposure. Additionally, denial of service attacks could be leveraged as part of multi-stage attacks or to distract incident response teams.

1. Monitor official Monkey HTTP server channels for security advisories and apply patches promptly once available. 2. In the absence of patches, 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_http_index_lookup function. 3. Restrict access to Monkey HTTP server instances by limiting exposure to trusted networks and employing network segmentation to reduce attack surface. 4. Conduct thorough input validation and sanitization on HTTP requests at the application or proxy level to prevent malformed requests from reaching the server. 5. For embedded devices, coordinate with device vendors to obtain firmware updates or mitigations addressing this vulnerability. 6. Implement robust monitoring and alerting for unusual HTTP traffic patterns indicative of exploitation attempts. 7. Consider deploying rate limiting to mitigate potential DoS attack volumes. 8. Review and harden server configurations to disable unnecessary HTTP features or modules that could be exploited. 9. Maintain an inventory of all systems running Monkey HTTP server to prioritize mitigation efforts. 10. Prepare incident response plans specifically addressing DoS scenarios impacting embedded web servers.