CVE-2025-63656: Monkey HTTP Server DoS Vulnerability

CVE-2025-63656 Overview

CVE-2025-63656 is an out-of-bounds read vulnerability in the header_cmp function located in mk_server/mk_http_parser.c of the Monkey HTTP Server. This vulnerability allows remote attackers to cause a Denial of Service (DoS) condition by sending specially crafted HTTP requests to a vulnerable server instance.

Critical Impact

Remote attackers can crash Monkey HTTP Server instances by exploiting this out-of-bounds read vulnerability, leading to service disruption without authentication.

Affected Products

• Monkey HTTP Server commit f37e984 and potentially surrounding commits
• Monkey HTTP Server versions containing the vulnerable header_cmp function

Discovery Timeline

• 2026-01-29 - CVE CVE-2025-63656 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-63656

Vulnerability Analysis

This vulnerability represents an out-of-bounds read memory corruption issue in the Monkey HTTP Server's HTTP parsing component. The flaw exists within the header_cmp function, which is responsible for comparing HTTP headers during request processing.

When the server receives a maliciously crafted HTTP request, the header_cmp function fails to properly validate the boundaries of the data being read. This allows the function to read memory beyond the allocated buffer, potentially accessing uninitialized or protected memory regions.

The impact is primarily a Denial of Service condition, as the out-of-bounds read can trigger a crash or cause the server process to terminate unexpectedly. While the vulnerability does not appear to allow arbitrary code execution or data exfiltration based on the current analysis, it can be exploited remotely without authentication, making it a significant availability concern for deployments using Monkey HTTP Server.

Root Cause

The root cause of this vulnerability lies in insufficient bounds checking within the header_cmp function in mk_server/mk_http_parser.c. When processing HTTP request headers, the function does not adequately validate the length of input data before performing memory read operations. This allows attackers to supply oversized or malformed header values that cause the function to read past the end of the allocated buffer.

Attack Vector

The attack vector for CVE-2025-63656 involves sending a crafted HTTP request to a Monkey HTTP Server instance. The malicious request contains specially constructed headers designed to trigger the out-of-bounds read condition in the header_cmp function. Since this vulnerability can be exploited over the network through standard HTTP traffic, no authentication or prior access to the target system is required.

The vulnerability manifests during HTTP header parsing operations. Attackers can craft HTTP requests with specific header structures that cause the parser to read beyond buffer boundaries. For detailed technical analysis, refer to the GitHub Security Advisory and the GitHub Issue Discussion.

Detection Methods for CVE-2025-63656

Indicators of Compromise

• Unexpected crashes or restarts of Monkey HTTP Server processes
• Segmentation fault errors in server logs related to HTTP parsing operations
• Unusual HTTP requests with abnormally long or malformed headers in access logs
• Core dump files indicating memory access violations in mk_http_parser.c

Detection Strategies

• Monitor Monkey HTTP Server process stability for unexpected terminations
• Implement application-level monitoring to detect repeated crashes or restarts
• Deploy network intrusion detection rules to identify malformed HTTP request patterns
• Review server logs for segmentation faults or memory access errors related to header parsing

Monitoring Recommendations

• Configure process monitoring to alert on Monkey HTTP Server crashes
• Enable core dump collection for post-incident forensic analysis
• Implement log aggregation to correlate HTTP access logs with server error logs
• Deploy network traffic analysis to baseline normal HTTP header patterns and detect anomalies

How to Mitigate CVE-2025-63656

Immediate Actions Required

• Update Monkey HTTP Server to a version that includes the security fix for the header_cmp function
• Review deployment configurations and limit exposure of vulnerable instances to untrusted networks
• Implement network-level filtering to restrict access to Monkey HTTP Server from trusted sources only
• Monitor for crash events and unusual HTTP traffic patterns targeting affected servers

Patch Information

A security fix for this vulnerability has been discussed in the Monkey project. System administrators should check the GitHub Issue Discussion for the latest patch information and update to a commit that includes the boundary checking fix for the header_cmp function in mk_server/mk_http_parser.c.

Workarounds

• Place a reverse proxy or Web Application Firewall (WAF) in front of Monkey HTTP Server to filter malicious requests
• Implement rate limiting to reduce the impact of potential DoS attempts
• Restrict network access to the Monkey HTTP Server using firewall rules to allow only trusted IP addresses
• Consider temporarily switching to an alternative HTTP server until a patched version is deployed

# Example: Restrict access to Monkey HTTP Server using iptables
# Allow only trusted network (adjust IP range as needed)
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP