GHSA-mpf7-p9x7-96r3: Mailpit is Vulnerable to Server-Side Request Forgery (SSRF) via Link Check API
SummaryThe 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.DetailsThe 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:The call chain is:GET /api/v1/message/{ID}/link-check hits LinkCheck() in
https://github.com/axllent/mailpit/blob/v1.29.0/server/apiv1/other.go#L84Which calls linkcheck.RunTests() in https://github.com/axllent/mailpit/blob/v1.29.0/internal/linkcheck/main.go#L16Which 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#L14Which spawns goroutines calling doHead() for each URL with no filteringThere 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).PoCPrerequisites: 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: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:-- This behavior can be identified by creating a email txt file asStart a Python server on port 8408execute the command `mailpit sendmail < email.txt `Observe a request to your python server and link status on the UI as OKThe attacker now knows both internal services are reachable and gets their exact HTTP status codes, this allows internal port scanningImpactWho 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.RemediationThen standard Go library can be used to identify a local address being requested and deny it.And the safe dialer that uses it:Then the doHead() change — replace the bare transport with one that uses the safe dialer, and re-validate URLs on
  redirect hops:This fix should mitigate the reported SSRF.
CVSS v3.1: 5.8