urllib3 is a user-friendly HTTP client library for Python. Prior to 2.5.0, urllib3 does not control redirects in browsers and Node.js. urllib3 supports being used in a Pyodide runtime utilizing the JavaScript Fetch API or falling back on XMLHttpRequest. This means Python libraries can be used to make HTTP requests from a browser or Node.js. Additionally, urllib3 provides a mechanism to control redirects, but the retries and redirect parameters are ignored with Pyodide; the runtime itself determines redirect behavior. This issue has been patched in version 2.5.0.
Weakness
A web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a Redirect. This simplifies phishing attacks.
Potential Mitigations
- Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
- When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
- Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Use a list of approved URLs or domains to be used for redirection.
- When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
- For example, ID 1 could map to “/login.asp” and ID 2 could map to “http://www.example.com/". Features such as the ESAPI AccessReferenceMap [REF-45] provide this capability.
- Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.
- Many open redirect problems occur because the programmer assumed that certain inputs could not be modified, such as cookies and hidden form fields.
References