zap.Open: Invalidate relative path roots

Add validation to ensure file schema paths passed to zap.Open are
absolute since this is already documented.

Tests are also added to demonstrate that double dot segements within
file schema URIs passed to zap.Open remaining within the specified file
hierarchy.

This change addresses https://cwe.mitre.org/data/definitions/23.html

ref #1390

sink.go

@@ -103,6 +103,11 @@ func (sr *sinkRegistry) newSink(rawURL string) (Sink, error) {
 	if err != nil {
 		return nil, fmt.Errorf("can't parse %q as a URL: %v", rawURL, err)
 	}
+
+	if u.Scheme == schemeFile && !filepath.IsAbs(u.Path) {
+		return nil, fmt.Errorf("file URI %q attempts a relative path", rawURL)
+	}
+
 	if u.Scheme == "" {
 		u.Scheme = schemeFile
 	}

writer_test.go

@@ -224,6 +224,85 @@ func TestOpenOtherErrors(t *testing.T) {
 	}
 }
 
+func TestOpenRelativeValidated(t *testing.T) {
+	tests := []struct {
+		msg     string
+		paths   []string
+		wantErr string
+	}{
+		{
+			msg: "invalid relative path root",
+			paths: []string{
+				"file:../some/path",
+			},
+			// url.Parse's Path for this path value is "" which would result
+			// in a file not found error if not validated.
+			wantErr: `open sink "file:../some/path": file URI "file:../some/path" attempts a relative path`,
+		},
+		{
+			msg: "invalid double dot as the host element",
+			paths: []string{
+				"file://../some/path",
+			},
+			wantErr: `open sink "file://../some/path": file URLs must leave host empty or use localhost: got file://../some/path`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.msg, func(t *testing.T) {
+			_, _, err := Open(tt.paths...)
+			assert.EqualError(t, err, tt.wantErr)
+		})
+	}
+}
+
+func TestOpenDotSegmentsSanitized(t *testing.T) {
+	tempName := filepath.Join(t.TempDir(), "test.log")
+	assert.False(t, fileExists(tempName))
+	require.True(t, filepath.IsAbs(tempName), "Expected absolute temp file path.")
+
+	tests := []struct {
+		paths            []string
+		toWrite          []byte
+		wantFileContents string
+	}{
+		{
+			paths:            []string{"file:/.." + tempName},
+			toWrite:          []byte("a"),
+			wantFileContents: "a",
+		},
+		{
+			paths:            []string{"file:/../.." + tempName},
+			toWrite:          []byte("b"),
+			wantFileContents: "ab",
+		},
+		{
+			paths:            []string{"file:///.." + tempName},
+			toWrite:          []byte("c"),
+			wantFileContents: "abc",
+		},
+		{
+			paths:            []string{"file:///../.." + tempName},
+			toWrite:          []byte("d"),
+			wantFileContents: "abcd",
+		},
+	}
+
+	for _, tt := range tests {
+		ws, cleanup, err := Open(tt.paths...)
+		require.NoError(t, err)
+		defer cleanup()
+
+		_, err = ws.Write(tt.toWrite)
+		require.NoError(t, err)
+
+		b, err := os.ReadFile(tempName)
+		require.NoError(t, err)
+
+		assert.Equal(t, string(b), tt.wantFileContents)
+	}
+}
+
 type testWriter struct {
 	expected string
 	t        testing.TB