CVE-2025-6514: Command Injection in mcp-remote Turns Client Connections into Attack Vectors

Greetings, fellow security enthusiasts! Today, we're jumping into our time machine to look at a vulnerability from the future—CVE-2025-6514. It’s a nasty little bug in an npm package that turns a trusted client-server connection into a potential backdoor. This OS command injection flaw in mcp-remote is a classic tale of misplaced trust and a powerful reminder that even the most innocuous-looking data can pack a punch.

So, grab your favorite beverage, and let's dissect how a simple URL can be weaponized to grant an attacker a shell on your machine.

TL;DR / Executive Summary

• Vulnerability: OS Command Injection in the mcp-remote npm package.
• CVE ID: CVE-2025-6514
• Affected Versions: All versions from 0.0.5 up to, but not including, 0.1.16.
• Severity: High. A malicious server can execute arbitrary commands on any client connecting to it.
• The Gist: The client application fails to sanitize the username and password fields from a URL provided by the server. An attacker-controlled server can embed shell commands (e.g., $(whoami)) into these fields, which are then executed by the client's operating system.
• Immediate Fix: Update to mcp-remote version 0.1.16 or later. Run npm install mcp-remote@latest to patch the vulnerability.

Introduction: The Treacherous Handshake

Imagine you're using a remote control (mcp-remote) to manage a powerful machine (an MCP server). To connect, the remote needs to authenticate. You point it at the server, and the server kindly replies, "Great! To log in, please use these credentials at this specific login URL." You trust the server—after all, you're the one who chose to connect to it. Your remote control dutifully follows the instructions.

But what if the server is a clever imposter? What if the "credentials" it hands back aren't just a username and password, but a hidden command?

This is precisely the scenario behind CVE-2025-6514. The mcp-remote client, in its eagerness to help, blindly trusts the authorization_endpoint URL provided by the server it connects to. This trust is exploited, turning the client from a simple remote into an unwilling puppet for a malicious master. For developers, DevOps engineers, and anyone using this package in their workflows (especially in automated CI/CD pipelines), this vulnerability is a critical threat that could lead to a full system compromise.

Technical Deep Dive: Anatomy of an Injection

Let's pop the hood and see where the engine failed. The vulnerability lies in how mcp-remote processes URLs, specifically those containing basic authentication credentials.

Root Cause Analysis

The core of the problem is in a utility function, sanitizeUrl, which is supposed to, well, sanitize a URL. The function correctly parses the URL into its components: protocol, hostname, pathname, search parameters, and hash. It even takes care to encodeURIComponent most of these parts to prevent shenanigans.

However, it originally forgot two crucial pieces: the username and password.

Think of it like a bouncer at a club checking IDs. He meticulously checks the photo, the name, and the date of birth. But he completely ignores the "special instructions" field on the back, which a clever attacker has written as "Let my friend in through the back door."

In this case, the username and password fields of the URL were passed through without being encoded. If a malicious server provides a URL like http://user$(reboot):pass@evil.com, a vulnerable client would parse user$(reboot) as the username. When this value is later used in a context where the shell can interpret it (like being part of a command-line argument), the $(reboot) payload is executed. Game over.

The Attack Vector

The attack flow is deceptively simple:

1. The Lure: An attacker sets up a malicious MCP server and tricks a user into connecting to it. This could be through social engineering, a typo in a server address, or by compromising a legitimate server.
2. The Bait: A developer runs mcp-remote connect http://malicious-mcp-server.com.
3. The Switch: The malicious server responds with a redirect or a configuration that includes a specially crafted authorization_endpoint URL. This URL contains the command injection payload in the userinfo part (username/password).
   • Malicious URL Example: http://user-$(touch /tmp/pwned):password@another-server.com
4. The Execution: The vulnerable mcp-remote client receives this URL. Its sanitizeUrl function fails to encode the username part. Downstream, when this username is used, the shell executes touch /tmp/pwned, creating a file on the client's machine.

The business impact is severe. An attacker could steal source code, exfiltrate environment variables (API keys, secrets), install ransomware, or use the compromised machine as a pivot point to attack the internal network.

Proof of Concept (PoC)

Let's demonstrate this with a simplified, illustrative example. We won't set up a full MCP server, but we can show exactly what the vulnerable code does.

