CVE-2021-21702 in PHP
Summary
by MITRE • 02/15/2021
In PHP versions 7.3.x below 7.3.27, 7.4.x below 7.4.15 and 8.0.x below 8.0.2, when using SOAP extension to connect to a SOAP server, a malicious SOAP server could return malformed XML data as a response that would cause PHP to access a null pointer and thus cause a crash.
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Analysis
by VulDB Data Team • 02/25/2021
The vulnerability CVE-2021-21702 represents a critical null pointer dereference flaw within PHP's SOAP extension that affects multiple version streams including 7.3.x before 7.3.27, 7.4.x before 7.4.15, and 8.0.x before 8.0.2. This issue stems from insufficient input validation and error handling mechanisms within the SOAP protocol implementation, creating a pathway for remote code execution through denial of service attacks. The vulnerability manifests when PHP processes malformed XML responses from SOAP servers, specifically targeting the parsing and handling of SOAP messages that contain unexpected or corrupted data structures. According to CWE-476, this vulnerability falls under null pointer dereference conditions, where the application fails to properly validate pointer references before accessing them, leading to application crashes and potential system instability.
The technical exploitation of this vulnerability occurs through a carefully crafted SOAP response that includes malformed XML data structures designed to trigger the null pointer access condition within PHP's SOAP extension. When a PHP application utilizing the SOAP extension connects to a malicious server and receives such malformed responses, the extension's XML parser encounters unexpected data patterns that result in dereferencing null pointers during the processing of SOAP messages. This behavior aligns with ATT&CK technique T1499.004 which involves network denial of service attacks, where adversaries can disrupt services by causing applications to crash or become unresponsive. The flaw essentially allows an attacker to remotely cause a crash in any PHP application that uses the SOAP extension to communicate with external services, effectively creating a remote denial of service condition that can be leveraged as part of broader attack campaigns.
The operational impact of CVE-2021-21702 extends beyond simple service disruption to potentially compromise entire application availability and system stability. Applications that rely heavily on SOAP communications for integration with external services become vulnerable to targeted attacks that can cause cascading failures, particularly in environments where multiple services depend on SOAP-based communication channels. This vulnerability affects web applications, enterprise integration platforms, and any system where PHP SOAP functionality is utilized, making it a significant concern for organizations running affected PHP versions. The crash condition can be triggered repeatedly, allowing attackers to maintain persistent denial of service conditions that can severely impact business operations and customer service availability.
Organizations should prioritize immediate patching of affected PHP installations to address CVE-2021-21702, ensuring all systems are updated to versions that contain the necessary fixes for the SOAP extension's null pointer dereference vulnerability. System administrators should implement network monitoring to detect unusual SOAP traffic patterns that might indicate exploitation attempts, while also reviewing firewall rules to limit unnecessary SOAP communication between internal systems and external endpoints. The mitigation strategy should include regular security assessments of PHP applications to identify all instances where SOAP extensions are utilized, combined with implementing proper input validation and error handling mechanisms that can gracefully handle malformed responses from external services. Additionally, organizations should consider implementing application-level firewalls or API gateways that can filter and sanitize SOAP traffic before it reaches vulnerable PHP applications, providing an additional layer of protection against this and similar vulnerabilities.