CVE-2025-63655 is a denial of service vulnerability found in the Monkey HTTP server, specifically within the mk_http_range_parse function located in the mk_server/mk_http.c source file. The vulnerability arises due to a NULL pointer dereference, which occurs when the server processes a crafted HTTP request containing malformed or malicious range headers. When this function attempts to parse the HTTP Range header, it fails to properly validate input, leading to dereferencing a NULL pointer and causing the server process to crash. This results in a denial of service condition, rendering the server unavailable to legitimate users. The vulnerability does not require any authentication or user interaction, making it easier for remote attackers to exploit. Although no specific affected versions are listed, the flaw is tied to a particular commit (f37e984) in the Monkey server codebase, suggesting it affects versions containing this commit. No patches or fixes have been published yet, and no exploits have been observed in the wild. The lack of a CVSS score means the severity must be assessed based on the impact on availability and ease of exploitation. Given the direct crash caused by a crafted HTTP request, the vulnerability poses a significant risk to service continuity for any deployments of Monkey HTTP server.

The primary impact of CVE-2025-63655 is on the availability of services running the Monkey HTTP server. Successful exploitation results in a server crash, causing denial of service to legitimate users. For European organizations relying on Monkey server for web services, this could lead to downtime, loss of business continuity, and potential reputational damage. Critical infrastructure or public-facing services using Monkey server are particularly vulnerable to disruption. Although no data confidentiality or integrity issues are indicated, the availability impact alone can be significant, especially for high-traffic or mission-critical environments. The ease of exploitation without authentication increases the risk of widespread attacks. Additionally, attackers could leverage this vulnerability as part of a larger attack chain to cause service outages or distract from other malicious activities. The lack of known exploits currently limits immediate risk, but the vulnerability remains a concern until patched.

1. Monitor network traffic for unusual or malformed HTTP Range headers that could indicate exploitation attempts. 2. Implement web application firewalls (WAFs) or intrusion prevention systems (IPS) with rules to detect and block suspicious HTTP requests targeting the Range header. 3. Restrict external access to Monkey HTTP servers where possible, limiting exposure to untrusted networks. 4. Maintain up-to-date backups and ensure rapid recovery procedures are in place to minimize downtime in case of a DoS event. 5. Engage with Monkey server developers or community to track the release of patches or updates addressing this vulnerability and apply them promptly once available. 6. Consider deploying rate limiting or connection throttling to reduce the impact of potential DoS attempts. 7. Conduct internal audits to identify all instances of Monkey HTTP server deployments and assess their exposure. 8. Use network segmentation to isolate vulnerable servers from critical infrastructure to contain potential disruptions.