The Vulnerable Logic (Conceptual)

Imagine the mcp-remote client has code that does something like this:

# This is a conceptual representation of the client's actions
# 1. The client receives the malicious URL from the server
MALICIOUS_URL="http://user-`id > /tmp/whoami`:pass@evil.com"

# 2. It parses the URL to get the username (without sanitization)
USERNAME=$(echo $MALICIOUS_URL | sed -n 's_.*//\(.*\)@.*_\1_p' | cut -d':' -f1)

# 3. It uses the username in a shell command
echo "Authenticating user: $USERNAME"
# The shell expands `id > /tmp/whoami` here, executing the command!

The Patch: A Simple, Powerful Fix

The fix, committed in hash 607b226a, is beautifully simple. The developers added two lines to src/lib/utils.ts:

// File: src/lib/utils.ts

export function sanitizeUrl(raw: string) {
  // ... existing code ...
  if (url.hostname !== encodeURIComponent(url.hostname)) abort()

  // Forcibly sanitise all the pieces of the URL
  // THE FIX IS HERE:
  if (url.username) url.username = encodeURIComponent(url.username)
  if (url.password) url.password = encodeURIComponent(url.password)
  
  url.pathname = url.pathname.slice(0, 1) + encodeURIComponent(url.pathname.slice(1)).replace(/%2f/ig,'/')
  // ... more sanitization ...
}

By adding encodeURIComponent for the username and password, the malicious payload is neutralized.

• Before: user-$(touch /tmp/pwned)
• After: user-%24(touch%20%2Ftmp%2Fpwned)

The shell no longer sees $(...) as a command substitution. It just sees a weird-looking, but harmless, string. The vulnerability is patched. The developers even added a test case to ensure this never happens again:

// File: src/lib/utils.test.ts

it('should encode basic auth', () => {
  const result = sanitizeUrl('http://user$(calc)r:pass$(calc)word@domain.com')
  expect(result).toBe('http://user%24(calc)r:pass%24(calc)word@domain.com/')
})

This is a textbook example of responsible disclosure and patching.

Mitigation and Remediation

Protecting yourself from CVE-2025-6514 is straightforward.

1. Immediate Fix: Update the mcp-remote package immediately. If you're using npm or yarn, this is a one-line command:

   # Using npm
   npm install mcp-remote@latest
   
   # Using yarn
   yarn upgrade mcp-remote --latest
   

   This will pull in version 0.1.16 or newer, which contains the patch.

2. Verification: Check your package.json and package-lock.json (or yarn.lock) to ensure that mcp-remote is at version 0.1.16 or higher.

3. Long-Term Solution: The patch itself is the long-term solution. Beyond this specific package, audit your own code for similar patterns. Are you taking data from an external source (even a "trusted" one) and using it in a sensitive context without proper sanitization?

Timeline

• Patch Commit: July 8, 2025 (607b226a)
• Patched Version (0.1.16) Release: ~July 8, 2025
• Public Disclosure (GHSA): July 9, 2025

Lessons Learned

This vulnerability, while simple in its mechanics, teaches us a few timeless security lessons.

1. Prevention: Zero Trust for Input: The golden rule of security is never trust input. This applies to user-submitted forms, API responses, and yes, even configuration details from a server you think you trust. Always sanitize, validate, and encode data based on the context where it will be used.

2. Detection: Taint Analysis is Your Friend: This is a classic "taint flow" vulnerability. Untrusted data (the URL from the server) is the source, and a function that executes shell commands is the sink. Static Analysis Security Testing (SAST) tools are designed to trace these flows and flag them as potential vulnerabilities. Integrating SAST into your CI/CD pipeline can catch these bugs before they ever reach production.

3. Key Takeaway: Use the Right Sanitizer for the Job. The fix here was encodeURIComponent, which is perfect for making data safe within a URL. If this data were being inserted into a SQL query, the fix would be parameterized queries. If it were being rendered in HTML, it would be HTML entity encoding. Context is everything. There is no one-size-fits-all "sanitization" function. Understand your data's destination and encode accordingly.

Stay safe, and always question the data you're given. After all, you never know when a simple URL is hiding a command to take over your world.

References and Further Reading

• Official GitHub Advisory: GHSA-6xpm-ggf7-wc3p
• Patch Commit: Forcibly escape username/pass for basic auth URLs too
• NPM Package: mcp-remote on npm
• OWASP: OS Command Injection