Attack Chain

1. The attacker identifies a self-hosted Next.js application utilizing WebSocket upgrades.
2. The attacker crafts a malicious WebSocket upgrade request. This request contains a target destination (internal or external) to which the server will be forced to proxy the request.
3. The attacker sends the crafted WebSocket upgrade request to the Next.js server.
4. The Next.js server, lacking proper validation, processes the malicious upgrade request.
5. The server initiates a connection to the attacker-specified destination.
6. The server proxies data between the attacker and the target destination.
7. The attacker gains access to internal services, cloud metadata endpoints, or other sensitive resources.
8. The attacker exfiltrates sensitive information or leverages the access for further malicious activities.

Impact

Successful exploitation of this SSRF vulnerability (CVE-2026-44578) can lead to the exposure of internal services, cloud metadata endpoints, or other sensitive resources. Attackers can potentially gain unauthorized access to sensitive data or internal systems, leading to data breaches, privilege escalation, or further compromise of the affected infrastructure. The number of victims and the specific sectors targeted depend on the deployment and configurations of the vulnerable Next.js applications. Vercel-hosted deployments are not affected.

Recommendation

• Upgrade Next.js to a patched version (>= 15.5.16 or >= 16.2.5) to remediate the vulnerability as described in GHSA-c4j6-fc7j-m34r.
• Deploy the Sigma rule Detect Suspicious WebSocket Upgrade Requests to detect potential exploitation attempts by monitoring for suspicious target domains in WebSocket upgrade requests.
• If immediate upgrade is not possible, implement the suggested workarounds: do not expose the origin server directly to untrusted networks and block WebSocket upgrades at the reverse proxy or load balancer if they are not required, as described in GHSA-c4j6-fc7j-m34r.
• Restrict origin egress to internal networks and metadata services where possible, as recommended in GHSA-c4j6-fc7j-m34r.