diff --git a/ext/phar/phar_object.c b/ext/phar/phar_object.c
index 98efcf701c6c4..39526ce4b235b 100644
--- a/ext/phar/phar_object.c
+++ b/ext/phar/phar_object.c
@@ -4306,7 +4306,7 @@ static int extract_helper(phar_archive_data *archive, zend_string *search, char
 			if (FAILURE == phar_extract_file(overwrite, entry, pathto, pathto_len, error)) return -1;
 			extracted++;
 		} ZEND_HASH_FOREACH_END();
-	} else if ('/' == ZSTR_VAL(search)[ZSTR_LEN(search) - 1]) {
+	} else if (ZSTR_LEN(search) > 0 && '/' == ZSTR_VAL(search)[ZSTR_LEN(search) - 1]) {
 		/* ends in "/" -- extract all entries having that prefix */
 		ZEND_HASH_MAP_FOREACH_PTR(&archive->manifest, entry) {
 			if (0 != strncmp(ZSTR_VAL(search), entry->filename, ZSTR_LEN(search))) continue;
diff --git a/ext/phar/tests/gh17518.phpt b/ext/phar/tests/gh17518.phpt
new file mode 100644
index 0000000000000..6a45e390edd0a
--- /dev/null
+++ b/ext/phar/tests/gh17518.phpt
@@ -0,0 +1,23 @@
+--TEST--
+GH-17518 (offset overflow phar extractTo())
+--EXTENSIONS--
+phar
+--INI--
+phar.readonly=0
+--FILE--
+<?php
+$fname = __DIR__.'/gh17518.phar.php';
+$phar = new Phar($fname);
+$phar['a'] = 'b';
+try {
+    $phar->extractTo(__DIR__ . '/gh17518', '');
+} catch (Throwable $e) {
+    echo $e::class, ": ", $e->getMessage(), "\n";
+}
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__.'/gh17518.phar.php');
+?>
+--EXPECTF--
+PharException: phar error: attempted to extract non-existent file or directory "" from phar "%sgh17518.phar.php"