How ReDoS happens in Node.js MCP SDK and how to fix it

Introduction

In a web service using Anthropic's Model Context Protocol (MCP) TypeScript SDK, security scanner Trivy flagged CVE-2026-0621—a high-severity ReDoS vulnerability in @modelcontextprotocol/sdk version 1.24.0. This dependency, declared in package-lock.json, processes input in request handlers that are directly exposed to remote attackers.

The vulnerable code path handles user-influenced input, meaning an attacker could craft a malicious payload that exploits inefficient regular expression patterns within the SDK. When the regex engine encounters this input, it enters a state of catastrophic backtracking, effectively freezing the Node.js event loop and rendering the entire service unresponsive.

For developers building AI-powered applications with the MCP SDK, this vulnerability represents a critical availability risk—your service could be taken offline with a single carefully crafted request.

The Vulnerability Explained

What is ReDoS?

Regular Expression Denial of Service (ReDoS) occurs when a regex pattern exhibits exponential time complexity for certain inputs. The issue stems from how backtracking regex engines (like the one in Node.js) handle ambiguous patterns.

Consider a regex with nested quantifiers or overlapping alternatives. When the engine fails to find a match, it backtracks through an exponentially growing number of possible paths. For a vulnerable pattern, an input string of just 30-40 characters can require billions of operations to process.

The Specific Vulnerability in MCP SDK

The vulnerable dependency was declared in package-lock.json:

"node_modules/@modelcontextprotocol/sdk": {
  "version": "1.24.0",
  "resolved": "https://registry.npmjs.org/@modelcontextprotocol/sdk/-/sdk-1.24.0.tgz",
  "integrity": "sha512-D8h5KXY2vHFW8zTuxn2vuZGN0HGrQ5No6LkHwlEA9trVgNdPL3TF1dSqKA7Dny6BbBYKSW/rOBDXdC8KJAjUCg==",
  ...
}

The MCP SDK processes protocol messages that may contain user-controlled content. When these messages pass through regex-based parsing or validation logic with vulnerable patterns, an attacker can exploit the flaw.

Attack Scenario

Imagine this web service accepts MCP protocol messages from clients:

1. Attacker identifies the endpoint: The service exposes an MCP-compatible API endpoint
2. Crafts malicious payload: The attacker creates a message containing a string designed to trigger catastrophic backtracking (e.g., "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa!")
3. Sends the request: The malicious message is sent to the web service
4. Service freezes: The Node.js event loop blocks while processing the regex, making the entire service unresponsive
5. Denial of Service achieved: Legitimate users cannot access the service

Because this is a web service with request handlers directly exposed to remote attackers, exploitation requires no authentication or special access—just the ability to send HTTP requests.

Real-World Impact

For applications using the MCP SDK:

• Complete service unavailability: A single malicious request can freeze the entire Node.js process
• No automatic recovery: The event loop remains blocked until the regex completes (potentially hours) or the process is killed
• Resource exhaustion: CPU usage spikes to 100% on the affected core
• Cascading failures: In containerized environments, health checks fail, triggering restarts and potential cascade effects

The Fix

The fix involves upgrading @modelcontextprotocol/sdk from version 1.24.0 to 1.25.2, which contains patched regex patterns.

Changes Made

package.json (dependency declaration):

- "@modelcontextprotocol/sdk": "^1.24.0",
+ "@modelcontextprotocol/sdk": "^1.25.2",

package-lock.json (locked version):

  "node_modules/@modelcontextprotocol/sdk": {
-   "version": "1.24.0",
-   "resolved": "https://registry.npmjs.org/@modelcontextprotocol/sdk/-/sdk-1.24.0.tgz",
-   "integrity": "sha512-D8h5KXY2vHFW8zTuxn2vuZGN0HGrQ5No6LkHwlEA9trVgNdPL3TF1dSqKA7Dny6BbBYKSW/rOBDXdC8KJAjUCg==",
+   "version": "1.25.2",
+   "resolved": "https://registry.npmjs.org/@modelcontextprotocol/sdk/-/sdk-1.25.2.tgz",
+   "integrity": "sha512-LZFeo4F9M5qOhC/Uc1aQSrBHxMrvxett+9KLHt7OhcExtoiRN9DKgbZffMP/nxjutWDQpfMDfP3nkHI4X9ijww==",

