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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-23845?
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
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 | 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 include Axllent Mailpit.
How do I fix CVE-2026-23845?
Upgrade github.com/axllent/mailpit to 1.28.3 or later.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
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 Mailpit: Unauthenticated remote memory-exhaustion DoS via unlimited SMTP DATA and /api/v1/send body sizes
- CVE-2026-22689 Mailpit is vulnerable to Cross-Site WebSocket Hijacking (CSWSH) allowing unauthenticated access to emails
- CVE-2026-45711 Mailpit: Path traversal & arbitrary file write in mailpit dump --http via attacker-controlled message IDs
- CVE-2026-45712 Mailpit: Concurrent map read & write in proxy CSS rewriter - remote unauth crash (fatal error: concurrent map read and map write)
- 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
Sources
Also known as GHSA-6jxm-fv7w-rw5j, GO-2026-4345.