Mailpit has a Server-Side Request Forgery (SSRF) via HTML Check API

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.8 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N

EPSS

• CVE-2026-23845: 0.43% estimated exploitation activity in the next 30 days; percentile 37.42 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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. Affected Components - Primary File: internal/htmlcheck/css.go (lines 132-207) - API Endpoint: /api/v1/message/{ID}/html-check - Handler: server/apiv1/other.go (lines 38-75) - Vulnerable Functions: - inlineRemoteCSS() - line 132 - downloadToBytes() - line 193 - isURL() - line 221 Technical Details 1. Insufficient URL Validation (isURL() function): 2. Unrestricted Download (downloadToBytes() function): 3. Automatic CSS Processing: Attack Vectors Attack Vector 1: Cloud Metadata Credential Theft Attacker sends HTML email with: When HTML check is triggered: 1. Mailpit makes GET request to AWS metadata endpoint 2. Downloads IAM credentials as "CSS…

Published by the source

21 Jan 2026, 01:01 UTC

Last modified by the source

21 Jan 2026, 01:01 UTC

Source last checked

26 Oct 2026, 15:14 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-6jxm-fv7w-rw5j

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# Patch Next advisory brief: CVE-2026-23845

Advisory: Mailpit has a Server-Side Request Forgery (SSRF) via HTML Check API
IDs: CVE-2026-23845, GHSA-6jxm-fv7w-rw5j
Record: https://patchnext.com/advisories/cve-2026-23845 (revision 1, source checked 26 Oct 2026, 15:14 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/axllent/mailpit 1.28.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/axllent/mailpit: affected < 1.28.3; fixed in 1.28.3

## Suggested inspection (read-only)
- Go github.com/axllent/mailpit: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/axllent/mailpit.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-6jxm-fv7w-rw5j (GitHub-reviewed advisory GHSA-6jxm-fv7w-rw5j lists affected ranges: Go github.com/axllent/mailpit >=0 <1.28.3)

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