CVE-2016-9487 in EpubCheck
Summary
by MITRE
EpubCheck 4.0.1 does not properly restrict resolving external entities when parsing XML in EPUB files during validation. An attacker who supplies a specially crafted EPUB file may be able to exploit this behavior to read arbitrary files, or have the victim execute arbitrary requests on his behalf, abusing the victim's trust relationship with other entities.
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Analysis
by VulDB Data Team • 12/27/2024
The vulnerability identified as CVE-2016-9487 affects EpubCheck version 4.0.1, a widely used tool for validating EPUB file format compliance. This security flaw resides in the XML parsing mechanism of the validation process, specifically within how the tool handles external entity resolution. The issue represents a critical weakness in the tool's security architecture that could be exploited by malicious actors to compromise systems that rely on EpubCheck for EPUB file validation. The vulnerability stems from improper restrictions on external entity resolution during XML processing, creating a potential attack surface that allows unauthorized access to system resources.
The technical implementation of this vulnerability involves the XML parser's failure to adequately sanitize external entity references when processing EPUB files. When EpubCheck encounters an EPUB document containing malicious XML markup, it processes external entity declarations without sufficient validation or restriction mechanisms. This behavior aligns with CWE-611, which describes improper access control in XML parsers, and specifically manifests as a vulnerability in the XML external entity processing functionality. The flaw enables attackers to craft EPUB files that contain malicious external entity references, potentially leading to information disclosure or remote code execution scenarios. The vulnerability operates through the standard XML parsing mechanisms that are commonly implemented in validation tools, making it particularly dangerous as it leverages the legitimate functionality of XML processing.
The operational impact of CVE-2016-9487 extends beyond simple file reading capabilities, as it can enable attackers to exploit the victim's trust relationships with other network entities. An attacker could construct an EPUB file that, when processed by EpubCheck, triggers requests to internal network resources or external malicious servers. This capability represents a significant risk in environments where EpubCheck is used to validate documents from untrusted sources, such as content management systems, digital library platforms, or automated validation services. The vulnerability essentially allows attackers to bypass normal security boundaries and potentially access sensitive information or execute unauthorized operations through the validation process itself. The attack vector is particularly concerning because it leverages a legitimate tool used for content validation, making it difficult to detect and prevent.
Organizations utilizing EpubCheck for EPUB file validation should implement immediate mitigations to address this vulnerability. The most effective approach involves upgrading to a patched version of EpubCheck that properly restricts external entity resolution during XML parsing. Security administrators should also consider implementing network-level restrictions to prevent EpubCheck from accessing external resources during validation operations. Additional defensive measures include deploying content filtering mechanisms that scan EPUB files for suspicious XML patterns before processing, and establishing strict access controls on validation environments to limit potential attack surfaces. From a broader security perspective, this vulnerability highlights the importance of proper XML parser configuration and the need for security-conscious development practices that consider the full attack surface of parsing libraries. The issue also aligns with ATT&CK technique T1059.007, which covers XML external entity processing, emphasizing the need for robust input validation and secure parsing practices in software applications that handle structured data formats.