ssrfcheck SSRF Bypass Vulnerability

The ssrfcheck npm package, designed to protect against Server-Side Request Forgery (SSRF) attacks, contains a vulnerability due to an incomplete IP address denylist. Specifically, the package fails to classify the reserved IP address space 224.0.0.0/4 (Multicast) as invalid. This omission allows attackers to bypass the intended SSRF protection mechanisms. The vulnerability affects all versions of ssrfcheck up to and including version 1.1.1. This issue came to light in early May 2026. Although multicast addresses are typically used for local network communication, their acceptance by ssrfcheck deviates from established security practices and could be exploited in certain SSRF scenarios. The maintainers have released version 1.2.0 to address this vulnerability, incorporating the missing reserved IP range into the denylist.

Attack Chain

1. An attacker identifies a web application that utilizes the vulnerable ssrfcheck package for URL validation.
2. The attacker crafts a malicious URL containing an address within the 224.0.0.0/4 IP range (e.g., 239.255.255.250).
3. The web application uses ssrfcheck’s isSSRFSafeURL function to validate the URL.
4. Due to the missing IP range in ssrfcheck’s denylist, the function incorrectly returns true, indicating the URL is safe.
5. The web application proceeds to make a request to the attacker-controlled multicast address.
6. The request is routed within the internal network, potentially targeting internal services or resources that are not exposed to the public internet.
7. The attacker gains unauthorized access to sensitive data or functionality within the internal network.
8. The attacker exfiltrates the obtained information or uses the compromised service as a pivot point for further attacks within the network.

Impact

Successful exploitation of this SSRF vulnerability could allow attackers to bypass intended security controls and access internal network resources. While the use of multicast addresses may limit the scope of potential attacks, it still presents a risk of unauthorized access to sensitive information and systems. The vulnerability affects all users of the ssrfcheck package up to version 1.1.1. Web applications relying on ssrfcheck for SSRF protection are vulnerable until the package is updated to version 1.2.0 or later.

Recommendation

• Upgrade the ssrfcheck package to version 1.2.0 or later to remediate the vulnerability.
• Deploy the Sigma rule SSRFCheck_Multicast_Bypass to detect attempts to exploit this vulnerability in your environment.
• Review and audit any custom SSRF protection mechanisms that may be in place to ensure they adequately address reserved IP address spaces.
• Monitor network traffic for connections to multicast addresses originating from web applications that rely on URL validation.
• Update your vulnerability management system to include CVE-2025-8267 for tracking and remediation purposes.
• Implement network segmentation to limit the impact of potential SSRF attacks, even if the vulnerable package is exploited.