security: harden the narinfo route pattern

The narinfo hash was updated in #840 to reflect the upstream definition
in NixOS/nix#15004 and so the server should only allow narinfo requests
that match this pattern.

Author: kalbasit

pkg/server/security_test.go

@@ -97,13 +97,6 @@ L:
 			expectedStatus:      http.StatusNotFound, // Not found upstream, but reached it
 			shouldReachUpstream: true,
 		},
-		{
-			name:                "Valid 52-char narinfo hash",
-			method:              http.MethodGet,
-			path:                "/1lid9xrpirkzcpqsxfq02qwiq0yd70chfl860wzsqd1739ih0nri.narinfo",
-			expectedStatus:      http.StatusNotFound,
-			shouldReachUpstream: true,
-		},
 		{
 			name:                "Invalid hash length (31 chars)",
 			method:              http.MethodGet,
@@ -121,8 +114,8 @@ L:
 		{
 			name:                "Path traversal attempt (alphanumeric but malicious)",
 			method:              http.MethodGet,
-			path:                "/abcdefghijklmnopqrstuvwxyz0123456789.narinfo", // 44 chars
-			expectedStatus:      http.StatusBadRequest,                           // Rejected by helper.IsValidHash
+			path:                "/aeou456789abcdfghijklmnpqrsvwxy.narinfo", // contains all 4 chars not allowed aeou
+			expectedStatus:      http.StatusNotFound,
 			shouldReachUpstream: false,
 		},
 		{

pkg/server/server.go

@@ -29,14 +29,15 @@ import (
 	"github.com/kalbasit/ncps/pkg/cache/upstream"
 	"github.com/kalbasit/ncps/pkg/helper"
 	"github.com/kalbasit/ncps/pkg/nar"
+	"github.com/kalbasit/ncps/pkg/narinfo"
 	"github.com/kalbasit/ncps/pkg/storage"
 )
 
 const (
 	routeIndex          = "/"
 	routeNar            = "/nar/{hash:[a-z0-9]{32,52}}.nar"
 	routeNarCompression = "/nar/{hash:[a-z0-9]{32,52}}.nar.{compression:*}"
-	routeNarInfo        = "/{hash:[a-z0-9]{32,52}}.narinfo"
+	routeNarInfo        = "/{hash:" + narinfo.HashPattern + "}.narinfo"
 	routeCacheInfo      = "/nix-cache-info"
 	routeCachePublicKey = "/pubkey"
 
@@ -305,11 +306,6 @@ func (s *Server) getNixCachePublicKey(w http.ResponseWriter, r *http.Request) {
 func (s *Server) getNarInfo(withBody bool) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		hash := chi.URLParam(r, "hash")
-		if !helper.IsValidHash(hash) {
-			http.Error(w, http.StatusText(http.StatusBadRequest), http.StatusBadRequest)
-
-			return
-		}
 
 		ctx, span := tracer.Start(
 			r.Context(),