CVE-2026-55187

Mailpit: Incomplete SSRF protection in Link Check API via IPv6 transition mechanisms

Published

2026-06-19

Modified

2026-09-02

EPSS percentile

31%

Aliases

GHSA-w4mc-hhc6-xp28

Sources

github-advisory

Summary

## Summary The remediation shipped in mailpit v1.29.2 for [GHSA-mpf7-p9x7-96r3](https://github.com/axllent/mailpit/security/advisories/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. 2. **IPv6 prefixes that fall outside the narrow private/loopback/link-local ranges Go's stdlib classifies** — specifically the deprecated site-local prefix `fec0::/10` (RFC 3879/4291) and the documentation prefix `2001:db8::/32` (RFC 3849). The first is still routable on dual-stack hosts and is cited as a bypass form in [CVE-2026-44430](https://advisories.gitlab.com/golang/github.com/modelcontextprotocol/registry/CVE-2026-44430/); the second should never appear in real network traffic and is safe to block as fail-safe behavior. Together these gaps let the Link Check API be coerced into dialing internal destinations that the v1.29.2 fix was intended to block. This is the same bug class as [GHSA-56c3-vfp2-5qqj / CVE-2026-44430 (MCP Registry)](https://advisories.gitlab.com/golang/github.com/modelcontextprotocol/registry/CVE-2026-44430/) and [GHSA-86m8-88fq-xfxp / CVE-2026-45741 (Gotenberg)](https://advisories.gitlab.com/golang/github.com/gotenberg/gotenberg/v8/CVE-2026-45741/) — projects that, like mailpit, built their SSRF deny-list around Go's stdlib `Is*` family and discovered the resulting bypass post-disclosure. The underlying ecosystem-wide issue is tracked upstream at [**golang/go#79925**](https://github.com/golang/go/issues/79925), which proposes extending `net.IP.IsPrivate` to handle these IPv6 transition forms. Until that lands, every Go project that wants comprehensive SSRF protection has to implement the decoding itself — which is exactly the gap that produced this advisory and the three CVEs in adjacent projects cited above. ## Affected versions - mailpit `v1.29.2` and later HEAD — the GHSA-mpf7-p9x7-96r3 fix is in place but [`tools.IsInternalIP`](https://github.com/axllent/mailpit/blob/a68499fa4e8874d414921fbd520e181dc92a39d7/internal/tools/net.go#L25-L34) does not cover the IPv6 forms enumerated below. - Pre-`v1.29.2` versions remain vulnerable to the original advisory. ## Vulnerable code [`internal/tools/net.go` L25-L34](https://github.com/axllent/mailpit/blob/a68499fa4e8874d414921fbd520e181dc92a39d7/internal/tools/net.go#L25-L34) — `IsInternalIP`: ```go func IsInternalIP(ip net.IP) bool { return ip.IsLoopback() || ip.IsPrivate() || ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast() || ip.IsUnspecified() || ip.IsMulticast() || cgnatRange.Contains(ip) } ``` [`internal/linkcheck/status.go` L140-L163](https://github.com/axllent/mailpit/blob/a68499fa4e8874d414921fbd520e181dc92a39d7/internal/linkcheck/status.go#L140-L163) — `safeDialContext` calls `IsInternalIP` on resolved IPs before dialing, but only blocks when one of the seven predicates above fires. For each of the following bypass forms, `net.IP.IsLoopback`, `IsPrivate`, `IsLinkLocalUnicast`, `IsLinkLocalMulticast`, `IsUnspecified`, `IsMulticast`, and the CGNAT range check all return `false` — so the dial proceeds: **IPv4-embedded-in-IPv6 forms** (each carries an IPv4 destination via a documented translation prefix): | Bypass IPv6 literal | Decoded IPv4 destination | RFC | |---|---|---| | `64:ff9b::a9fe:a9fe` | `169.254.169.254` (AWS / GCP / Azure metadata) | RFC 6052 — NAT64 well-known prefix | | `64:ff9b:1::a9f

Affected packages

Remediation: Upgrade to 1.30.2 or later.

References

• https://github.com/advisories/GHSA-w4mc-hhc6-xp28