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Introduction

In early 2024, a critical vulnerability was disclosed in the Microsoft Azure SDK. Known as CVE-2024-21421, this flaw made it possible for attackers to spoof trusted Azure resources, potentially allowing unauthorized actions and service impersonation. If you build or maintain cloud applications using Azure SDK, understanding this vulnerability is crucial for safeguarding your services.

This long read will break down what the vulnerability is, how it works, show code snippets to explain the risk, and walk through a proof-of-concept exploit. All information here is explained in straightforward American language and provides exclusive practical details.

What Is CVE-2024-21421?

CVE-2024-21421 is a "spoofing vulnerability" in the Azure SDK for various languages (especially .NET and JavaScript). The flaw arises when client applications accept or process responses that an attacker could have tampered with or crafted, making them look like they genuinely came from an Azure service.

Impact:

Gain unauthorized access to data or functions

Microsoft published details and their patch here:
Microsoft Security Update Guide – CVE-2024-21421

How Does the Azure SDK Spoofing Happen?

Azure SDK clients often rely on secure tokens and signed responses to verify information comes from trusted Azure resources (like Azure Storage, Key Vault, or Identity endpoints).

CVE-2024-21421 specifically was caused because the Azure SDK did not rigorously validate all parts of response objects—certain claims or metadata could be spoofed if an attacker man-in-the-middled (MITM) the connection, or if a service endpoint was misconfigured.

Typical vulnerable flow

1. Application requests a resource via the Azure SDK client (for example, getting a blob from Azure Storage).

Azure SDK accepts the response and parses resource details.

3. If response validation is insufficient, an attacker could inject forged data (like resource URIs, user IDs, or claims).
4. The application might then treat this forged data as legitimate, acting on it or exposing it to users.

Code Example: Simulating the Vulnerability

Let’s see a code snippet in JavaScript (Node.js) using the Azure Identity SDK, vulnerable to spoofed claims.

// vulnerable.js
const { DefaultAzureCredential } = require("@azure/identity");
const { SecretClient } = require("@azure/keyvault-secrets");

const url = "https://my-key-vault.vault.azure.net/";;
const credential = new DefaultAzureCredential();
const client = new SecretClient(url, credential);

async function getSecret(name) {
  // No extra response validation
  const secret = await client.getSecret(name);
  // Suppose this client trusts properties from response blindly
  console.log("Secret value:", secret.value);
  console.log("Secret metadata (subject):", secret.properties.subject); // can be spoofed!
}

getSecret("my-secret").catch(console.error);

Here, secret.properties.subject could be spoofed if an attacker controls the network and the SDK doesn't correctly validate the response.

Exploit Details: What Would an Attacker Do?

1. Position as MITM: The attacker tricks the victim into talking to a compromised endpoint, or uses DNS poisoning to reroute Azure traffic through an attacker-controlled proxy.
2. Forge Response: The attacker crafts a response containing malicious or fake claims. For example, they can set the subject or identity fields in the secret’s metadata to impersonate another user or service.
3. Abuse Trust: The client application trusts these claims, possibly making decisions (such as showing secrets to the wrong user, or escalating permissions) based on attacker-supplied data.

A simplified proof-of-concept script for a MITM proxy (Python, using mitmproxy) that modifies the Key Vault response:

# mitmproxy_addon.py
def response(flow):
    if "my-key-vault.vault.azure.net" in flow.request.pretty_url:
        # Inject forged subject field
        data = flow.response.text
        data = data.replace('"subject":"original-user"', '"subject":"attacker-user"')
        flow.response.text = data

With this, if the vulnerable application above fetches a secret, it will see attacker-user as the subject!

Releasing updated SDK versions

Microsoft’s official fix guidance:
Microsoft Security Advisory CVE-2024-21421

Update Your SDKs!

Make sure you are running the patched versions listed in the above advisory—older versions remain vulnerable.

Do Not Blindly Trust SDK Response

If possible, validate important claims and fields yourself, especially when used for sensitive authorization or logic.

References

- Official Microsoft CVE-2024-21421 Page
- Azure SDK Github Issues and Changelog
- Azure SDK Security Documentation
- Sample exploit code and more info: GitHub Gist (external)

Conclusion

*CVE-2024-21421* stands as a reminder: even in high-level, officially supported SDKs, trust boundaries matter. Always keep your dependencies updated and know what you’re trusting in your code. By understanding flaws like this, you can build more secure and reliable cloud applications.

Timeline

Published on: 03/12/2024 17:15:50 UTC
Last modified on: 03/12/2024 17:46:17 UTC