diff --git a/ext/posix/posix.c b/ext/posix/posix.c
index b7acf8c751270..76e14f6ecb0c6 100644
--- a/ext/posix/posix.c
+++ b/ext/posix/posix.c
@@ -744,6 +744,15 @@ PHP_FUNCTION(posix_access)
 		RETURN_FALSE;
 	}
 
+	if (mode < 0 || (mode & ~(F_OK | R_OK | W_OK | X_OK))) {
+		zend_argument_value_error(
+			2,
+			"must be a bitmask of POSIX_F_OK, POSIX_R_OK, POSIX_W_OK, and POSIX_X_OK"
+		);
+		efree(path);
+		RETURN_THROWS();
+	}
+
 	ret = access(path, mode);
 	efree(path);
 
diff --git a/ext/posix/tests/posix_access_flags.phpt b/ext/posix/tests/posix_access_flags.phpt
new file mode 100644
index 0000000000000..37c7522a2a3b1
--- /dev/null
+++ b/ext/posix/tests/posix_access_flags.phpt
@@ -0,0 +1,63 @@
+--TEST--
+posix_access() flag (mode) validation
+--SKIPIF--
+<?php
+if (!function_exists("posix_access")) {
+    die("skip no posix_access()");
+}
+?>
+--FILE--
+<?php
+
+$dir = __DIR__;
+$testfile = "$dir/testfile.txt";
+
+// Create a temporary file for valid access tests
+file_put_contents($testfile, "hello");
+
+// Invalid: negative mode
+try {
+    posix_access($testfile, -1);
+} catch (ValueError $e) {
+    echo $e->getMessage(), "\n";
+}
+
+// Invalid: mode with garbage bits
+try {
+    posix_access($testfile, 01000); // S_ISVTX bit (sticky)
+} catch (ValueError $e) {
+    echo $e->getMessage(), "\n";
+}
+
+// Invalid: mode with unrelated high bits
+try {
+    posix_access($testfile, 02000); // S_ISGID bit
+} catch (ValueError $e) {
+    echo $e->getMessage(), "\n";
+}
+
+// Valid: check read and write access
+if (posix_access($testfile, POSIX_R_OK | POSIX_W_OK)) {
+    echo "Read/write access OK\n";
+}
+
+// Valid: check file existence
+if (posix_access($testfile, POSIX_F_OK)) {
+    echo "File exists OK\n";
+}
+
+?>
+--CLEAN--
+<?php
+$dir = __DIR__;
+$testfile = "$dir/testfile.txt";
+if (file_exists($testfile)) {
+    unlink($testfile);
+}
+?>
+--EXPECTF--
+posix_access(): Argument #2 ($flags) must be a bitmask of POSIX_F_OK, POSIX_R_OK, POSIX_W_OK, and POSIX_X_OK
+posix_access(): Argument #2 ($flags) must be a bitmask of POSIX_F_OK, POSIX_R_OK, POSIX_W_OK, and POSIX_X_OK
+posix_access(): Argument #2 ($flags) must be a bitmask of POSIX_F_OK, POSIX_R_OK, POSIX_W_OK, and POSIX_X_OK
+Read/write access OK
+File exists OK