CVE-2025-63652: Monkey HTTP Server DoS Vulnerability

CVE-2025-63652 Overview

A use-after-free vulnerability has been identified in the mk_http_request_end function within the Monkey HTTP Server. This memory corruption flaw exists in the mk_server/mk_http.c source file at commit f37e984. The vulnerability allows remote attackers to cause a Denial of Service (DoS) condition by sending specially crafted HTTP requests to the affected server.

Use-after-free vulnerabilities occur when a program continues to use a pointer after the memory it references has been freed. In the context of an HTTP server, this can lead to unpredictable behavior, crashes, and service unavailability when processing malicious requests.

Critical Impact
Remote attackers can crash the Monkey HTTP Server by sending crafted HTTP requests, causing service disruption and potential downstream availability impacts for applications relying on this lightweight web server.

Affected Products

Monkey HTTP Server (commit f37e984 and potentially adjacent versions)
Applications and services built on or embedding Monkey HTTP Server

Discovery Timeline

2026-01-29 - CVE-2025-63652 published to NVD
2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-63652

Vulnerability Analysis

The vulnerability resides in the HTTP request handling logic of the Monkey web server, specifically within the mk_http_request_end function. Use-after-free conditions in HTTP servers are particularly dangerous because they can be triggered remotely without authentication, simply by sending malformed or specially crafted HTTP requests.

When the server processes an HTTP request, it allocates memory structures to track the request state and associated data. The flaw in mk_http_request_end causes the server to reference memory that has already been deallocated during request finalization. This leads to undefined behavior—typically manifesting as a crash that terminates the server process.

The vulnerability is exploitable over the network, requiring no prior authentication or special privileges. An attacker simply needs network access to the Monkey HTTP server to trigger the condition.

Root Cause

The root cause is improper memory lifecycle management in the mk_http_request_end function located in mk_server/mk_http.c. The code fails to properly track whether memory associated with an HTTP request has been freed before attempting to access it. This creates a dangling pointer situation where subsequent operations on the request object access invalid memory regions.

Such issues typically arise from complex control flow paths where memory may be freed in one code branch but still referenced in another, or from incorrect ordering of cleanup operations during request termination.

Attack Vector

The attack vector involves sending crafted HTTP requests to the Monkey server. The specific request structure that triggers the use-after-free condition causes the server to enter a code path where memory is prematurely freed while still being referenced. This can be accomplished by:

Establishing a TCP connection to the target Monkey HTTP server
Sending a specially crafted HTTP request designed to trigger the vulnerable code path
The server processes the request and encounters the use-after-free condition
The server crashes, resulting in denial of service

The attack requires no authentication and can be performed by any network client that can reach the server. For detailed technical information about the vulnerability, refer to the GitHub Security Advisory and GitHub Issue #426.

Detection Methods for CVE-2025-63652

Indicators of Compromise

Unexpected Monkey HTTP server process crashes or restarts
Core dump files generated by the Monkey server process containing use-after-free crash signatures
Abnormal or malformed HTTP requests in server access logs preceding crashes
Service monitoring alerts indicating repeated HTTP server unavailability

Detection Strategies

Monitor Monkey HTTP server process stability and implement automatic restart detection
Deploy network intrusion detection rules to identify potentially malformed HTTP requests targeting Monkey servers
Enable and analyze core dumps to identify use-after-free crash patterns in the mk_http_request_end function
Implement log correlation to identify attack patterns across multiple server instances

Monitoring Recommendations

Configure system monitoring to alert on unexpected Monkey server process terminations
Implement service health checks with appropriate timeout thresholds to detect DoS conditions
Monitor system resources for signs of repeated crash/restart cycles indicating active exploitation
Track HTTP error rates and connection patterns for anomalies that may indicate exploitation attempts

How to Mitigate CVE-2025-63652

Immediate Actions Required

Identify all instances of Monkey HTTP server in your environment, particularly those running commit f37e984 or nearby versions
Assess network exposure of affected Monkey servers and implement access restrictions where possible
Monitor affected servers for signs of exploitation or unusual crash activity
Review the GitHub Security Advisory for the latest remediation guidance

Patch Information

Check the official Monkey HTTP server repository for security patches addressing this use-after-free vulnerability. Monitor the GitHub Issue #426 for updates on fix availability. Update to a patched version as soon as one becomes available.

Organizations should subscribe to the Monkey project's security announcements and monitor the repository for commits addressing this specific vulnerability in mk_server/mk_http.c.

Workarounds

Place affected Monkey servers behind a reverse proxy or web application firewall (WAF) that can filter malformed HTTP requests
Implement network segmentation to limit exposure of Monkey servers to trusted networks only
Deploy rate limiting and connection throttling to reduce the impact of potential exploitation attempts
Consider temporarily switching to an alternative HTTP server if the service is critical and no patch is available