Backport security fix from PHPOffice#4119 to v1

src/PhpSpreadsheet/Reader/Security/XmlScanner.php

@@ -113,15 +113,12 @@ private static function forceString($arg): string
      */
     private function toUtf8($xml)
     {
-        $pattern = '/encoding="(.*?)"/';
-        $result = preg_match($pattern, $xml, $matches);
-        $charset = strtoupper($result ? $matches[1] : 'UTF-8');
+        $charset = $this->findCharSet($xml);
 
         if ($charset !== 'UTF-8') {
             $xml = self::forceString(mb_convert_encoding($xml, 'UTF-8', $charset));
 
-            $result = preg_match($pattern, $xml, $matches);
-            $charset = strtoupper($result ? $matches[1] : 'UTF-8');
+            $charset = $this->findCharSet($xml);
             if ($charset !== 'UTF-8') {
                 throw new Reader\Exception('Suspicious Double-encoded XML, spreadsheet file load() aborted to prevent XXE/XEE attacks');
             }
@@ -169,4 +166,20 @@ public function scanFile($filestream)
     {
         return $this->scan(file_get_contents($filestream));
     }
+
+    private function findCharSet(string $xml): string
+    {
+        $patterns = [
+            '/encoding="([^"]*]?)"/',
+            "/encoding='([^']*?)'/",
+        ];
+
+        foreach ($patterns as $pattern) {
+            if (preg_match($pattern, $xml, $matches)) {
+                return strtoupper($matches[1]);
+            }
+        }
+
+        return 'UTF-8';
+    }
 }

tests/data/Reader/Xml/XEETestInvalidUTF-7-single-quote.xml

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding='UTF-7' standalone="yes"?>
+    +ADw-+ACE-DOCTYPE+ACA-foo+ACA-+AFs-+ADw-+ACE-ENTITY+ACA-toreplace+ACA-+ACI-xxe+AF8-test+ACI-+AD4-+ACA-+AF0-+AD4-+AAo-+ADw-sst+ACA-xmlns+AD0-+ACI-http://schemas.openxmlformats.org/spreadsheetml/2006/main+ACI-+ACA-count+AD0-+ACI-2+ACI-+ACA-uniqueCount+AD0-+ACI-1+ACI-+AD4-+ADw-si+AD4-+ADw-t+AD4-+ACY-toreplace+ADs-+ADw-/t+AD4-+ADw-/si+AD4-+ADw-/sst+AD4-

tests/data/Reader/Xml/XEETestValidUTF-8-single-quote.xml

@@ -0,0 +1,4 @@
+<?xml version='1.0' encoding='UTF-8' standalone='yes'?>
+<root>
+	test: Valid
+</root>