CVE-2017-5644 in POI
Summary
by MITRE
Apache POI in versions prior to release 3.15 allows remote attackers to cause a denial of service (CPU consumption) via a specially crafted OOXML file, aka an XML Entity Expansion (XEE) attack.
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Analysis
by VulDB Data Team • 09/14/2020
The vulnerability identified as CVE-2017-5644 represents a critical security flaw within Apache POI library versions prior to 3.15, specifically targeting the processing of OOXML files through XML Entity Expansion attacks. This vulnerability falls under the broader category of XML External Entity processing issues, which have been extensively documented in cybersecurity literature and classified under CWE-611. The Apache POI library serves as a popular Java-based solution for reading and writing Microsoft Office file formats including Word documents, Excel spreadsheets, and PowerPoint presentations. When processing specially crafted OOXML documents, the library fails to properly validate and restrict XML entity expansion, creating a pathway for malicious actors to exploit this weakness. The flaw enables attackers to craft documents containing recursive XML entities that cause the parser to consume excessive CPU resources during processing, leading to system resource exhaustion and potential denial of service conditions.
The technical mechanism behind this vulnerability involves the XML parser's handling of entity declarations within OOXML files. When an attacker constructs a document with maliciously designed entity references that reference other entities in a recursive manner, the parser expands these entities repeatedly until system resources are depleted. This process creates a massive increase in CPU consumption as the parser attempts to resolve and expand each entity reference, effectively creating an infinite loop or extremely long-running process. The vulnerability affects the XML parsing functionality within Apache POI's OOXML processing components, particularly impacting the way the library handles external entity references during document parsing operations. This weakness is consistent with the attack patterns described in the MITRE ATT&CK framework under the technique of "Exploitation for Privilege Escalation" and "Resource Exhaustion" where adversaries leverage application vulnerabilities to consume system resources.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it can affect any system or application that utilizes Apache POI for processing Office documents. Organizations relying on automated document processing, email filtering systems, or any service that accepts user-uploaded Office files become vulnerable to this attack vector. The CPU consumption can escalate rapidly, potentially causing system instability, application crashes, or complete service unavailability. This vulnerability is particularly dangerous in environments where automated processing occurs, such as web applications, document management systems, or enterprise email gateways that process large volumes of Office documents. The attack requires minimal sophistication from the attacker's perspective, as it only requires crafting a specific XML structure within an OOXML document, making it an attractive target for both automated attacks and targeted exploitation attempts. The vulnerability's impact is amplified in cloud environments or shared hosting scenarios where resource exhaustion on one system could affect neighboring applications or services.
Mitigation strategies for CVE-2017-5644 primarily focus on upgrading to Apache POI version 3.15 or later, which includes proper XML entity expansion controls and restrictions. Organizations should implement comprehensive patch management processes to ensure all systems utilizing Apache POI are updated promptly. Additional protective measures include implementing XML parser configuration settings that disable external entity resolution andDTD processing, as recommended by the OWASP XML Security Guidelines. Network-level controls such as content filtering and file type validation can provide additional defense-in-depth layers, particularly for applications that must process untrusted Office documents. The implementation of input validation and sanitization techniques for all Office document processing workflows helps reduce the attack surface. Security monitoring should include detection of unusual CPU consumption patterns during document processing, which could indicate exploitation attempts. Organizations should also consider implementing application-level restrictions on document processing timeouts and resource limits to prevent complete system exhaustion. These measures align with the defensive strategies outlined in the NIST Cybersecurity Framework and provide comprehensive protection against XML entity expansion attacks. Regular security assessments and vulnerability scanning should include checks for outdated Apache POI versions to prevent exploitation of this and similar vulnerabilities.