CVE-2026-55187: Mailpit: Incomplete SSRF protection in Link Check API via IPv6 transition mechanisms

The remediation shipped in mailpit v1.29.2 for GHSA-mpf7-p9x7-96r3 (CVE-2026-27808) is incomplete. The tools.IsInternalIP deny-list relies on Go’s stdlib classification helpers (IsLoopback, IsPrivate, IsLinkLocalUnicast, IsLinkLocalMulticast, IsUnspecified, IsMulticast) plus an inline CGNAT range, but those helpers do not match two classes of IPv6 address that should be blocked for SSRF purposes:

1. IPv6 forms that embed an IPv4 destination via documented translation mechanisms — 6to4, NAT64, IPv4-compatible IPv6, ISATAP, or (in older Go versions) IPv4-mapped IPv6. These let an attacker reach internal IPv4 destinations by supplying an IPv6 literal that encodes the desired IPv4.