net/http: ignore directory path when parsing multipart forms

Fixes #45789

Change-Id: Id588f5dbbecf5fbfb54e957c53903aaa900171f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/313809
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>

src/mime/multipart/multipart.go

@@ -20,6 +20,7 @@ import (
 	"mime"
 	"mime/quotedprintable"
 	"net/textproto"
+	"path/filepath"
 	"strings"
 )
 
@@ -67,13 +68,20 @@ func (p *Part) FormName() string {
 	return p.dispositionParams["name"]
 }
 
-// FileName returns the filename parameter of the Part's
-// Content-Disposition header.
+// FileName returns the filename parameter of the Part's Content-Disposition
+// header. If not empty, the filename is passed through filepath.Base (which is
+// platform dependent) before being returned.
 func (p *Part) FileName() string {
 	if p.dispositionParams == nil {
 		p.parseContentDisposition()
 	}
-	return p.dispositionParams["filename"]
+	filename := p.dispositionParams["filename"]
+	if filename == "" {
+		return ""
+	}
+	// RFC 7578, Section 4.2 requires that if a filename is provided, the
+	// directory path information must not be used.
+	return filepath.Base(filename)
 }
 
 func (p *Part) parseContentDisposition() {

src/net/http/request_test.go

@@ -245,6 +245,29 @@ func TestParseMultipartForm(t *testing.T) {
 	}
 }
 
+// Issue 45789: multipart form should not include directory path in filename
+func TestParseMultipartFormFilename(t *testing.T) {
+	postData :=
+		`--xxx
+Content-Disposition: form-data; name="file"; filename="../usr/foobar.txt/"
+Content-Type: text/plain
+
+--xxx--
+`
+	req := &Request{
+		Method: "POST",
+		Header: Header{"Content-Type": {`multipart/form-data; boundary=xxx`}},
+		Body:   io.NopCloser(strings.NewReader(postData)),
+	}
+	_, hdr, err := req.FormFile("file")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if hdr.Filename != "foobar.txt" {
+		t.Errorf("expected only the last element of the path, got %q", hdr.Filename)
+	}
+}
+
 // Issue #40430: Test that if maxMemory for ParseMultipartForm when combined with
 // the payload size and the internal leeway buffer size of 10MiB overflows, that we
 // correctly return an error.