Security: Prevent Server-Side Request Forgery (SSRF) via Link Check API ([GHSA-mpf7-p9x7-96r3](https://github.com/axllent/mailpit/security/advisories/GHSA-mpf7-p9x7-96r3))

By default all internal HTTP requests are now blocked, unless mailpit is started with the `--allow-internal-http-requests` flag (env  `MP_ALLOW_INTERNAL_HTTP_REQUESTS=true`).

cmd/root.go

@@ -105,6 +105,7 @@ func init() {
 	rootCmd.Flags().StringVar(&config.UITLSKey, "ui-tls-key", config.UITLSKey, "TLS key for web UI (HTTPS) - requires ui-tls-cert")
 	rootCmd.Flags().StringVar(&server.AccessControlAllowOrigin, "api-cors", server.AccessControlAllowOrigin, "Set CORS origin(s) for the API, comma-separated (eg: example.com,foo.com)")
 	rootCmd.Flags().BoolVar(&config.BlockRemoteCSSAndFonts, "block-remote-css-and-fonts", config.BlockRemoteCSSAndFonts, "Block access to remote CSS & fonts")
+	rootCmd.Flags().BoolVar(&config.AllowInternalHTTPRequests, "allow-internal-http-requests", config.AllowInternalHTTPRequests, "Allow link-checker & screenshots to access internal IP addresses")
 	rootCmd.Flags().StringVar(&config.EnableSpamAssassin, "enable-spamassassin", config.EnableSpamAssassin, "Enable integration with SpamAssassin")
 	rootCmd.Flags().BoolVar(&config.AllowUntrustedTLS, "allow-untrusted-tls", config.AllowUntrustedTLS, "Do not verify HTTPS certificates (link checker & screenshots)")
 	rootCmd.Flags().BoolVar(&config.DisableHTTPCompression, "disable-http-compression", config.DisableHTTPCompression, "Disable HTTP compression support (web UI & API)")
@@ -250,6 +251,9 @@ func initConfigFromEnv() {
 	if getEnabledFromEnv("MP_BLOCK_REMOTE_CSS_AND_FONTS") {
 		config.BlockRemoteCSSAndFonts = true
 	}
+	if getEnabledFromEnv("MP_ALLOW_INTERNAL_HTTP_REQUESTS") {
+		config.AllowInternalHTTPRequests = true
+	}
 	if len(os.Getenv("MP_ENABLE_SPAMASSASSIN")) > 0 {
 		config.EnableSpamAssassin = os.Getenv("MP_ENABLE_SPAMASSASSIN")
 	}

config/config.go

@@ -127,6 +127,10 @@ var (
 	// BlockRemoteCSSAndFonts used to disable remote CSS & fonts
 	BlockRemoteCSSAndFonts = false
 
+	// AllowInternalHTTPRequests will allow HTTP requests to internal IP addresses (e.g., loopback, private, link-local, or multicast) when set to true.
+	// This policy applies to both link checking and screenshot generation (proxy) features and is disabled by default for security reasons.
+	AllowInternalHTTPRequests = false
+
 	// CLITagsArg is used to map the CLI args
 	CLITagsArg string
 

internal/linkcheck/status.go

@@ -1,14 +1,20 @@
 package linkcheck
 
 import (
+	"context"
 	"crypto/tls"
+	"errors"
+	"fmt"
+	"net"
 	"net/http"
 	"regexp"
+	"strings"
 	"sync"
 	"time"
 
 	"github.com/axllent/mailpit/config"
 	"github.com/axllent/mailpit/internal/logger"
+	"github.com/axllent/mailpit/internal/tools"
 )
 
 func getHTTPStatuses(links []string, followRedirects bool) []Link {
@@ -34,6 +40,10 @@ func getHTTPStatuses(links []string, followRedirects bool) []Link {
 			if err != nil {
 				l.StatusCode = 0
 				l.Status = httpErrorSummary(err)
+				if strings.Contains(l.Status, "private/reserved address") {
+					l.Status = "Blocked private/reserved address"
+					l.StatusCode = 451
+				}
 			} else {
 				l.StatusCode = code
 				l.Status = http.StatusText(code)
@@ -57,23 +67,37 @@ func getHTTPStatuses(links []string, followRedirects bool) []Link {
 
 // Do a HEAD request to return HTTP status code
 func doHead(link string, followRedirects bool) (int, error) {
+	if !tools.IsValidLinkURL(link) {
+		return 0, fmt.Errorf("invalid URL: %s", link)
+	}
 
-	timeout := time.Duration(10 * time.Second)
+	dialer := &net.Dialer{
+		Timeout:   10 * time.Second,
+		KeepAlive: 30 * time.Second,
+	}
 
