Cybersecurity researchers recently discovered a vulnerability in the login page of OPNsense, the popular open-source firewall and network router platform, which allows attackers to redirect users to an arbitrary web site via a specifically crafted URL. The vulnerability, identified as CVE-2023-38998, exists in versions of OPNsense before 23.7. In this post, we will dive into the details of this vulnerability, provide the exploit code snippet, and discuss potential impacts and mitigation steps.

Vulnerability Details

The vulnerability lies in the login page of OPNsense when validating user input during the authentication process. An attacker can craft a URL containing a malicious redirect, which is processed by the OPNsense login page and unsuspectingly navigated to by an unsuspecting user. This type of vulnerability is known as an "open redirect." The underlying issue is due to OPNsense's lack of proper user input validation and sanitization.

To exploit this vulnerability, an attacker can create a malicious URL with the following format

https://<OPNsense-URL>/ui/login?redirect_url=<malicious-URL>;

Replace <OPNsense-URL> with the target OPNsense instance's URL and <malicious-URL> with the attacker's chosen malicious website.

An example of a malicious URL exploiting this vulnerability may look like this

https://192.168.1.1/ui/login?redirect_url=https://maliciouswebsite.com

Once the crafted URL is clicked, the user will be redirected to the specified malicious website after successful authentication.

Original References and Resources

1. https://www.open-sense.org/security-advisories
The OPNsense project's official security advisories page contains information about all known vulnerabilities, including CVE-2023-38998.

2. https://github.com/opnsense/core/commit/a5168a90d
This GitHub commit indicates the patch applied to fix this vulnerability in the OPNsense core repository. The patch includes changes to the source code for proper input validation.

Potential Impact

The open redirect vulnerability can be exploited by attackers to perform phishing attacks, tricking users into providing sensitive information or even installing malicious software on their systems. With proper social engineering, unaware users may be misled into interacting with a site designed to look identical to the legitimate OPNsense login page, leading to the potential theft of authentication credentials or the delivery of exploits targeting their systems.

To protect against this vulnerability, users should

1. Upgrade the OPNsense installation to version 23.7 or later, which contains the security patch that addresses this issue.

2. Apply secure coding practices and validate user input to ensure that all URLs are properly sanitized before redirecting users.

3. Educate users about the risk of phishing attacks and encourage them to verify URLs before clicking any links, especially those provided from untrusted sources.

Conclusion

CVE-2023-38998 is a serious vulnerability in the login page of OPNsense that allows an open redirect, putting user's sensitive information and systems at risk. It is crucial for administrators to upgrade their OPNsense installations to version 23.7 or later and follow best security practices to protect themselves and their users. By staying informed about emerging threats and vulnerabilities, and taking swift action to mitigate them, we can build a more secure digital world.

Timeline

Published on: 08/09/2023 19:15:00 UTC
Last modified on: 08/15/2023 15:08:00 UTC