CVE-2017-5992 in Openpyxl
Summary
by MITRE
Openpyxl 2.4.1 resolves external entities by default, which allows remote attackers to conduct XXE attacks via a crafted .xlsx document.
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Analysis
by VulDB Data Team • 08/15/2020
The vulnerability identified as CVE-2017-5992 affects openpyxl version 2.4.1, a popular python library for reading and writing excel files. This security flaw resides in the library's default handling of external entity resolution within xml documents, creating a significant attack surface for remote adversaries. The issue manifests when the library processes crafted .xlsx files that contain malicious xml external entity declarations, enabling attackers to exploit the underlying xml parser's default behavior of resolving external references without proper sanitization.
The technical implementation of this vulnerability stems from openpyxl's xml parsing mechanism which, by default, does not disable external entity resolution. When a malicious .xlsx file is processed, the xml parser encounters external entity references that are automatically resolved, potentially allowing attackers to access local files, perform server-side request forgery attacks, or conduct denial of service conditions. This behavior aligns with CWE-611, which specifically addresses improper restriction of XML external entity reference, and represents a classic example of XML External Entity processing vulnerabilities that have plagued numerous applications over the years.
The operational impact of this vulnerability extends beyond simple data theft, as it provides attackers with multiple attack vectors through the xml processing pipeline. Remote attackers can leverage this weakness to access sensitive files on the target system, potentially leading to information disclosure, privilege escalation, or system compromise. The vulnerability is particularly concerning because it affects a widely-used library that processes spreadsheet files in numerous applications, making it an attractive target for mass exploitation campaigns. According to ATT&CK framework, this vulnerability maps to T1059.007 for script execution and T1190 for exploitation of remote services, demonstrating how the vulnerability can be leveraged in broader attack chains.
Mitigation strategies for CVE-2017-5992 primarily focus on updating to patched versions of openpyxl where external entity resolution has been disabled by default. Organizations should immediately upgrade to openpyxl version 2.4.2 or later, which includes proper xml parser configuration to prevent external entity resolution. Additionally, administrators should implement strict input validation policies for all spreadsheet file processing, particularly in web applications that accept user-uploaded .xlsx files. The vulnerability can also be addressed through xml parser hardening, where external entity declarations are explicitly disabled in the parsing configuration. Security teams should also consider implementing network-level protections such as firewalls that restrict access to internal resources from external systems, and application-level sandboxing for spreadsheet processing to limit the potential impact of successful exploitation attempts.