Patch & Verify SAP Cloud SDK Python OAuth2 SSRF

This guide walks you through patching and verifying a Server-Side Request Forgery (SSRF) vulnerability in the SAP Cloud SDK for Python’s OAuth2 client-credentials flow. You’ll learn exactly which code to change, how to apply the patch, and how to run a verification test to confirm the fix blocks malicious URL redirects. This is relevant for developers using the SAP Cloud SDK for Python (versions prior to 3.3.0) who are exposing OAuth2 token endpoints to untrusted input.

What You’ll Need

A Python project using the SAP Cloud SDK for Python (version 3.3.0 or earlier).
Access to the source code repository (e.g., GitHub) or the local installed package.
Python 3.7+ installed on your machine.
A basic understanding of OAuth2 client-credentials flow and HTTP requests.
An IDE or text editor to modify code.
Optional: A test HTTP server (like http.server) to simulate a malicious endpoint.

Step-by-Step

Identify the vulnerable code – In your project, locate the OAuth2 client-credentials token retrieval logic. This is typically in the sap/cds/oauth2/client.py file within the SDK package, or in your own code if you’ve implemented a custom token fetcher. The vulnerability lies in the handling of the token_url parameter – the SDK does not validate the URL before making a request, allowing an attacker to supply a URL pointing to internal services (e.g., http://169.254.169.254/ for cloud metadata).

Patch the URL validation – Modify the code to validate the token_url against a allowlist of trusted hosts. If you’re using the SDK’s built-in class, you’ll need to override it or use a custom token provider. Here’s a minimal patch example for a custom function that fetches a token:

import requests
from urllib.parse import urlparse

def fetch_token(token_url, client_id, client_secret):
    # Validate the URL to prevent SSRF
    parsed = urlparse(token_url)
    if parsed.scheme not in ('https',) or parsed.hostname not in ('your-oauth-server.com',):
        raise ValueError("Invalid token URL")
    
    response = requests.post(
        token_url,
        data={'grant_type': 'client_credentials'},
        auth=(client_id, client_secret),
        timeout=5
    )
    return response.json()

If you must allow multiple hosts, use a list of allowed domains and check the hostname against it.

Apply the patch to the SDK (if using the built-in) – If you’re using the SDK’s OAuth2ClientCredentialsTokenProvider, you’ll need to subclass it and override the get_token method to add the validation. Alternatively, you can monkey-patch the requests.Session.request method to intercept and validate the URL, but that’s more invasive. The cleanest approach is to create a custom token provider that wraps the SDK’s provider and validates the URL before delegating.
Update your project dependencies – After patching, ensure your project uses the patched code. If you’re using a virtual environment, you may need to reinstall the package or copy the modified file into your project’s source (if you’re not using a forked version). Use pip freeze to check the current version.
Write a verification test – Create a Python test script (e.g., test_ssrf.py) that attempts to fetch a token from a malicious URL pointing to a local service (like http://127.0.0.1:8080/) and asserts that the request is blocked. Example:
```
import pytest
from your_module import fetch_token

def test_ssrf_blocked():
    with pytest.raises(ValueError):
        fetch_token('http://127.0.0.1:8080/', 'client', 'secret')
```
Run the test with pytest test_ssrf.py.
Run the full test suite – Execute your project’s existing tests to ensure the patch doesn’t break normal OAuth2 flows. Use pytest or your preferred test runner.
Verify in a live environment – If possible, deploy the patched code to a staging environment and attempt to trigger the SSRF by providing a malicious token URL in a configuration or request. Confirm that the request is rejected and the application logs an error.

Common Mistakes

Only checking the scheme – Some developers only validate that the URL uses HTTPS, but an attacker can still point to an internal HTTPS service (e.g., https://169.254.169.254/). Always validate the hostname.
Using a blocklist instead of an allowlist – Blocking known internal IPs (like 169.254.169.254) is insufficient because attackers can use other internal IPs or DNS rebinding. Use an allowlist of trusted hosts.
Not handling redirects – The requests library follows redirects by default. An attacker could use a redirect from a trusted host to an internal one. Disable redirects (allow_redirects=False) or validate the final URL after redirects.
Forgetting to update the SDK version – If you’re relying on the SDK’s built-in provider, you must ensure you’re using the patched version (3.3.0 or later). Simply patching your own code may not be enough if you’re using the SDK’s class.

Tips

Use a dedicated configuration for token URLs – Store the allowed token endpoints in a configuration file or environment variable, so it’s easy to update without code changes.
Log all token requests – Enable logging for OAuth2 requests to detect any unusual activity. Use the logging module and log the token URL (but never the client secret).
Consider using a library that already handles this – If you’re not deeply integrated with the SDK, consider using a more secure OAuth2 library that validates URLs by default.
Run security scans – After patching, run a static analysis tool (like Bandit) to check for other SSRF vulnerabilities in your code.

Troubleshooting

The patch doesn’t block the request – Check if you’re validating the URL in the correct place. If you’re using a custom token provider, ensure the validation happens before any request is made. Also, check if redirects are being followed – disable them if necessary.
Tests fail because the valid token URL is rejected – Your allowlist may be too restrictive. Make sure you’ve included all legitimate token endpoints. Use a list of allowed hostnames and compare against the hostname (case-insensitively).
The SDK’s built-in provider is still vulnerable – If you’re using the SDK’s class and can’t modify it, you must upgrade to version 3.3.0 or later. If you’re stuck on an older version, you’ll need to implement a custom provider as described.
Redirects cause SSRF despite validation – Set allow_redirects=False in the requests.post call and manually handle redirects, validating each redirect URL.
DNS rebinding attacks – If the token URL uses a domain that resolves to an internal IP, your allowlist check on the hostname may pass. To mitigate, resolve the hostname and verify the IP is not in a private range (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 169.254.0.0/16). This is more complex but adds a strong layer of defense.

Frequently Asked Questions

What exactly is the SSRF vulnerability in the SAP Cloud SDK for Python?

The vulnerability is in the OAuth2 client-credentials token request. If an attacker can control the token_url parameter (e.g., via a crafted configuration or user input), the SDK will make a request to that URL, potentially reaching internal services that should not be accessible from the internet.

Which versions of the SAP Cloud SDK for Python are affected?

Versions prior to 3.3.0 are affected. The fix was included in version 3.3.0, which added URL validation for token endpoints. If you’re using an older version, you should upgrade or apply a manual patch.

Can I just disable redirects to fix the SSRF?

Disabling redirects helps but is not sufficient on its own. You still need to validate the initial URL’s hostname. An attacker could point directly to an internal IP without using a redirect. Always combine redirect handling with an allowlist.

How do I know if my token URL is safe?

Check that the hostname is one you explicitly trust. Avoid using IP addresses directly; use domain names that you control. Also, ensure the URL uses HTTPS to prevent man-in-the-middle attacks.

What should I do if I can’t upgrade the SDK immediately?

Apply the manual patch described in this guide by creating a custom token provider that validates the URL. This is a temporary measure until you can upgrade to a patched version.

Are there any other security considerations for OAuth2 in SAP Cloud SDK?

Yes, always store client secrets securely (e.g., in a secret manager), use short-lived tokens, and rotate them regularly. Also, ensure your token endpoint is not exposed to untrusted users.

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