CVE-2025-63657: Monkey HTTP Server DoS Vulnerability

CVE-2025-63657 Overview

An out-of-bounds read vulnerability has been identified in the mk_mimetype_find function located in mk_server/mk_mimetype.c of the Monkey HTTP Server. This vulnerability exists in commit f37e984 and allows remote attackers to cause a Denial of Service (DoS) condition by sending specially crafted HTTP requests to the server.

Critical Impact

Remote attackers can exploit this out-of-bounds read vulnerability to crash the Monkey HTTP Server, causing service disruption for legitimate users without requiring authentication.

Affected Products

• Monkey HTTP Server (commit f37e984)

Discovery Timeline

• 2026-01-29 - CVE CVE-2025-63657 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-63657

Vulnerability Analysis

This vulnerability is classified as an out-of-bounds read, a memory corruption flaw where the application reads data from a memory location outside the intended buffer boundaries. In the context of the Monkey HTTP Server, the mk_mimetype_find function fails to properly validate input boundaries when processing MIME type lookups during HTTP request handling.

When a malformed HTTP request is received, the function attempts to access memory beyond the allocated buffer, leading to undefined behavior. In most cases, this results in the server process crashing, effectively denying service to all connected clients. While out-of-bounds read vulnerabilities typically do not allow code execution, they can potentially leak sensitive memory contents in some scenarios.

Root Cause

The root cause of this vulnerability lies in insufficient bounds checking within the mk_mimetype_find function. When processing HTTP requests, the function performs MIME type lookups based on file extensions or content type headers. The implementation does not adequately validate the length of input data before accessing memory, allowing an attacker to trigger memory access outside the intended buffer.

This type of vulnerability commonly occurs when string handling operations assume well-formed input without implementing defensive checks against maliciously crafted data.

Attack Vector

The vulnerability can be exploited remotely over the network by sending a specially crafted HTTP request to a Monkey HTTP Server instance. No authentication is required to trigger this vulnerability, making it particularly dangerous for public-facing servers.

The attack scenario involves:

1. An attacker identifies a target running Monkey HTTP Server
2. A crafted HTTP request is constructed with malformed data designed to trigger the out-of-bounds read
3. The malicious request is sent to the server
4. The mk_mimetype_find function processes the request and accesses memory outside valid bounds
5. The server crashes, resulting in denial of service

For detailed technical information about this vulnerability, refer to the GitHub Security Advisory and the GitHub Issue Discussion.

Detection Methods for CVE-2025-63657

Indicators of Compromise

• Unexpected Monkey HTTP Server crashes or process terminations
• Unusual HTTP requests with malformed headers or abnormally long file extensions
• Server log entries showing parsing errors immediately before service interruption
• Memory-related error messages in system logs from the Monkey process

Detection Strategies

• Monitor Monkey HTTP Server process stability and implement automatic restart mechanisms with alerting
• Deploy network intrusion detection rules to identify malformed HTTP requests targeting MIME type parsing
• Implement web application firewall (WAF) rules to filter requests with suspicious characteristics
• Review server access logs for patterns of unusual requests preceding server crashes

Monitoring Recommendations

• Configure real-time alerting for Monkey HTTP Server process crashes
• Enable verbose logging to capture details of requests that may trigger the vulnerability
• Monitor memory usage patterns for anomalies in the Monkey server process
• Set up automated service health checks to detect DoS conditions quickly

How to Mitigate CVE-2025-63657

Immediate Actions Required

• Update Monkey HTTP Server to a version newer than commit f37e984 that includes the security fix
• Review the GitHub Issue Discussion for patch availability and remediation guidance
• Consider temporarily disabling public access to affected Monkey HTTP Server instances until patched
• Implement network-level filtering to restrict access to trusted sources only

Patch Information

Security patches and remediation details are tracked in the official Monkey HTTP Server repository. Administrators should consult the GitHub Security Advisory for specific patch information and version recommendations.

It is critical to update to a version that includes the fix for the bounds checking issue in the mk_mimetype_find function.

Workarounds

• Place the Monkey HTTP Server behind a reverse proxy that can filter and validate incoming HTTP requests
• Implement request rate limiting to reduce the impact of potential DoS attacks
• Restrict network access to the server using firewall rules, allowing only trusted IP ranges
• Deploy a web application firewall with rules to detect and block malformed HTTP requests