CVE-2025-63653: Monkey HTTP Server DoS Vulnerability

CVE-2025-63653 Overview

CVE-2025-63653 is an out-of-bounds read vulnerability discovered in the Monkey HTTP Server, specifically within the mk_vhost_fdt_close function located in mk_server/mk_vhost.c. This vulnerability affects commit f37e984 of the Monkey project and allows remote attackers to cause a Denial of Service (DoS) condition by sending specially crafted HTTP requests to the server.

The out-of-bounds read occurs when the server improperly handles certain input during virtual host file descriptor table close operations, resulting in memory access beyond allocated boundaries. This can lead to application crashes and service disruption.

Critical Impact

Remote attackers can cause service disruption by sending malicious HTTP requests, potentially impacting availability of web applications hosted on vulnerable Monkey HTTP Server instances.

Affected Products

• Monkey HTTP Server (commit f37e984 and potentially related versions)

Discovery Timeline

• 2026-01-29 - CVE-2025-63653 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-63653

Vulnerability Analysis

This vulnerability is classified as an out-of-bounds read, which occurs when the mk_vhost_fdt_close function in the Monkey HTTP Server attempts to read memory outside the bounds of an allocated buffer. The vulnerable code resides in mk_server/mk_vhost.c and is triggered during virtual host file descriptor table cleanup operations.

When processing certain malformed HTTP requests, the server fails to properly validate array indices or buffer boundaries before performing read operations. This leads to accessing memory locations that were not intended to be read, which can cause the application to crash or behave unpredictably.

The vulnerability can be exploited remotely without authentication, as it only requires the attacker to send a crafted HTTP request to the vulnerable server. While out-of-bounds read vulnerabilities typically do not allow code execution, they can leak sensitive information from memory or cause denial of service conditions when the invalid memory access triggers a segmentation fault.

Root Cause

The root cause of this vulnerability lies in insufficient bounds checking within the mk_vhost_fdt_close function. When closing virtual host file descriptor table entries, the function does not properly validate that the index or pointer being accessed falls within the allocated memory region. This missing validation allows an attacker to trigger reads from arbitrary memory locations by manipulating HTTP request parameters that influence the file descriptor table operations.

Attack Vector

An attacker can exploit this vulnerability by sending a specially crafted HTTP request to a Monkey HTTP Server instance. The malicious request is designed to manipulate internal state in a way that causes the mk_vhost_fdt_close function to access memory outside its intended boundaries.

The attack is network-based and does not require authentication or user interaction. The attacker simply needs network access to the target server's HTTP port. Upon receiving the malformed request, the server processes it through the virtual host handling code, eventually triggering the out-of-bounds read when the file descriptor table close operation is invoked.

For technical details regarding the exploitation mechanism, refer to the GitHub Security Advisory and the GitHub Issue Discussion #426.

Detection Methods for CVE-2025-63653

Indicators of Compromise

• Unexpected Monkey HTTP Server crashes or restarts, particularly during periods of HTTP request processing
• Segmentation fault errors in server logs referencing mk_vhost_fdt_close or mk_vhost.c
• Unusual HTTP requests with malformed headers or abnormal content targeting the server
• Core dump files generated by the Monkey process indicating memory access violations

Detection Strategies

• Deploy network intrusion detection systems (IDS) with signatures for malformed HTTP requests targeting Monkey HTTP Server
• Monitor application logs for crash events and memory access violations related to virtual host operations
• Implement web application firewalls (WAF) to inspect and filter anomalous HTTP traffic patterns
• Use runtime application self-protection (RASP) solutions to detect out-of-bounds memory access attempts

Monitoring Recommendations

• Enable verbose logging for the Monkey HTTP Server to capture detailed request information during crash events
• Configure system monitoring to alert on unexpected process terminations or high restart frequency
• Implement centralized log aggregation to correlate crash events across multiple server instances
• Monitor memory usage patterns for anomalies that may indicate exploitation attempts

How to Mitigate CVE-2025-63653

Immediate Actions Required

• Review deployment of Monkey HTTP Server instances and identify those running vulnerable versions (commit f37e984)
• Consider temporarily taking vulnerable servers offline or placing them behind additional network filtering if they are internet-facing
• Implement network-level controls to restrict access to the Monkey HTTP Server from untrusted sources
• Monitor server logs closely for any signs of exploitation attempts

Patch Information

Organizations should monitor the official Monkey HTTP Server repository and the referenced GitHub Issue Discussion #426 for updates regarding patches addressing this vulnerability. The GitHub Security Advisory may also contain updated guidance on remediation.

As specific patch information is not available in the CVE data, it is recommended to:

• Subscribe to the Monkey project's security announcements
• Regularly check for new commits that address the mk_vhost_fdt_close function
• Apply updates as soon as they become available from the project maintainers

Workarounds

• Place vulnerable Monkey HTTP Server instances behind a reverse proxy or load balancer that can filter malicious requests
• Restrict network access to the server using firewall rules, limiting connections to trusted IP ranges only
• Consider migrating to an alternative web server until a patch is available if the service is critical
• Implement request rate limiting to reduce the impact of potential DoS attacks