From 0.1.1 until 1.0.30, 2.0.55, and 3.0.54, when an application validates an untrusted X.509 certificate with phpseclib, X509::validateSignature()… (CVE-2026-55599)

phpseclib is a PHP library for secure communications. In affected versions (from 0.1.1 until 1.0.30, 2.0.55, and 3.0.54), the X509::validateSignature() method automatically fetches a URL specified in the Authority Information Access (AIA) extension of an untrusted X.509 certificate. Since the attacker controls the certificate, they can specify any host, port, and path, causing the server to make arbitrary network connections. This behavior constitutes a server-side request forgery (SSRF) vulnerability due to an insecure default setting. The vulnerability is resolved by disabling or restricting URL fetching in the fixed versions.

An unauthenticated attacker can exploit this vulnerability to make a server validating an untrusted X.509 certificate initiate network connections to arbitrary internal or external hosts and ports. This can be used to access internal-only services such as loopback addresses (127.0.0.1), cloud metadata services (169.254.169.254), or other restricted network resources. The impact is limited to information disclosure or network scanning within the internal environment, with no direct code execution or data modification indicated.

This vulnerability is fixed in phpseclib versions 1.0.30, 2.0.55, and 3.0.54. Users should upgrade to these or later versions to remediate the issue. Until upgrading, disabling or restricting URL fetching in the X509::validateSignature() function, if configurable, can mitigate the risk. No vendor advisory content contradicts this guidance.