Add 5s timeout to DNS auth lookup to prevent resource exhaustion (#825)

The DNS auth endpoint was vulnerable to resource exhaustion if an
attacker controlled a domain with a slow/non-responding authoritative
DNS server. Without a timeout, handler goroutines would block on DNS
resolution indefinitely, allowing an attacker to pile up goroutines
until OOM.

Production is protected by NGINX ingress's default 60s proxy timeout,
but this adds defense-in-depth with a tighter 5s bound.

internal/api/handlers/v0/auth/dns.go

@@ -6,6 +6,7 @@ import (
 	"net"
 	"net/http"
 	"strings"
+	"time"
 
 	"github.com/danielgtaylor/huma/v2"
 	v0 "github.com/modelcontextprotocol/registry/internal/api/handlers/v0"
@@ -77,11 +78,15 @@ func RegisterDNSEndpoint(api huma.API, pathPrefix string, cfg *config.Config) {
 // ExchangeToken exchanges DNS signature for a Registry JWT token
 func (h *DNSAuthHandler) ExchangeToken(ctx context.Context, domain, timestamp, signedTimestamp string) (*auth.TokenResponse, error) {
 	keyFetcher := func(ctx context.Context, domain string) ([]string, error) {
+		// Apply a timeout to DNS lookup to prevent resource exhaustion from slow/malicious DNS servers
+		timeoutCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
+		defer cancel()
+
 		// Lookup DNS TXT records
 		// DNS implies a hierarchy where subdomains are treated as part of the parent domain,
 		// therefore we grant permissions for all subdomains (e.g., com.example.*)
 		// This is in line with other DNS-based authentication methods e.g. ACME DNS-01 challenges
-		txtRecords, err := h.resolver.LookupTXT(ctx, domain)
+		txtRecords, err := h.resolver.LookupTXT(timeoutCtx, domain)
 		if err != nil {
 			return nil, fmt.Errorf("failed to lookup DNS TXT records: %w", err)
 		}