sftp: fix command injection via crafted filenames on PowerShell remotes GHSA-2m8m-jhrm-w6j2 CVE-PENDING

PowerShell treats several Unicode smart-quote characters (U+2018, U+2019,
U+201A, U+201B) as single-quote delimiters in addition to the ASCII
apostrophe. The quoting helper only doubled the ASCII apostrophe, so a
remote filename containing one of these could close the quoted path and
inject statements that ran as the SSH account during server-side hashing.

Double all five delimiters when wrapping a PowerShell path so a filename
is always treated as data.

Fixes GHSA-2m8m-jhrm-w6j2

backend/sftp/sftp.go

@@ -2447,12 +2447,27 @@ func (o *Object) Hash(ctx context.Context, r hash.Type) (string, error) {
 	return hashString, nil
 }
 
+// powerShellQuoteEscaper doubles every character PowerShell accepts as a
+// single-quote string delimiter. As well as the ASCII apostrophe, PowerShell
+// treats the Unicode smart quotes U+2018, U+2019, U+201A and U+201B as single
+// quotes, so a path wrapped in apostrophes must double all of them or an
+// attacker controlled filename could close the literal and inject a statement.
+// Doubling a delimiter is PowerShell's escape for a literal occurrence of it,
+// and preserves the exact character.
+var powerShellQuoteEscaper = strings.NewReplacer(
+	"'", "''",
+	"‘", "‘‘",
+	"’", "’’",
+	"‚", "‚‚",
+	"‛", "‛‛",
+)
+
 // quoteOrEscapeShellPath makes path a valid string argument in configured shell
 // and also ensures it cannot cause unintended behavior.
 func quoteOrEscapeShellPath(shellType string, shellPath string) (string, error) {
 	// PowerShell
 	if shellType == "powershell" {
-		return "'" + strings.ReplaceAll(shellPath, "'", "''") + "'", nil
+		return "'" + powerShellQuoteEscaper.Replace(shellPath) + "'", nil
 	}
 	// Windows Command Prompt
 	if shellType == "cmd" {

backend/sftp/sftp_internal_test.go

@@ -64,6 +64,11 @@ func TestShellEscapeCmd(t *testing.T) {
 }
 
 func TestShellEscapePowerShell(t *testing.T) {
+	// PowerShell treats U+2018, U+2019, U+201A and U+201B as single-quote
+	// delimiters in addition to the ASCII apostrophe, so all of them are
+	// doubled inside the wrapping apostrophes. Doubling is PowerShell's escape
+	// for a literal delimiter and preserves the exact character.
+	unquote := strings.NewReplacer("''", "'", "‘‘", "‘", "’’", "’", "‚‚", "‚", "‛‛", "‛")
 	for i, test := range []struct {
 		unescaped, escaped string
 	}{
@@ -72,10 +77,23 @@ func TestShellEscapePowerShell(t *testing.T) {
 		{"c:/test&notepad", "'c:/test&notepad'"},
 		{"c:/test\"&\"notepad", "'c:/test\"&\"notepad'"},
 		{"c:/test'&'notepad", "'c:/test''&''notepad'"},
+		// injection attempts via the ASCII apostrophe and each smart quote
+		{"x';calc;#", "'x'';calc;#'"},
+		{"x’;calc;#", "'x’’;calc;#'"},
+		{"x‘;calc;#", "'x‘‘;calc;#'"},
+		{"x‚;calc;#", "'x‚‚;calc;#'"},
+		{"x‛;calc;#", "'x‛‛;calc;#'"},
 	} {
 		got, err := quoteOrEscapeShellPath("powershell", test.unescaped)
 		assert.NoError(t, err)
 		assert.Equal(t, test.escaped, got, fmt.Sprintf("Test %d unescaped = %q", i, test.unescaped))
+		// Every single-quote delimiter must appear an even number of times so
+		// none is left unpaired to close the literal early.
+		for _, q := range []string{"'", "‘", "’", "‚", "‛"} {
+			assert.Zero(t, strings.Count(got, q)%2, fmt.Sprintf("Test %d odd %q count in %q", i, q, got))
+		}
+		// Undoubling the quoted body recovers the original path exactly.
+		assert.Equal(t, test.unescaped, unquote.Replace(got[1:len(got)-1]), fmt.Sprintf("Test %d round-trip", i))
 	}
 }