CVE-2026-4430: Unauthenticated SSRF bypassing IPv6 restrictions in ModelContextProtocol

Vulnerability Summary: Unauthenticated SSRF Bypasses IPv6 Address Restrictions Vulnerability Overview Title: Unauthenticated SSRF: HTTP namespace verification dials 6to4 / NAT64 / site-local IPv6 addresses, bypassing private-address allowlist CVE ID: CVE-2026-4430 Severity: CVSS 6.3 / 10 (Moderate) Vulnerability Type: Server-Side Request Forgery (SSRF) Core Issue: Registry's HTTP namespace verification logic (used to fetch well-known public keys) uses to prevent access to private/internal addresses. However, the existing IP blacklist ( ) has a flaw and fails to cover the following IPv6 address prefixes, allowing attackers to bypass restrictions: 1. 6to4 (2002::/16): Allows embedding arbitrary IPv4 addresses. 2. NAT64 (64:ff9b::/96, 64:ff9b::/48): Allows converting IPv4 addresses to IPv6 for routing. 3. Site-local (fec0::/10): Deprecated but still supported by some systems. Exploitation Method: Attackers can construct malicious requests, using the aforementioned IPv6 addresses as domain resolution targets, thereby probing internal network services or accessing cloud metadata (such as AWS/Azure/GKE internal IPs). Scope of Impact Affected Versions: (Go) = 1.7.7 Affected Environments: Registry instances deployed in network environments supporting IPv6 or NAT64. Kubernetes nodes or internal services in cloud environments including AWS, Azure, GKE, etc. Deployments that have not enabled strict IPv4-only policies. Attack Conditions: Unauthenticated, but requires knowledge of the target Registry's domain name. Remediation 1. Update Code: Upgrade to version 1.7.7 or higher. 2. Code Patch Logic: Expand the function to explicitly check and block the following IPv6 prefixes: (6to4) and (NAT64) (Site-local) Call the new helper function within . POC / Exploitation Code 1. HTTP Request Triggering SSRF (Step 2 - Trigger the dial) Note: The must be within ±15s of the server clock, and can be any hex string (as signature verification occurs after dialing). 2. DNS Configuration Example (Step 1 - Set up attacker DNS) The attacker needs to configure DNS records to resolve the domain name to a malicious IPv6 address: 3. Post-Fix Code Logic (Expected behaviour after fix) 4. Invocation Point Modification

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CVE-2026-4430: Unauthenticated SSRF bypassing IPv6 restrictions in ModelContextProtocol

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