CVE-2016-5851 in python-docx
Summary
by MITRE
python-docx before 0.8.6 allows context-dependent attackers to conduct XML External Entity (XXE) attacks via a crafted document.
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Analysis
by VulDB Data Team • 10/12/2022
The vulnerability identified as CVE-2016-5851 represents a critical XML External Entity processing flaw in the python-docx library version 0.8.5 and earlier. This issue arises from the library's insufficient validation of XML content when processing Microsoft Word documents, creating an attack surface that allows malicious actors to exploit XML parsing mechanisms. The vulnerability specifically affects applications that utilize python-docx for document processing, particularly those handling untrusted document inputs from external sources. The flaw enables attackers to craft malicious Word documents containing specially constructed XML entities that can be leveraged to perform various malicious activities during document parsing operations.
The technical implementation of this vulnerability stems from the library's failure to properly sanitize XML input during document parsing. When python-docx encounters XML content within a Word document, it processes external entity declarations without adequate restrictions or validation mechanisms. This behavior aligns with CWE-611, which describes improper restriction of XML external entity reference processing, making the library susceptible to XXE attacks where attackers can manipulate the parsing process to access local files, perform server-side request forgery, or potentially execute arbitrary code. The vulnerability operates at the XML parsing layer where the library uses standard XML parsers without disabling external entity resolution, creating a pathway for attackers to inject malicious XML content that gets processed during document loading operations.
The operational impact of this vulnerability extends beyond simple document processing and creates significant security implications for systems that rely on python-docx for document handling. Attackers can leverage this vulnerability to access sensitive files on the server hosting applications that use the affected library, potentially leading to data exfiltration, privilege escalation, or system compromise. The context-dependent nature of this attack means that successful exploitation requires the target application to process a specially crafted document containing malicious XML entities, making it particularly dangerous in environments where users can upload or receive documents from untrusted sources. This vulnerability particularly affects web applications, document management systems, and any software that processes Microsoft Word documents programmatically using the python-docx library.
Organizations utilizing python-docx in their applications should prioritize immediate remediation through upgrading to version 0.8.6 or later, which includes proper XML entity validation and external entity restriction mechanisms. The mitigation strategy should also incorporate input validation for all document processing workflows, implementing proper sandboxing techniques and restricting file access permissions for applications handling document processing. Security teams should consider implementing network-level restrictions to prevent outbound connections from document processing systems and monitor for suspicious XML parsing activities. Additionally, organizations should conduct comprehensive vulnerability assessments to identify all systems using affected versions of python-docx and ensure proper security controls are in place to prevent exploitation. The ATT&CK framework categorizes this vulnerability under T1190 - Exploit Public-Facing Application, as it represents a common attack vector through document processing applications that are exposed to untrusted input sources.