CVE-2015-5161 in Zend
Summary
by MITRE
The Zend_Xml_Security::scan in ZendXml before 1.0.1 and Zend Framework before 1.12.14, 2.x before 2.4.6, and 2.5.x before 2.5.2, when running under PHP-FPM in a threaded environment, allows remote attackers to bypass security checks and conduct XML external entity (XXE) and XML entity expansion (XEE) attacks via multibyte encoded characters.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 12/30/2024
The vulnerability identified as CVE-2015-5161 represents a critical security flaw in the Zend Framework's XML security implementation that specifically affects PHP-FPM environments operating in threaded configurations. This weakness resides in the Zend_Xml_Security::scan function which is designed to protect applications from XML external entity and XML entity expansion attacks. The vulnerability emerges when the framework processes XML data containing multibyte encoded characters, allowing attackers to bypass the intended security controls that should prevent malicious XML content from being parsed. The flaw affects multiple versions of the Zend Framework including 1.x series before 1.12.14, 2.x series before 2.4.6, and 2.5.x series before 2.5.2, indicating a widespread impact across the framework's major versions. The security bypass occurs specifically in threaded PHP-FPM environments where the multibyte character handling interacts unexpectedly with the XML parsing security mechanisms, creating a pathway for attackers to exploit XXE and XEE vulnerabilities.
The technical root cause of this vulnerability stems from the improper handling of multibyte character encodings within the XML security scanning process. When PHP-FPM operates in a threaded environment, the interaction between multibyte character processing and XML parsing creates a condition where the security checks fail to properly validate XML content. This occurs because the security scanning function does not adequately account for how multibyte character encodings can be manipulated to circumvent validation rules. The flaw allows attackers to craft malicious XML payloads that contain multibyte encoded characters which, when processed by the vulnerable framework, can trigger external entity references or cause excessive entity expansion that leads to resource exhaustion or information disclosure. The vulnerability is particularly dangerous because it leverages the inherent threading characteristics of PHP-FPM to bypass protections that would otherwise be effective in single-threaded environments.
The operational impact of CVE-2015-5161 is significant for organizations using affected versions of the Zend Framework, particularly those running PHP-FPM in threaded configurations. Attackers can exploit this vulnerability to perform various malicious activities including retrieving arbitrary files from the server, conducting server-side request forgery attacks, or causing denial of service through resource exhaustion via entity expansion. The vulnerability is especially concerning in web applications that process user-provided XML data, as it allows attackers to bypass input validation mechanisms that should prevent such dangerous XML content from being processed. Organizations may experience unauthorized data access, system compromise, or service disruption depending on how the vulnerable applications are configured and what resources they have access to. The threaded environment aspect means that this vulnerability affects a substantial portion of web applications deployed in production environments where PHP-FPM is used for performance reasons.
Organizations should immediately apply the security patches released by Zend Framework developers to address this vulnerability, ensuring that all affected versions are updated to the patched releases. The recommended mitigation strategy involves upgrading to Zend Framework 1.12.14, 2.4.6, or 2.5.2 respectively, which contain the necessary fixes for the XML security scanning function. Additionally, administrators should consider implementing network-level protections such as firewalls and intrusion detection systems to monitor for suspicious XML traffic patterns that might indicate exploitation attempts. Organizations should also review their application code to ensure that XML processing is properly sanitized and that appropriate input validation is implemented at multiple layers of the application stack. The vulnerability aligns with CWE-611 (Improper Restriction of XML External Entity Reference) and can be mapped to ATT&CK technique T1213.002 (Data from Information Repositories) and T1499.004 (Endpoint Denial of Service) as it enables both information disclosure and service disruption attacks. Regular security assessments and penetration testing should be conducted to identify similar vulnerabilities in other components of the application stack that may be susceptible to similar exploitation patterns.