CVE-2026-55599: phpseclib SSRF via Untrusted X.509 Certificate Validation

Technical summary

The vulnerability exists in the X509::validateSignature() method across phpseclib versions 0.1.1 through 1.0.29, 2.0.54, and 3.0.53. During certificate validation, the method parses the Authority Information Access (AIA) extension (OID 1.3.6.1.5.5.7.1.1) and automatically initiates HTTP/HTTPS connections to URLs specified within it. The library performs no validation of the destination host, port, or path before making the connection. An attacker supplying a malicious certificate with a crafted AIA URL can redirect validating servers to probe internal networks, cloud metadata services (169.254.169.254), or localhost interfaces (127.0.0.1), enabling reconnaissance and potential credential theft or service abuse. The root cause is an insecure default that enables URL fetching without explicit opt-in or destination allowlisting.

Business impact

Organizations running web applications or services that validate untrusted X.509 certificates—such as API gateways, certificate pinning validators, or PKI infrastructure—face exposure to internal network reconnaissance. Attackers can map internal services, access cloud metadata endpoints to steal credentials and configuration, or trigger denial-of-service conditions on internal systems. The vulnerability is particularly dangerous in containerized and cloud environments where metadata endpoints are common attack vectors. Because the flaw is an insecure default, many developers may not realize their applications are vulnerable simply by using recent versions of phpseclib without explicit hardening.

Affected systems

Any application using phpseclib versions 0.1.1 through 1.0.29, 2.0.54, or 3.0.53 that validates certificates—whether for TLS handshake verification, certificate chain validation, or certificate pinning checks—is potentially affected. This includes web services, API clients, VPN implementations, and certificate management tools written in PHP. The vulnerability does not require the application to explicitly enable AIA fetching; it happens by default. Verify your phpseclib version and usage patterns: if your application calls X509::validateSignature() or related certificate validation methods on untrusted input, assume exposure.

Exploitability

Exploitation requires an attacker to supply a certificate that the target application will validate. This is feasible in scenarios where certificates are uploaded by users, retrieved from untrusted sources, or used in protocols that accept arbitrary certificates (such as ACME validators or certificate transparency monitors). The attack requires no authentication, no user interaction, and no special network position—only the ability to control a certificate and knowledge that the target uses phpseclib. The CVSS score of 5.8 (Medium) reflects the network-accessible attack vector and potential for information disclosure, though the impact is limited to confidentiality; no code execution or authentication bypass occurs.

Remediation

Upgrade phpseclib to version 1.0.30, 2.0.55, or 3.0.54 or later, depending on your current version branch. These releases fix the vulnerability by disabling unsafe AIA URL fetching by default and requiring explicit allowlisting of trusted destinations. After patching, audit any code that validates untrusted certificates and consider implementing additional controls: restrict certificate sources, disable AIA fetching unless absolutely necessary, or use a Web Application Firewall to block outbound connections to sensitive internal ranges (127.0.0.0/8, 169.254.0.0/16, etc.). Test thoroughly in a staging environment before rolling out patches to production.

Patch guidance

1. Identify all servers and applications using phpseclib—check composer.json, vendor directories, and dependency trees. 2. For each major version line, upgrade to the patched release: 1.0.30+, 2.0.55+, or 3.0.54+. 3. Review release notes and changelogs to understand any breaking changes or deprecated methods. 4. Test certificate validation workflows in staging, including edge cases with AIA extensions. 5. Deploy patches using your standard change management process, prioritizing applications that accept untrusted certificates. 6. Verify patches are applied by checking version output and running phpunit tests if available. 7. Monitor for any certificate validation errors or unexpected behavior post-patch; AIA handling changes may affect some legitimate certificate chains.

Detection guidance

Monitor for unexpected outbound connections from PHP application servers, particularly to internal IP ranges (127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 169.254.0.0/16) or cloud metadata endpoints. Log and alert on certificate validation operations that trigger HTTP connections. Review application logs for error messages related to certificate parsing or AIA extension processing. If you have network access logs, correlate certificate validation operations with new outbound connections. For development and testing, enable verbose logging in phpseclib by checking $this->log if available in the library. In web application firewalls and intrusion detection systems, look for patterns where certificate uploads or imports precede internal network probes.

Why prioritize this

Although the CVSS score is Medium (5.8), this vulnerability warrants prompt attention because: (1) it is an insecure default affecting all users of the library without explicit opt-in; (2) it enables attackers to probe internal infrastructure and cloud metadata endpoints, a common precursor to lateral movement and credential theft; (3) the barrier to exploitation is low—merely supplying a crafted certificate; (4) many organizations may not realize their applications are vulnerable simply by using phpseclib for certificate validation. Prioritize patching if your application validates certificates from untrusted sources or is exposed to the internet.

Risk score, explained

The CVSS 3.1 score of 5.8 reflects an attack requiring network access and no privileges or user interaction, coupled with limited impact (confidentiality only—information disclosure via SSRF). However, context matters: in cloud and containerized environments, SSRF access to metadata endpoints can yield high-impact credential theft, effectively raising the practical risk. Organizations should assess their own environment: if internal services or cloud metadata are reachable from applications using phpseclib, treat this as a higher-priority finding.

Frequently asked questions