# 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.

- **Severity:** Medium 5.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 10 Jul 2026
- **Updated:** 13 Jul 2026
- **Also known as:** GHSA-w4mc-hhc6-xp28, GO-2026-5688

## 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 (Server-Side Request Forgery)

## Which versions are affected by CVE-2026-55187?

3 open source packages ship an affected version, including github.com/axllent/mailpit.


| Package | Ecosystem | 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.

## Is CVE-2026-55187 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55187 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55187 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55187 exists.

Affected packages and fixed versions:
- github.com/axllent/mailpit (Go): affected all versions; fixed in 1.30.2
- github.com/axllent/mailpit (Go): affected all versions; fixed in 1.30.2
- github.com/axllent/mailpit (Go): affected all versions; fixed in 1.30.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-55187/ (Markdown: https://insights.sbomify.com/CVE-2026-55187/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55187
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55187
- OSV: https://osv.dev/vulnerability/CVE-2026-55187

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55187/
Summaries are generated and may contain mistakes; the official description above is the record.