Even if the PHP core is "stable," the underlying libraries (OpenSSL, libxml2) used by PHP 5.6.40 are likely also outdated and contain their own critical vulnerabilities. The Danger of "Hidden" Vulnerabilities
This is not alarmist. In 2023-2025, multiple ransomware groups (e.g., LockBit 3.0 variants) explicitly target PHP 5.6.40 as an initial foothold.
Several vulnerabilities were verified in PHP version 5.6.40, including:
on December 31, 2018. Since then, no official security patches have been released by the PHP Group, leaving any newly discovered vulnerabilities completely unaddressed. Verified Vulnerabilities and Risks php version 5640 vulnerabilities verified
Restrict dangerous functions in your php.ini file to minimize the impact of a potential remote code execution vulnerability:
The PHP Archive (PHAR) file handling mechanism suffers from an unauthenticated memory exploitation vulnerability in phar_detect_phar_fname_ext . If an attacker persuades a application to parse a maliciously structured filename, it can cause a memory overflow and expose data. ⚠️ The Severe Risks of Remaining on PHP 5.6.40
Unfortunately, patching individual CVEs or relying on Web Application Firewalls (WAFs) is a losing battle when it comes to EOL software. The liabilities of using PHP 5.6.40 extend far beyond a list of specific CVEs: Even if the PHP core is "stable," the
PHP version was the final release of the PHP 5.6 branch, which reached its end-of-life (EOL) on December 31, 2018 . Despite being a maintenance release intended to address final security concerns, it remains vulnerable to several critical flaws discovered post-release. Verified Vulnerabilities in PHP 5.6.40
Running PHP 5.6.40 in a production environment is no longer a viable option according to Influential Software .
Vulnerabilities in the xmlrpc_decode function can lead to system instability or information disclosure when processing malicious requests. Several vulnerabilities were verified in PHP version 5
There is no patch. No backport. No savior. Here is your action plan.
Consider partnering with vendors who provide commercial Long Term Support (LTS) for End-Of-Life PHP versions (such as Sury.org for Debian/Ubuntu environments or Remi's RPM repository for CentOS/RHEL).