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URL redirection from remote source

ID: py/url-redirection
Kind: path-problem
Severity: error
Precision: high
Tags:
   - security
   - external/cwe/cwe-601
Query suites:
   - python-code-scanning.qls
   - python-security-extended.qls
   - python-security-and-quality.qls

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Directly incorporating user input into a URL redirect request without validating the input can facilitate phishing attacks. In these attacks, unsuspecting users can be redirected to a malicious site that looks very similar to the real site they intend to visit, but which is controlled by the attacker.

Recommendation

To guard against untrusted URL redirection, it is advisable to avoid putting user input directly into a redirect URL. Instead, maintain a list of authorized redirects on the server; then choose from that list based on the user input provided.

Example

The following example shows an HTTP request parameter being used directly in a URL redirect without validating the input, which facilitates phishing attacks:

from flask import Flask, request, redirect

app = Flask(__name__)

@app.route('/')
def hello():
    target = request.args.get('target', '')
    return redirect(target, code=302)

One way to remedy the problem is to validate the user input against a known fixed string before doing the redirection:

from flask import Flask, request, redirect

VALID_REDIRECT = "http://cwe.mitre.org/data/definitions/601.html"

app = Flask(__name__)

@app.route('/')
def hello():
    target = request.args.get('target', '')
    if target == VALID_REDIRECT:
        return redirect(target, code=302)
    else:
        ... # Error

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