-	tr := &http.Transport{}
+	tr := &http.Transport{
+		DialContext: safeDialContext(dialer),
+	}
 
 	if config.AllowUntrustedTLS {
 		tr.TLSClientConfig = &tls.Config{InsecureSkipVerify: true} // #nosec
 	}
 
 	client := http.Client{
-		Timeout:   timeout,
+		Timeout:   10 * time.Second,
 		Transport: tr,
 		CheckRedirect: func(req *http.Request, via []*http.Request) error {
-			if followRedirects {
-				return nil
+			if len(via) >= 3 {
+				return errors.New("too many redirects")
 			}
-			return http.ErrUseLastResponse
+			if !followRedirects {
+				return http.ErrUseLastResponse
+			}
+			if !tools.IsValidLinkURL(req.URL.String()) {
+				return fmt.Errorf("blocked redirect to invalid URL: %s", req.URL)
+			}
+			return nil
 		},
 	}
 
@@ -92,7 +116,6 @@ func doHead(link string, followRedirects bool) (int, error) {
 		}
 
 		return 0, err
-
 	}
 
 	return res.StatusCode, nil
@@ -107,8 +130,33 @@ func httpErrorSummary(err error) string {
 	if !re.MatchString(e) {
 		return e
 	}
-
 	parts := re.FindAllStringSubmatch(e, -1)
 
 	return parts[0][len(parts[0])-1]
 }
+
+// SafeDialContext is a custom dialer that checks if the resolved IP addresses are internal before allowing the connection.
+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
+		}
+
+		if !config.AllowInternalHTTPRequests {
+			for _, ip := range ips {
+				if tools.IsInternalIP(ip.IP) {
+					logger.Log().Warnf("[link-check] Blocked HEAD request to private/reserved address: %s (%s)", host, ip)
+					return nil, fmt.Errorf("blocked request to %s (%s): private/reserved address", host, ip)
+				}
+			}
+		}
+
+		return dialer.DialContext(ctx, network, net.JoinHostPort(ips[0].IP.String(), port))
+	}
+}

internal/tools/net.go

@@ -0,0 +1,28 @@
+package tools
+
+import (
+	"net"
+	"net/url"
+)
+
+// IsInternalIP checks if the given IP address is an internal IP address (e.g., loopback, private, link-local, or multicast).
+// 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
+func IsInternalIP(ip net.IP) bool {
+	return ip.IsLoopback() ||
+		ip.IsPrivate() ||
+		ip.IsLinkLocalUnicast() ||
+		ip.IsLinkLocalMulticast() ||
+		ip.IsUnspecified() ||
+		ip.IsMulticast()
+}
+
+// IsValidLinkURL checks if the provided string is a valid URL with http or https scheme and a non-empty hostname.
+func IsValidLinkURL(str string) bool {
+	u, err := url.Parse(str)
+	return err == nil && (u.Scheme == "http" || u.Scheme == "https") && u.Hostname() != ""
+}

server/handlers/proxy.go

@@ -2,10 +2,13 @@
 package handlers
 
 import (
+	"context"
 	"crypto/tls"
 	"encoding/base64"
+	"errors"
 	"fmt"
 	"io"
+	"net"
 	"net/http"
 	"net/url"
 	"regexp"
@@ -96,21 +99,37 @@ func ProxyHandler(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if !linkRe.MatchString(uri) {
-		logger.Log().Warnf("[proxy] invalid request %s", uri)
-		httpError(w, "Error: invalid request")
+	if !linkRe.MatchString(uri) || !tools.IsValidLinkURL(uri) {
+		logger.Log().Warnf("[proxy] invalid URL %s", uri)
+		httpError(w, "Error: invalid URL")
 		return
 	}
 
-	tr := &http.Transport{}
+	dialer := &net.Dialer{
+		Timeout:   10 * 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{
-		Transport: tr,
 		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 !tools.IsValidLinkURL(req.URL.String()) {
+				return fmt.Errorf("blocked redirect to invalid URL: %s", req.URL)
+			}
+			return nil
+		},
 	}
 
 	req, err := http.NewRequest("GET", uri, nil)
@@ -357,3 +376,28 @@ func supportedProxyContentType(ct string) bool {
 
 	return false
 }
+
+// SafeDialContext is a custom dialer that checks if the resolved IP addresses are internal before allowing the connection.
+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
+		}
+
+		if !config.AllowInternalHTTPRequests {
+			for _, ip := range ips {
+				if tools.IsInternalIP(ip.IP) {
+					return nil, fmt.Errorf("blocked request to %s (%s): private/reserved address", host, ip)
+				}
+			}
+		}
+
+		return dialer.DialContext(ctx, network, net.JoinHostPort(ips[0].IP.String(), port))
+	}
+}