CVE-2026-55187 Mailpit: Incomplete SSRF protection in Link Check API via IPv6 transition mechanisms (follow-up to CVE-2026-27808)
Attackers can bypass SSRF protections in Mailpit's Link Check API to access internal services and cloud metadata via IPv6 transition mechanisms.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55187?
CVE-2026-55187 is a medium-severity server-side request forgery vulnerability in axllent mailpit, published on 10 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Mailpit is an email testing tool and API for developers. Prior to 1.30.2, the remediation shipped for CVE-2026-27808 is incomplete because the tools.IsInternalIP deny-list in internal/tools/net.go relies on Go's standard library classification helpers and does not block IPv6 transition mechanisms or prefixes such as NAT64, 6to4, IPv4-compatible IPv6, ISATAP, fec0::/10, and 2001:db8::/32. An attacker who can deliver email and invoke POST /api/v1/message/{ID}/link-check can coerce the Link Check API's safeDialContext path into dialing internal destinations and can use status-code and error feedback to map internal service reachability, including cloud metadata endpoints. This issue is fixed in version 1.30.2.
Weakness: CWE-918
Which versions are affected by CVE-2026-55187?
3 open source packages ship an affected version, including github.com/axllent/mailpit.
| Package | Affected | Fixed in |
|---|---|---|
github.com/axllent/mailpit Go | All versions | 1.30.2 |
github.com/axllent/mailpit Go | All versions | 1.30.2 |
github.com/axllent/mailpit Go | All versions | 1.30.2 |
How do I fix CVE-2026-55187?
Upgrade github.com/axllent/mailpit to 1.30.2 or later.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-55187 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55187 exists.
Sources
Also known as GHSA-w4mc-hhc6-xp28, GO-2026-5688.