CVE-2025-63655: Monkey HTTP Server DoS Vulnerability

CVE-2025-63655 Overview

A NULL pointer dereference vulnerability has been identified in the mk_http_range_parse function within 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. The flaw occurs when the server improperly handles malformed Range headers, leading to a crash when dereferencing a NULL pointer.

Critical Impact

Remote attackers can crash the Monkey HTTP Server without authentication, causing service disruption for all users relying on the affected web server.

Affected Products

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

Discovery Timeline

• 2026-01-29 - CVE-2025-63655 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-63655

Vulnerability Analysis

This vulnerability is a classic NULL pointer dereference issue located in the mk_http_range_parse function within the mk_server/mk_http.c source file. When the Monkey HTTP Server processes incoming HTTP requests containing Range headers, the parsing function fails to properly validate pointer values before dereferencing them. This oversight allows an attacker to craft a malicious HTTP request that triggers the NULL pointer condition, causing the server process to crash.

The vulnerability is particularly concerning because it can be exploited remotely without any authentication requirements. Any attacker who can send HTTP requests to the server can potentially trigger the crash, making this a low-complexity attack vector with significant availability impact.

Root Cause

The root cause lies in insufficient input validation within the mk_http_range_parse function. The function processes HTTP Range header values but does not adequately check for NULL pointers before attempting to dereference them. When a specially crafted Range header is received that causes internal parsing to return NULL values, the subsequent code attempts to access memory through these NULL pointers, resulting in a segmentation fault and process termination.

Attack Vector

The attack is conducted remotely over the network by sending a malformed HTTP request to the Monkey HTTP Server. The attacker crafts an HTTP request with a specifically malformed Range header that exploits the parsing logic in mk_http_range_parse. When the server attempts to process this request, the NULL pointer dereference occurs, causing an immediate crash of the server process.

The vulnerability can be triggered by sending crafted HTTP Range headers to the server. When the mk_http_range_parse function in mk_server/mk_http.c processes malformed input, it fails to validate pointer values before dereferencing, leading to a segmentation fault. For technical details, see the GitHub Security Advisory.

Detection Methods for CVE-2025-63655

Indicators of Compromise

• Unexpected crashes or restarts of the Monkey HTTP Server process
• Segmentation fault entries in system logs associated with the mk_http_range_parse function
• Unusual HTTP requests with malformed or abnormal Range headers in access logs
• Service availability interruptions without apparent resource exhaustion

Detection Strategies

• Monitor HTTP access logs for requests containing unusual or malformed Range header patterns
• Implement application-level monitoring to detect repeated server crashes or restarts
• Deploy web application firewalls (WAF) with rules to detect anomalous Range header values
• Use intrusion detection systems (IDS) to identify patterns of DoS attempts targeting the server

Monitoring Recommendations

• Configure crash dump collection for the Monkey HTTP Server process to capture forensic data
• Set up alerting for service availability monitoring to detect unexpected downtime
• Implement log aggregation and analysis to correlate crash events with incoming HTTP requests
• Monitor for patterns of repeated requests from single sources targeting the server

How to Mitigate CVE-2025-63655

Immediate Actions Required

• Review the GitHub Issue Discussion for the latest patch information and apply available fixes
• Consider implementing network-level filtering to restrict access to the Monkey HTTP Server
• Deploy a reverse proxy or WAF in front of the server to filter malicious Range headers
• Monitor server logs closely for any exploitation attempts

Patch Information

Refer to the GitHub Security Advisory for official patch details and updated commits. Upgrade the Monkey HTTP Server to a version that includes the fix for the NULL pointer dereference in the mk_http_range_parse function.

Workarounds

• Deploy a reverse proxy (such as nginx or HAProxy) to sanitize and validate Range headers before forwarding requests
• Implement request filtering at the network perimeter to block requests with malformed Range headers
• Consider temporarily disabling Range header processing if the application does not require partial content delivery
• Restrict network access to the Monkey HTTP Server to trusted IP addresses only

If direct patching is not immediately possible, configure a reverse proxy to filter HTTP Range headers. Consult the GitHub Issue Discussion for community-provided mitigation configurations and workarounds.