SOAP XML Parser Stack Exhaustion in PHP

Executive Summary

This vulnerability involves a function called cleanup_xml_node() in the SOAP XML parser that recursively processes XML nesting without a depth limit. An attacker can exploit this by sending a SOAP request with tens of thousands of nested elements to any SoapServer endpoint, causing the system to exhaust its stack and crash the process. Similar unbounded recursion issues exist in the SOAP value decoder and WSDL node search helper.

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Detection Guidance

This vulnerability can be detected by monitoring for excessive recursion or stack exhaustion in PHP SOAP server processes. Check for unusually deep XML nesting in SOAP requests. Use system monitoring tools like top, htop, or strace to observe process crashes or high stack usage during SOAP requests.

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Impact Analysis

This vulnerability can lead to denial-of-service (DoS) attacks, where an attacker crashes the affected system by overwhelming it with deeply nested XML elements. This disrupts service availability for legitimate users and may require system restarts or manual intervention to recover.

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Compliance Impact

This vulnerability causes a denial-of-service (DoS) condition by crashing the process through stack exhaustion. It does not directly affect data confidentiality, integrity, or availability beyond service disruption. Compliance impacts would depend on whether the affected system is critical to operations covered by GDPR, HIPAA, or other standards requiring availability of services.

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Mitigation Strategies

Immediately update PHP to the latest patched version. Disable SOAP server endpoints if not in use. Implement input validation to reject deeply nested XML structures. Use a web application firewall to block malformed SOAP requests with excessive nesting.

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