5.8

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-23845

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

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.

(GitHub Advisory)

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CVE-2026-23845

CVE-2026-23845:

5.8

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
github.com/axllent/mailpit	go	< 1.28.3	1.28.3

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability is a Server-Side Request Forgery (SSRF) in Mailpit's HTML Check feature. When an email contains a link to an external CSS file (<link rel="stylesheet" href="...">), the server attempts to download it to perform an analysis. The root cause lies in the internal/htmlcheck/css.go file. The inlineRemoteCSS function would extract the URL and pass it to the downloadToBytes function after an insufficient check by isURL. The downloadToBytes function would then make an HTTP GET request to any URL, including internal network addresses (e.g., http://169.254.169.254/ for cloud metadata services). An attacker could craft an email with a malicious CSS link to make the Mailpit server issue requests to internal services, potentially exposing sensitive data. The patch addresses this by replacing the vulnerable functions with safer alternatives: isValidURL for stricter validation, downloadCSSToBytes which checks the response content type, and a new safe HTTP client that validates redirects.

Vulnerable functions

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WAF Protection Rules

WAF Rule

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Reasoning

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