diff --git a/internal/api/handlers/v0/auth/common.go b/internal/api/handlers/v0/auth/common.go
index 5d91047d..4c79bfc6 100644
--- a/internal/api/handlers/v0/auth/common.go
+++ b/internal/api/handlers/v0/auth/common.go
@@ -89,6 +89,12 @@ func ValidateDomainAndTimestamp(domain, timestamp string) (*time.Time, error) {
 		return nil, fmt.Errorf("invalid domain format")
 	}
 
+	if isGitHubPagesDomain(domain) {
+		return nil, fmt.Errorf(
+			"github.io domains cannot be used with DNS/HTTP authentication; " +
+				"use GitHub authentication for io.github.* namespaces")
+	}
+
 	ts, err := time.Parse(time.RFC3339, timestamp)
 	if err != nil {
 		return nil, fmt.Errorf("invalid timestamp format: %w", err)
@@ -382,6 +388,18 @@ func ReverseString(domain string) string {
 	return strings.Join(parts, ".")
 }
 
+// isGitHubPagesDomain reports whether domain is github.io or a subdomain of it.
+// GitHub Pages serves <name>.github.io from the <name>/<name>.github.io repository,
+// so the HTTP proof only demonstrates push access to that one repository. For an
+// organization that is a far weaker bar than the org-Owner ("admin") check the
+// GitHub authentication method enforces for io.github.<org>/* namespaces — accepting
+// the proof here would let any member with write access to the Pages repo mint the
+// whole org namespace. Users of io.github.* namespaces authenticate via GitHub.
+func isGitHubPagesDomain(domain string) bool {
+	d := strings.ToLower(domain)
+	return d == "github.io" || strings.HasSuffix(d, ".github.io")
+}
+
 func IsValidDomain(domain string) bool {
 	if len(domain) == 0 || len(domain) > 253 {
 		return false
diff --git a/internal/api/handlers/v0/auth/common_test.go b/internal/api/handlers/v0/auth/common_test.go
index 7662116a..35706464 100644
--- a/internal/api/handlers/v0/auth/common_test.go
+++ b/internal/api/handlers/v0/auth/common_test.go
@@ -2,6 +2,7 @@ package auth_test
 
 import (
 	"testing"
+	"time"
 
 	"github.com/modelcontextprotocol/registry/internal/api/handlers/v0/auth"
 )
@@ -47,3 +48,33 @@ func TestIsValidDomain(t *testing.T) {
 		})
 	}
 }
+
+func TestValidateDomainAndTimestampRejectsGitHubPages(t *testing.T) {
+	timestamp := time.Now().UTC().Format(time.RFC3339)
+	tests := []struct {
+		domain    string
+		wantError bool
+	}{
+		// GitHub Pages domains must not mint io.github.* namespaces via DNS/HTTP
+		{"my-org.github.io", true},
+		{"my-org.GitHub.IO", true},
+		{"github.io", true},
+		{"sub.my-org.github.io", true},
+
+		// Lookalikes and ordinary domains stay allowed
+		{"github.io.evil-example.com", false},
+		{"example.com", false},
+		{"my-org.github.io.example.com", false},
+	}
+	for _, tc := range tests {
+		t.Run(tc.domain, func(t *testing.T) {
+			_, err := auth.ValidateDomainAndTimestamp(tc.domain, timestamp)
+			if tc.wantError && err == nil {
+				t.Errorf("ValidateDomainAndTimestamp(%q) succeeded, want github.io rejection", tc.domain)
+			}
+			if !tc.wantError && err != nil {
+				t.Errorf("ValidateDomainAndTimestamp(%q) failed: %v", tc.domain, err)
+			}
+		})
+	}
+}