CVE-2017-7233

What it actually does

A flaw in Django's `is_safe_url()` function causes it to incorrectly validate URLs composed entirely of numbers as "safe". This bypasses security checks and creates an open redirect vulnerability. If an application uses the output of this function to construct hyperlinks, this vulnerability can also be leveraged to conduct a Cross-Site Scripting (XSS) attack.

Why it matters to the business

This vulnerability can be exploited in phishing campaigns to redirect unsuspecting users to malicious websites, damaging brand reputation and eroding user trust. Successful exploitation could lead to credential theft, financial fraud, or malware infections for users who believe they are interacting with a legitimate company website.

How an attack unfolds

An attacker identifies a Django application that uses a vulnerable version of the `is_safe_url()` function for handling redirects, typically after a login or other form submission. The attacker crafts a malicious URL containing a redirect parameter that points to a numeric representation of an IP address they control. This link is then distributed to victims via phishing emails or social engineering. When a victim clicks the link, the vulnerable application processes the request, incorrectly validates the numeric URL as safe, and issues a redirect. The victim's browser is then sent to the attacker's malicious site, which can be designed to harvest credentials or deliver malware.

How to fix it

Upgrade Django to a patched version: 1.10.7 or later, 1.9.13 or later, or 1.8.18 or later. If using a third-party application that bundles Django, such as Bitnami MediaWiki, upgrade it to a version confirmed to include the fix (e.g., version 1.35.2 or newer for Bitnami MediaWiki).

How to confirm the fix worked

To confirm remediation, verify that the installed Django version is 1.10.7+, 1.9.13+, or 1.8.18+. To test for exposure, identify a redirect parameter within the application and attempt to pass a numeric URL (e.g., a dotless decimal IP address like `http://1113982867`) as its value. A patched system should block the redirect or treat the URL as unsafe, whereas a vulnerable system will perform the redirect to the external site.

Typical attack path

1. Attacker identifies a vulnerable Django application that performs URL redirects.
2. Attacker crafts a malicious link containing a redirect parameter pointing to a numeric URL (e.g., a dotless decimal IP address) under their control.
3. Attacker distributes the link to victims through phishing campaigns or social media.
4. A victim clicks the link, sending a request to the legitimate but vulnerable application.
5. The application's `is_safe_url()` function incorrectly validates the numeric URL as a safe, local redirect.
6. The application responds with an HTTP redirect (301/302) to the victim's browser, pointing to the attacker's malicious site.
7. The victim's browser automatically navigates to the malicious site, which can then execute its payload, such as a fake login form or malware download.

How to detect it

Monitor web server access logs (e.g., Nginx, Apache) and application logs for HTTP requests where a redirect parameter (e.g., 'next', 'redirect_to', 'continue') contains a value that is a numeric-only URL or a dotless decimal IP address. A successful exploitation attempt would be visible as a request with such a parameter followed by a 301/302 redirect response to an external, potentially malicious, destination.

• ?next=//[0-9]+
• &next=//[0-9]+
• ?redirect_to=http://[0-9]+
• &redirect_to=http://[0-9]+
• ?continue=//[0-9]+

What your security team should do

• Immediately identify all applications using vulnerable Django versions (pre-1.10.7, pre-1.9.13, pre-1.8.18) and prioritize them for patching.
• Upgrade affected Django instances to the latest secure versions available in their respective release series.
• As a defense-in-depth measure, review application code to ensure that redirects are not solely reliant on `is_safe_url()` and consider implementing a strict allow-list of known-safe redirect domains.