diff --git a/cmd/rio/main.go b/cmd/rio/main.go
index 08eec7e..d02cdb9 100644
--- a/cmd/rio/main.go
+++ b/cmd/rio/main.go
@@ -97,7 +97,7 @@ func runServer(ctx *cli.Context) error {
 
 	// Setup the Gitea stuff
 	httpClient := http.Client{Timeout: 10 * time.Second}
-	client, err := gitea.NewClient(
+	giteaClient, err := gitea.NewClient(
 		giteaUrl,
 		gitea.SetHTTPClient(&httpClient),
 		gitea.SetToken(""),
@@ -162,11 +162,12 @@ func runServer(ctx *cli.Context) error {
 			certsFile,
 			&cache,
 			acmeClient,
+			giteaClient,
 		)
 		listener = tls.NewListener(listener, tlsConfig)
 	}
 
-	if err := http.Serve(listener, Handler(domain, giteaUrl, client)); err != nil {
+	if err := http.Serve(listener, Handler(domain, giteaUrl, giteaClient)); err != nil {
 		fmt.Printf("Listening failed")
 		return err
 	}
diff --git a/internal/repo/repo.go b/internal/repo/repo.go
index 74cac2b..ac7dc37 100644
--- a/internal/repo/repo.go
+++ b/internal/repo/repo.go
@@ -15,6 +15,9 @@ import (
 
 var (
 	pathCache = cache.New(1*time.Hour, 1*time.Hour)
+
+	// Caching the existence of an user
+	userCache = cache.New(24*time.Hour, 12*time.Hour)
 )
 
 type PageCacheEntry struct {
@@ -132,3 +135,19 @@ func RepoFromPath(username, host, cname, path string, giteaClient *gitea.Client)
 	)
 	return repo, path, err
 }
+
+// Checks if the username exists as an organisation or an user on the Gitea
+// instance, so that an attacker can't just request certificates for random
+// usernames.
+func CanRequestCertificate(username string, giteaClient *gitea.Client) bool {
+	if _, found := userCache.Get(username); found {
+		return true
+	}
+
+	user, _, err := giteaClient.GetUserInfo(username)
+	if user != nil && err == nil {
+		userCache.Set(username, true, cache.DefaultExpiration)
+		return true
+	}
+	return false
+}
diff --git a/internal/repo/repo_test.go b/internal/repo/repo_test.go
new file mode 100644
index 0000000..80f02b6
--- /dev/null
+++ b/internal/repo/repo_test.go
@@ -0,0 +1,39 @@
+package repo
+
+import (
+	"net/http"
+	"testing"
+	"time"
+
+	"code.gitea.io/sdk/gitea"
+)
+
+var (
+	giteaClient, _ = gitea.NewClient(
+		"https://git.polynom.me",
+		gitea.SetHTTPClient(&http.Client{Timeout: 10 * time.Second}),
+		gitea.SetToken(""),
+		gitea.SetUserAgent("rio/testing"),
+	)
+)
+
+func TestCanRequestCertificatePositiveUser(t *testing.T) {
+	res := CanRequestCertificate("papatutuwawa", giteaClient)
+	if !res {
+		t.Fatalf("User papatutuwawa should be servable")
+	}
+}
+
+func TestCanRequestCertificatePositiveOrganisation(t *testing.T) {
+	res := CanRequestCertificate("moxxy", giteaClient)
+	if !res {
+		t.Fatalf("Organisation moxxy should be servable")
+	}
+}
+
+func TestCanRequestCertificateNegative(t *testing.T) {
+	res := CanRequestCertificate("user-who-does-not-exist", giteaClient)
+	if res {
+		t.Fatalf("User user-who-does-not-exist should not be servable")
+	}
+}
diff --git a/internal/server/tls.go b/internal/server/tls.go
index 160a93d..4b25e0c 100644
--- a/internal/server/tls.go
+++ b/internal/server/tls.go
@@ -7,7 +7,9 @@ import (
 
 	"git.polynom.me/rio/internal/certificates"
 	"git.polynom.me/rio/internal/dns"
+	"git.polynom.me/rio/internal/repo"
 
+	"code.gitea.io/sdk/gitea"
 	"github.com/go-acme/lego/v4/lego"
 
 	log "github.com/sirupsen/logrus"
@@ -41,16 +43,16 @@ func unlockDomain(domain string) {
 	delete(workingDomains, domain)
 }
 
-func MakeTlsConfig(pagesDomain, cachePath string, cache *certificates.CertificatesCache, acmeClient *lego.Client) *tls.Config {
+func MakeTlsConfig(pagesDomain, cachePath string, cache *certificates.CertificatesCache, acmeClient *lego.Client, giteaClient *gitea.Client) *tls.Config {
 	return &tls.Config{
-		InsecureSkipVerify: true,
 		GetCertificate: func(info *tls.ClientHelloInfo) (*tls.Certificate, error) {
 			// Validate that we should even care about this domain
+			cname := ""
 			if !strings.HasSuffix(info.ServerName, pagesDomain) {
 				// Note: We do not check err here because err != nil
 				// always implies that cname == "", which does not have
 				// pagesDomain as a suffix.
-				cname, _ := dns.LookupCNAME(info.ServerName)
+				cname, _ = dns.LookupCNAME(info.ServerName)
 				if !strings.HasSuffix(cname, pagesDomain) {
 					log.Warnf("Got ServerName for Domain %s that we're not responsible for", info.ServerName)
 					return cache.FallbackCertificate.TlsCertificate, nil
@@ -64,11 +66,31 @@ func MakeTlsConfig(pagesDomain, cachePath string, cache *certificates.Certificat
 				domain = "*." + pagesDomain
 			}*/
 
+			// Figure out a username for later username checks
+			username := ""
+			if cname == "" {
+				// domain ends on pagesDomain
+				username = strings.Split(domain, ".")[0]
+			} else {
+				// cname ends on pagesDomain
+				username = strings.Split(cname, ".")[0]
+			}
+
+			// Find the correct certificate
 			cert, found := cache.Certificates[info.ServerName]
 			if found {
 				if cert.IsValid() {
 					return cert.TlsCertificate, nil
 				} else {
+					if !repo.CanRequestCertificate(username, giteaClient) {
+						log.Warnf(
+							"Cannot renew certificate for %s because CanRequestCertificate(%s) returned false",
+							domain,
+							username,
+						)
+						return cert.TlsCertificate, nil
+					}
+
 					// If we're already working on the domain,
 					// return the old certificate
 					if lockIfUnlockedDomain(domain) {
@@ -76,7 +98,7 @@ func MakeTlsConfig(pagesDomain, cachePath string, cache *certificates.Certificat
 					}
 					defer unlockDomain(domain)
 
-					// Renew
+					// Renew the certificate
 					log.Infof("Certificate for %s expired, renewing", domain)
 					newCert, err := certificates.RenewCertificate(&cert, acmeClient)
 					if err != nil {
@@ -89,6 +111,15 @@ func MakeTlsConfig(pagesDomain, cachePath string, cache *certificates.Certificat
 					return newCert.TlsCertificate, nil
 				}
 			} else {
+				if !repo.CanRequestCertificate(username, giteaClient) {
+					log.Warnf(
+						"Cannot request certificate for %s because CanRequestCertificate(%s) returned false",
+						domain,
+						username,
+					)
+					return cache.FallbackCertificate.TlsCertificate, nil
+				}
+
 				// Don't request if we're already requesting.
 				if lockIfUnlockedDomain(domain) {
 					return cache.FallbackCertificate.TlsCertificate, nil