CVE-2025-63651: Monkey HTTP Server DoS Vulnerability

CVE-2025-63651 Overview

A use-after-free vulnerability has been discovered in the mk_string_char_search function located in mk_core/mk_string.c of the Monkey HTTP Server (commit f37e984). This memory corruption flaw allows remote attackers to cause a Denial of Service (DoS) condition by sending specially crafted HTTP requests to the affected server.

Critical Impact

Remote attackers can crash the Monkey HTTP Server through malicious HTTP requests, causing service disruption without requiring authentication.

Affected Products

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

Discovery Timeline

• 2026-01-29 - CVE-2025-63651 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-63651

Vulnerability Analysis

This vulnerability is a use-after-free (UAF) memory corruption issue affecting the string processing functionality of the Monkey HTTP Server. The flaw resides in the mk_string_char_search function within the mk_core/mk_string.c source file.

Use-after-free vulnerabilities occur when a program continues to use a memory reference after that memory has been freed and potentially reallocated for other purposes. In this case, the vulnerable function fails to properly manage memory lifecycle when processing certain string operations during HTTP request handling.

When an attacker sends a maliciously crafted HTTP request, the server triggers a code path where previously freed memory is accessed through the mk_string_char_search function. This leads to undefined behavior that manifests as a crash, causing complete service disruption.

Root Cause

The root cause of this vulnerability lies in improper memory management within the mk_string_char_search function. The function appears to retain a reference to memory that has been freed elsewhere in the codebase. When the function subsequently attempts to read or operate on this dangling pointer, the program accesses invalid memory, leading to a crash condition.

This type of flaw typically arises from:

• Missing checks to verify memory validity before access
• Incorrect ordering of memory free and usage operations
• Failure to nullify pointers after freeing associated memory
• Complex control flow that makes memory lifecycle tracking difficult

Attack Vector

The attack can be executed remotely over the network by sending a specially crafted HTTP request to a Monkey HTTP Server instance. No authentication is required to exploit this vulnerability, making it accessible to any attacker who can reach the server over the network.

The exploitation mechanism involves:

1. The attacker identifies a target running a vulnerable version of Monkey HTTP Server
2. A crafted HTTP request is constructed to trigger the vulnerable code path in mk_string_char_search
3. When the server processes this request, it attempts to access freed memory
4. The memory access violation causes the server process to crash
5. Service is disrupted until the server is restarted

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

Detection Methods for CVE-2025-63651

Indicators of Compromise

• Unexpected Monkey HTTP Server process crashes or restarts
• Abnormal HTTP request patterns in server access logs, particularly malformed or unusually structured requests
• Core dump files generated by the server process indicating memory access violations
• System logs showing segmentation faults or SIGABRT signals from the Monkey process

Detection Strategies

• Monitor server logs for repeated crash events or abnormal termination signals
• Implement network intrusion detection rules to identify malformed HTTP request patterns targeting the server
• Deploy application-level monitoring to track server uptime and automatic restart events
• Review core dumps and crash reports for stack traces involving mk_string_char_search or related string functions

Monitoring Recommendations

• Enable detailed logging on Monkey HTTP Server instances to capture request characteristics before crashes
• Configure alerting for server process terminations and automatic restarts
• Implement network traffic analysis to detect anomalous HTTP request patterns
• Set up health check endpoints to quickly detect service unavailability

How to Mitigate CVE-2025-63651

Immediate Actions Required

• Identify all Monkey HTTP Server instances running vulnerable versions (commit f37e984 or earlier)
• Apply rate limiting and request filtering at the network perimeter to reduce exposure
• Consider placing a reverse proxy or web application firewall in front of affected servers
• Monitor for patch releases from the Monkey project maintainers
• Evaluate temporary service isolation if the server is not critical to operations

Patch Information

A specific patch version has not been identified in the available CVE data. Organizations should monitor the GitHub Issue #426 and the security advisory for official remediation guidance from the project maintainers.

Check the official Monkey HTTP Server repository for commits addressing memory management in mk_core/mk_string.c following commit f37e984.

Workarounds

• Deploy a reverse proxy (such as nginx or HAProxy) in front of the Monkey HTTP Server to filter malicious requests
• Implement network-level access controls to restrict which clients can connect to the server
• Apply request validation and sanitization at the load balancer or WAF level
• Consider migrating to an alternative HTTP server if patches are not available in a timely manner

# Example: Rate limiting with iptables to reduce DoS attack surface
iptables -A INPUT -p tcp --dport 2001 -m connlimit --connlimit-above 50 -j DROP
iptables -A INPUT -p tcp --dport 2001 -m limit --limit 25/minute --limit-burst 100 -j ACCEPT