CVE-2016-9180 in perl-XML-Twig
Summary
by MITRE
perl-XML-Twig: The option to `expand_external_ents`, documented as controlling external entity expansion in XML::Twig does not work. External entities are always expanded, regardless of the option's setting.
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Analysis
by VulDB Data Team • 10/12/2022
The vulnerability identified as CVE-2016-9180 resides within the perl-XML-Twig module, a widely used Perl library for processing XML documents. This flaw represents a critical security issue that directly impacts the library's ability to control external entity expansion, which is a fundamental aspect of XML security. The vulnerability specifically affects the `expand_external_ents` option that is documented to govern whether external entities should be expanded during XML processing. When this option is set to disable external entity expansion, the system fails to honor this configuration and continues to expand external entities regardless of the user's intent.
The technical nature of this vulnerability stems from a failure in the XML processing logic where the module does not properly validate or enforce the `expand_external_ents` parameter. This represents a classic case of improper input validation and control flow management within the XML parser implementation. The flaw operates at the parsing layer where external entity references are processed, bypassing the intended security controls that should prevent unauthorized access to external resources. This behavior aligns with CWE-444, which describes improper handling of XML external entity references, and specifically relates to the CWE-611 vulnerability category concerning improper access control of XML external entities.
The operational impact of this vulnerability is severe and far-reaching, particularly in environments where XML processing is performed on untrusted input. Attackers can exploit this weakness to perform server-side request forgery attacks, where external entity references can be used to access internal network resources or trigger denial-of-service conditions. The vulnerability enables what is commonly known as XML external entity injection, allowing malicious actors to potentially exfiltrate sensitive data from the target system or consume system resources through resource exhaustion attacks. This type of vulnerability is particularly dangerous in web applications that process XML data from users, as it can be leveraged to bypass security controls and perform unauthorized operations.
Organizations using perl-XML-Twig in their applications face significant risk from this vulnerability, as it can be exploited to compromise system integrity and availability. The flaw is particularly concerning in environments where XML processing is used for data integration, web services, or document processing where external entities might reference internal resources. Security practitioners should consider this vulnerability in their threat modeling exercises and assess the impact on their specific implementations. The vulnerability also aligns with ATT&CK technique T1213.002, which involves data from external remote services, and T1059.007, which covers scripting through XML processing.
Mitigation strategies for CVE-2016-9180 should include immediate patching of the perl-XML-Twig module to the latest version that addresses this specific flaw. Organizations should also implement additional security controls such as input validation, XML schema validation, and network segmentation to limit the potential impact of exploitation. The use of XML parsers with proper external entity handling capabilities should be considered as a long-term solution. Security monitoring should be enhanced to detect suspicious XML processing activities, particularly those involving external entity references. Additionally, organizations should conduct comprehensive vulnerability assessments to identify all systems using affected versions of the module and ensure proper remediation across their infrastructure.