PHP version 5.6.40 was released on January 10, 2019 , as a final security release for the 5.6 branch. While 5.6.40 itself addressed several issues, it has since reached its official End of Life (EOL)
Here is the official migration link from PHP.net: php version 5640 vulnerabilities link
PHP 5.6.40 in a production environment is a major security risk because it reached its End of Life (EOL) on December 31, 2018 PHP version 5
Surviving PHP 7 End of Life: Best Practices for a Secure Transition Why "Final Security Release" is a Misnomer A
A remote code execution (RCE) vulnerability that affects PHP running on Windows in CGI configurations. Attackers can bypass previous protections to execute arbitrary commands. Buffer Overflows & Underflows: CVE-2016-10166: An integer underflow in the gd_interpolation.c CVE-2019-6977: A heap-based buffer overflow in gdImageColorMatch Memory Corruption: CVE-2019-9020: A heap-based buffer over-read in xmlrpc_decode that can lead to system compromise. CVE-2019-9021:
As of April 2026, PHP 5.6.40 has been officially unsupported for over seven years. While it was intended to be the most secure version of the 5.6 series at the time of its release, the threat landscape has evolved drastically since then. Why "Final Security Release" is a Misnomer
A heap-based buffer over-read in the PHAR extension allowing attackers to read memory past actual data. Out-of-Bounds Reads: CVE-2019-9024: An out-of-bounds read error in xmlrpc_decode triggered by a hostile XMLRPC server. Regular Expression Vulnerabilities: CVE-2019-9023: Multiple heap-based buffer over-read instances in regular expression functions. Security Risks of Continued Use