CVE-2026-39313: mcp-framework HTTP Server DoS Vulnerability

CVE-2026-39313 Overview

CVE-2026-39313 is a Denial of Service vulnerability affecting mcp-framework, a framework for building Model Context Protocol (MCP) servers. The vulnerability exists in the readRequestBody() function within the HTTP transport layer, which concatenates request body chunks into a string without enforcing any size limit. Although a maxMessageSize configuration value exists in the codebase, it is never enforced during the body reading process, allowing attackers to exhaust server memory with oversized requests.

Critical Impact

A remote unauthenticated attacker can crash any mcp-framework HTTP server by sending a single large POST request to the /mcp endpoint, causing memory exhaustion and complete denial of service.

Affected Products

• mcp-framework versions 0.2.21 and below
• Any HTTP-based MCP server built using vulnerable mcp-framework versions
• Model Context Protocol servers accepting POST requests on /mcp endpoint

Discovery Timeline

• 2026-04-16 - CVE CVE-2026-39313 published to NVD
• 2026-04-16 - Last updated in NVD database

Technical Details for CVE-2026-39313

Vulnerability Analysis

This vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling). The core issue lies in the HTTP transport implementation of mcp-framework, where the readRequestBody() function processes incoming HTTP request bodies by concatenating data chunks into a single string buffer. While the framework includes a maxMessageSize configuration option intended to prevent oversized payloads, this limit was never actually enforced during the body reading process.

The vulnerability enables resource exhaustion attacks where an attacker can send arbitrarily large HTTP POST requests to the /mcp endpoint. Since no size validation occurs during chunk processing, the server will continue allocating memory to store the incoming data until system resources are exhausted, resulting in application crash or system instability.

Root Cause

The root cause is a missing implementation of size validation in the readRequestBody() function. The maxMessageSize configuration parameter exists but is ignored during request processing. The function concatenates all incoming data chunks without tracking the accumulated size or comparing it against configured limits, creating an unbounded resource allocation scenario.

Attack Vector

The attack can be executed remotely over the network without authentication. An attacker simply needs to send a single HTTP POST request to the /mcp endpoint with an extremely large body payload. The server will attempt to buffer the entire request body in memory, leading to memory exhaustion and denial of service. The attack requires no special privileges or user interaction, making it trivially exploitable against any exposed mcp-framework HTTP server.

// Security patch demonstrating the fix - Source: GitHub Commit
   }
 
   private async readRequestBody(req: IncomingMessage): Promise<any> {
+    const maxSize = this._config.maxMessageSize ?? 4 * 1024 * 1024;
     return new Promise((resolve, reject) => {
       let body = '';
+      let size = 0;
       req.on('data', (chunk) => {
+        size += chunk.length;
+        if (size > maxSize) {
+          req.destroy();
+          reject(new Error(`Request body exceeds maximum size of ${maxSize} bytes`));
+          return;
+        }
         body += chunk.toString();
       });
       req.on('end', () => {

Source: GitHub Commit Update

Detection Methods for CVE-2026-39313

Indicators of Compromise

• Unusual memory consumption spikes on servers running mcp-framework HTTP transport
• Large HTTP POST requests targeting the /mcp endpoint exceeding normal payload sizes
• Server crashes or out-of-memory errors in mcp-framework application logs
• Incomplete or abnormally long-running HTTP requests to MCP server endpoints

Detection Strategies

• Monitor HTTP request body sizes to the /mcp endpoint and alert on requests exceeding expected thresholds
• Implement application performance monitoring to detect sudden memory usage increases in mcp-framework processes
• Review web server access logs for unusually large Content-Length headers on POST requests
• Configure network intrusion detection to flag HTTP requests with excessive payload sizes

Monitoring Recommendations

• Set up memory usage alerts for mcp-framework server processes with thresholds based on normal operation baselines
• Enable request logging with body size metrics to identify potential exploitation attempts
• Monitor for repeated server restarts or crashes that may indicate ongoing DoS attacks
• Implement rate limiting on the /mcp endpoint to reduce the impact of sustained attack attempts

How to Mitigate CVE-2026-39313

Immediate Actions Required

• Upgrade mcp-framework to version 0.2.22 or later immediately
• If immediate upgrade is not possible, implement network-level request size limits as a temporary measure
• Review and configure the maxMessageSize setting appropriately for your deployment after upgrading
• Monitor server memory usage closely until patches are applied

Patch Information

The vulnerability has been fixed in mcp-framework version 0.2.22. The patch modifies the readRequestBody() function to track accumulated request body size and enforce the maxMessageSize configuration limit. When the limit is exceeded, the request is immediately destroyed and an error is returned, preventing memory exhaustion. The fix implements a default maximum size of 4MB if no custom configuration is specified.

For technical details, see the GitHub Security Advisory GHSA-353c-v8x9-v7c3 and the commit containing the fix.

Workarounds

• Deploy a reverse proxy (nginx, HAProxy) in front of mcp-framework servers with request body size limits configured
• Implement network firewall rules to limit maximum HTTP request sizes to the /mcp endpoint
• Use a Web Application Firewall (WAF) to enforce payload size restrictions
• Restrict network access to the MCP server endpoint to trusted sources only

# Configuration example - nginx reverse proxy with body size limit
# Add to nginx server block protecting mcp-framework endpoint
location /mcp {
    client_max_body_size 4m;
    proxy_pass http://mcp-backend;
}