PHP 8.5.7 `mb_substr()` 'SJIS-mac' size_t underflow

Posted by Khashayar Fereidani on Jun 20

# PHP 8.5.7 `mb_substr()` 'SJIS-mac' size_t underflow



**Author:** Khashayar Fereidani

**Disclosure Date:** 2026-06-18

**Advisory:**



## Description



The `mb_get_substr()` function in `ext/mbstring/mbstring.c`

deliberately skips an early empty return guard for the `SJIS-mac`

encoding when `from >= in_len`. As a result, it falls...