fileserver: Reject ADS and short name paths; trim trailing dots and spaces on Windows (#5148)

* fileserver: Reject ADS and short name paths

* caddyhttp: Trim trailing space and dot on Windows

Windows ignores trailing dots and spaces in filenames.

* Fix test

* Adjust path filters

* Revert Windows test

* Actually revert the test

* Just check for colons

1 files changed, 14 insertions, 0 deletions

diff --git a/modules/caddyhttp/fileserver/staticfiles.go b/modules/caddyhttp/fileserver/staticfiles.go
index 04728ce..fe1a4fc 100644
--- a/modules/caddyhttp/fileserver/staticfiles.go
+++ b/modules/caddyhttp/fileserver/staticfiles.go
@@ -26,6 +26,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 	"time"
@@ -232,6 +233,19 @@ func (fsrv *FileServer) Provision(ctx caddy.Context) error {
 func (fsrv *FileServer) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
 	repl := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
 
+	if runtime.GOOS == "windows" {
+		// reject paths with Alternate Data Streams (ADS)
+		if strings.Contains(r.URL.Path, ":") {
+			return caddyhttp.Error(http.StatusBadRequest, fmt.Errorf("illegal ADS path"))
+		}
+		// reject paths with "8.3" short names
+		trimmedPath := strings.TrimRight(r.URL.Path, ". ") // Windows ignores trailing dots and spaces, sigh
+		if len(path.Base(trimmedPath)) <= 12 && strings.Contains(trimmedPath, "~") {
+			return caddyhttp.Error(http.StatusBadRequest, fmt.Errorf("illegal short name"))
+		}
+		// both of those could bypass file hiding or possibly leak information even if the file is not hidden
+	}
+
 	filesToHide := fsrv.transformHidePaths(repl)
 
 	root := repl.ReplaceAll(fsrv.Root, ".")