(CodeQL) Sandboxed URL creation to prevent SSRF attacks

[pixeebot-edge]

Pixee Fix ID: 075b03a4-09ef-44f3-8d2c-d91a2e6a0afc

Confidence: HIGH

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Remediation

This change fixes findings identified by CodeQL.

Details

This change sandboxes the creation of java.net.URL objects so they will be more resistant to Server-Side Request Forgery (SSRF) attacks.

Most of the time when you create a URL, you're intending to reference an HTTP endpoint, like an internal microservice. However, URLs can point to local file system files, a Gopher stream in your local network, a JAR file on a remote Internet site, and all kinds of other unexpected and undesirable stuff. When the URL values are influenced by attackers, they can trick your application into fetching internal resources, running malicious code, or otherwise harming the system. Consider the following code:

String url = userInput.getServiceAddress();
return IOUtils.toString(new URL(url).openConnection());

In this case, an attacker could supply a value like jar:file:/path/to/appserver/lib.jar and attempt to read the contents of your application's code.

Our changes introduce sandboxing around URL creation that force the developers to specify some boundaries on the types of URLs they expect to create:

+ import io.github.pixee.security.Urls;
+ import io.github.pixee.security.HostValidator;
  ...
  String url = userInput.getServiceAddress();
- URL u = new URL(url);
+ URL u = Urls.create(url, Urls.HTTP_PROTOCOLS, HostValidator.DENY_COMMON_INFRASTRUCTURE_TARGETS);
  InputStream is = u.openConnection();

This change alone reduces attack surface significantly, but can be enhanced to create even more security by specifying some controls around the hosts we expect to connect with:

+ import io.github.pixee.security.Urls;
+ import io.github.pixee.security.HostValidator;
  ...
  HostValidator allowsOnlyGoodDotCom = HostValidator.fromAllowedHostPattern(Pattern.compile("good\\.com"));
  URL u = Urls.create(url, Urls.HTTP_PROTOCOLS, allowsOnlyGoodDotCom);

Note: Beware temptation to write some validation on your own. Parsing URLs is difficult and differences between parsers in validation and execution will certainly lead to exploits as attackers have repeatedly proven.

More reading

• https://www.hacksplaining.com/prevention/ssrf
• https://portswigger.net/web-security/ssrf
• https://cheatsheetseries.owasp.org/cheatsheets/Server_Side_Request_Forgery_Prevention_Cheat_Sheet.html
• https://www.rapid7.com/blog/post/2021/11/23/owasp-top-10-deep-dive-defending-against-server-side-request-forgery/
• https://blog.assetnote.io/2021/01/13/blind-ssrf-chains/

To fix the SSRF vulnerability, we need to validate the user-provided URL against a predefined list of allowed hosts or URL prefixes. This ensures that the server only makes requests to trusted endpoints. The best approach is to maintain a list of allowed hosts in the configuration and validate the uri.getHost() against this list before constructing the HTTP request.

Steps to implement the fix:

1. Define a list of allowed hosts as a constant or retrieve it from a configuration file.
2. Replace the placeholder "REPLACE_WITH_ALLOWED_HOST" with logic to check if the host of the URI is in the list of allowed hosts.
3. Throw an exception if the host is not in the allowed list.
4. Ensure that the validation is applied consistently across all methods that use the fetchFeed function.

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