CVE-2026-21859 Mailpit Proxy Endpoint is Vulnerable to Server-Side Request Forgery (SSRF)
Attackers can exploit the /proxy endpoint to access internal network resources via Server-Side Request Forgery in Mailpit.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-21859?
CVE-2026-21859 is a medium-severity server-side request forgery vulnerability in axllent mailpit, published on 7 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 1.28.0 and below have a Server-Side Request Forgery (SSRF) vulnerability in the /proxy endpoint, allowing attackers to make requests to internal network resources. The /proxy endpoint validates http:// and https:// schemes, but it does not block internal IP addresses, enabling attackers to access internal services and APIs. This vulnerability is limited to HTTP GET requests with minimal headers. The issue is fixed in version 1.28.1.
Weakness: CWE-918
Which versions are affected by CVE-2026-21859?
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 | 1.28.1 |
github.com/axllent/mailpit Go | All versions | 1.28.1 |
github.com/axllent/mailpit Go | All versions | 1.28.1 |
Products include Axllent Mailpit.
How do I fix CVE-2026-21859?
Upgrade github.com/axllent/mailpit to 1.28.1 or later.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-21859 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-21859 exists.
Public exploit code: 1 Nuclei template.
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-8v65-47jx-7mfr, GO-2026-4284.