CVE-2026-45709: Mailpit has an incomplete fix for GHSA-6jxm: HTML check still permits SSRF to private/loopback/IMDS via missing IP-filter dialer

The fix for GHSA-6jxm-fv7w-rw5j (CVE-2026-23845, “Server-Side Request Forgery (SSRF) via HTML Check API”), shipped in mailpit v1.28.3, hardened internal/htmlcheck/css.go::downloadCSSToBytes with a 5MB size cap, a text/css content-type check, login-info stripping in isValidURL, and an opt-in --block-remote-css-and-fonts config flag — but did not add the IP-filtering dialer that the same codebase already uses on the two sister SSRF endpoints (the proxy handler and link-check). At HEAD 8bc966e61834a24c48b4465da418f75e73be0afd (2026-05-06), internal/htmlcheck/css.go::newSafeHTTPClient is mis-named — it builds an http.Client whose Transport.DialContext calls net.Dialer.DialContext directly with no IP allowlisting. As a result, the SSRF originally reported by Bao Anh Phan still permits the server to dial:

• loopback (127.0.0.0/8, ::1),
• private (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, fc00::/7),
• link-local incl. cloud IMDS (169.254.0.0/16, especially 169.254.169.254),
• CGNAT (100.64.0.0/10),
• and any other reserved/multicast range,

— provided the target replies with HTTP/200 and a content-type beginning with text/css. With redirect-following (CheckRedirect allows redirects to any isValidURL URL with no IP filter), an attacker-controlled public site can redirect mailpit’s request into the private network without ever appearing in the email’s HTML.

In the default mailpit deploy (no UI auth, no SMTP auth, port 1025/8025 exposed), this is an unauthenticated, network-reachable SSRF triggered by sending an HTML email and then issuing one HTTP GET to /api/v1/message/{id}/html-check.