gzip: Error out if reserved bits are set (#891)

RFC 1952 states: Reserved FLG bits must be zero.

gzip/gunzip.go

@@ -238,6 +238,11 @@ func (z *Reader) readHeader() (hdr Header, err error) {
 		}
 	}
 
+	// Reserved FLG bits must be zero.
+	if flg>>5 != 0 {
+		return hdr, ErrHeader
+	}
+
 	z.digest = 0
 	if z.decompressor == nil {
 		z.decompressor = flate.NewReader(z.r)

gzip/gunzip_test.go

@@ -9,6 +9,7 @@ import (
 	"bytes"
 	oldgz "compress/gzip"
 	"crypto/rand"
+	"errors"
 	"io"
 	"os"
 	"strings"
@@ -408,11 +409,14 @@ func TestDecompressor(t *testing.T) {
 }
 
 func TestIssue6550(t *testing.T) {
-	f, err := os.Open("testdata/issue6550.gz")
+	b, err := os.ReadFile("testdata/issue6550.gz")
 	if err != nil {
 		t.Fatal(err)
 	}
-	gzip, err := NewReader(f)
+	// issue6550.gz has invalid flag bit set.
+	// TestReservedBits tests that. Fix it up for this test.
+	b[3] &= 0x1f
+	gzip, err := NewReader(bytes.NewReader(b))
 	if err != nil {
 		t.Fatalf("NewReader(testdata/issue6550.gz): %v", err)
 	}
@@ -435,6 +439,21 @@ func TestIssue6550(t *testing.T) {
 	}
 }
 
+func TestReservedBits(t *testing.T) {
+	f, err := os.Open("testdata/issue6550.gz")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	_, err = NewReader(f)
+	if err == nil {
+		t.Fatalf("Reserved bits did not result in error")
+	}
+	if !errors.Is(err, ErrHeader) {
+		t.Fatal("Expected ErrHeader, got", err)
+	}
+}
+
 func TestInitialReset(t *testing.T) {
 	var r Reader
 	if err := r.Reset(bytes.NewReader(gunzipTests[1].gzip)); err != nil {