# CVE-2026-23845: Mailpit Vulnerable to Server-Side Request Forgery (SSRF) via HTML Check API

> An attacker can exploit the HTML Check API to force the server to download external CSS files, enabling Server-Side Request Forgery attacks.

- **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:** 19 Jan 2026
- **Updated:** 20 Jan 2026
- **Also known as:** GHSA-6jxm-fv7w-rw5j, GO-2026-4345

## What is CVE-2026-23845?

CVE-2026-23845 is a medium-severity server-side request forgery vulnerability in axllent mailpit, published on 19 Jan 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. Versions prior to 1.28.3 are vulnerable to Server-Side Request Forgery (SSRF) via HTML Check CSS Download. The HTML Check feature (`/api/v1/message/{ID}/html-check`) is designed to analyze HTML emails for compatibility. During this process, the `inlineRemoteCSS()` function automatically downloads CSS files from external `<link rel="stylesheet" href="...">` tags to inline them for testing. Version 1.28.3 fixes the issue.

Weakness: CWE-918 (Server-Side Request Forgery)

## Which versions are affected by CVE-2026-23845?

3 open source packages ship an affected version, including github.com/axllent/mailpit. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/axllent/mailpit` | Go | All versions | 1.28.3 |
| `github.com/axllent/mailpit` | Go | All versions | 1.28.3 |
| `github.com/axllent/mailpit` | Go | All versions | 1.28.3 |

Products: Axllent Mailpit.

## How do I fix CVE-2026-23845?

Upgrade github.com/axllent/mailpit to 1.28.3 or later.

## Is CVE-2026-23845 being exploited?

There is no known exploitation of CVE-2026-23845 in the wild and no public exploit code.

## Other vulnerabilities in axllent mailpit

- [CVE-2026-45713](https://insights.sbomify.com/CVE-2026-45713/): Mailpit: Unauthenticated remote memory-exhaustion DoS via unlimited SMTP DATA and /api/v1/send body sizes (high 7.5)
- [CVE-2026-22689](https://insights.sbomify.com/CVE-2026-22689/): Mailpit is vulnerable to Cross-Site WebSocket Hijacking (CSWSH) allowing unauthenticated access to emails (medium 6.5)
- [CVE-2026-45711](https://insights.sbomify.com/CVE-2026-45711/): Mailpit: Path traversal & arbitrary file write in mailpit dump --http via attacker-controlled message IDs (medium 5.9)
- [CVE-2026-45712](https://insights.sbomify.com/CVE-2026-45712/): Mailpit: Concurrent map read & write in proxy CSS rewriter - remote unauth crash (fatal error: concurrent map read and map write) (medium 5.9)
- [CVE-2026-45709](https://insights.sbomify.com/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 (medium 5.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-23845 and fix it if it is affected.

What it is: CVE-2026-23845 is a medium-severity server-side request forgery vulnerability in axllent mailpit, published on 19 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-23845 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/axllent/mailpit (Go): affected all versions; fixed in 1.28.3
- github.com/axllent/mailpit (Go): affected all versions; fixed in 1.28.3
- github.com/axllent/mailpit (Go): affected all versions; fixed in 1.28.3

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-2026-23845/ (Markdown: https://insights.sbomify.com/CVE-2026-23845/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23845
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23845
- OSV: https://osv.dev/vulnerability/CVE-2026-23845

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23845/
Summaries are generated and may contain mistakes; the official description above is the record.