Why This Fix Works

Version 1.25.2 of the MCP SDK addresses CVE-2026-0621 by:

1. Replacing vulnerable regex patterns: The problematic patterns were rewritten to avoid catastrophic backtracking
2. Adding input validation: Additional checks prevent oversized inputs from reaching regex processing
3. Architectural improvements: The update also adds @hono/node-server as a dependency, suggesting potential changes to how HTTP handling is performed

The fix is minimal and targeted—only the dependency versions in package.json and package-lock.json were modified. Existing tests continue to pass, confirming that the upgrade maintains behavioral compatibility while eliminating the vulnerability.

Prevention & Best Practices

Avoiding ReDoS in Your Code

1. Audit regex patterns for dangerous constructs:
   - Nested quantifiers: (a+)+
   - Overlapping alternatives: (a|a)+
   - Greedy quantifiers followed by overlapping patterns: .*.*

2. Use regex analysis tools:
   bash # Check regex patterns for ReDoS vulnerability npx redos-detector "your-regex-pattern"

3. Set execution timeouts:
   javascript // Use libraries that support regex timeouts const SafeRegex = require('safe-regex'); if (!SafeRegex(pattern)) { throw new Error('Potentially unsafe regex pattern'); }

4. Validate input before regex processing:
   javascript const MAX_INPUT_LENGTH = 1000; if (input.length > MAX_INPUT_LENGTH) { throw new Error('Input too long'); }

5. Consider linear-time regex engines:
   - RE2 (Google's regex library) guarantees linear time complexity
   - Available for Node.js via the re2 package

Dependency Management

• Enable automated security scanning: Tools like Trivy, Snyk, or Dependabot can catch vulnerable dependencies
• Keep dependencies updated: Regular updates reduce exposure to known vulnerabilities
• Lock dependency versions: Use package-lock.json to ensure reproducible builds
• Review transitive dependencies: Vulnerabilities often hide in indirect dependencies

Key Takeaways

• The MCP SDK v1.24.0 contained regex patterns vulnerable to catastrophic backtracking—always check CVE databases for your dependencies
• Web services using MCP SDK were directly exploitable—remote attackers could freeze servers without authentication
• Dependency upgrades are often the only fix for third-party vulnerabilities—you can't patch code you don't control
• ReDoS is particularly dangerous in Node.js due to its single-threaded event loop—one blocked regex blocks everything
• Automated scanning (Trivy) caught this before exploitation—integrate security scanners into your CI/CD pipeline

How Orbis AppSec Detected This

• Source: User-controlled input entering through MCP protocol message handlers
• Sink: Regex processing within @modelcontextprotocol/sdk v1.24.0 request handling logic
• Missing control: No protection against catastrophic backtracking in regex patterns; no input length validation before regex processing
• CWE: CWE-1333 (Inefficient Regular Expression Complexity)
• Fix: Upgraded @modelcontextprotocol/sdk from 1.24.0 to 1.25.2, which contains patched regex patterns

Orbis AppSec automatically detected this vulnerability and opened a pull request with the fix. Try Orbis AppSec on your repositories to find and fix issues like this automatically.

Conclusion

CVE-2026-0621 demonstrates how a seemingly innocuous dependency can introduce critical availability risks to your application. The ReDoS vulnerability in @modelcontextprotocol/sdk v1.24.0 could have allowed attackers to completely disable web services with minimal effort.

The fix was straightforward—a dependency upgrade—but the lesson is broader: your application's security is only as strong as your weakest dependency. Implement automated vulnerability scanning, keep dependencies updated, and understand the attack surface that third-party code introduces to your application.

For developers working with regex in Node.js, remember that the single-threaded event loop makes ReDoS particularly devastating. Always analyze regex patterns for catastrophic backtracking potential, validate input lengths, and consider using linear-time regex engines for untrusted input.

References

• CWE-1333: Inefficient Regular Expression Complexity
• OWASP Regular Expression Denial of Service
• Node.js RE2 Package Documentation
• Semgrep ReDoS Rules
• fix: upgrade @modelcontextprotocol/sdk to 1.25.2 (CVE-2026-0621)