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 ## Summary The fix for…

high7.5

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

## Summary

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`.

## Affected versions

- `internal/htmlcheck/css.go` at HEAD `8bc966e61834a24c48b4465da418f75e73be0afd` (2026-05-06).
- All versions `>= v1.28.3` (the version that shipped the GHSA-6jxm fix).

Affected

1 ranges

Vendor	Product	Version range	Fixed in
github.com	axllent_mailpit	>= 1.28.3 < 1.30.0	1.30.0

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