CVE-2017-17762 in EPiServer
Summary
by MITRE
XML external entity (XXE) vulnerability in Episerver 7 patch 4 and earlier allows remote attackers to read arbitrary files via a crafted DTD in an XML request involving util/xmlrpc/Handler.ashx.
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Analysis
by VulDB Data Team • 03/19/2020
The CVE-2017-17762 vulnerability represents a critical XML external entity processing flaw discovered in Episerver versions 7 patch 4 and earlier, specifically affecting the util/xmlrpc/Handler.ashx endpoint. This vulnerability falls under the Common Weakness Enumeration category CWE-611, which classifies it as an improper restriction of XML external entity reference. The flaw arises from the application's insufficient validation of XML input, particularly when processing XML requests that contain external entity declarations. Attackers can exploit this weakness by crafting malicious DTD (Document Type Definition) content within XML requests that are processed by the vulnerable handler. The vulnerability is particularly concerning because it allows remote attackers to perform arbitrary file reads on the affected system, potentially exposing sensitive data, configuration files, and other system resources that should remain protected. The attack vector is accessible over the network, making it a significant risk for systems that are publicly exposed or accessible to untrusted users.
The technical implementation of this vulnerability occurs when the Episerver application processes XML requests through the Handler.ashx endpoint without proper sanitization of external entity references. When a malicious XML payload containing a crafted DTD is submitted, the XML parser within the application resolves external entities, which can point to local files on the server. This process, known as XML external entity processing, is commonly used for legitimate purposes such as accessing remote resources, but becomes dangerous when attackers can manipulate the entity references to access local files. The vulnerability is particularly dangerous because it can be exploited to read system files that contain sensitive information, including database connection strings, application configuration files, and potentially even operating system files. The exploitation requires no authentication and can be performed remotely, making it a severe threat to system security.
The operational impact of CVE-2017-17762 extends beyond simple data exfiltration, as it can enable attackers to gain deeper insights into the system architecture and potentially facilitate further attacks. Successful exploitation allows adversaries to read arbitrary files, which may include database credentials, application secrets, and configuration parameters that could be used for privilege escalation or lateral movement within the network. This vulnerability can also be leveraged as part of a broader attack chain, potentially leading to full system compromise. The attack can result in significant data breaches, regulatory compliance violations, and financial losses for organizations that rely on Episerver for content management. Organizations with web applications that process XML input through vulnerable endpoints face substantial risk, particularly those handling sensitive data or operating in regulated environments where such vulnerabilities could lead to compliance violations under standards like PCI DSS or GDPR.
The mitigation strategies for CVE-2017-17762 focus on implementing proper XML input validation and disabling external entity processing within the application. Organizations should immediately upgrade to Episerver versions that have patched this vulnerability, as the official release notes for patch releases typically include fixes that disable external entity resolution or implement proper input sanitization. Security configurations should enforce strict XML parsing rules that prevent the processing of external entities, particularly in the util/xmlrpc/Handler.ashx endpoint. Additionally, implementing web application firewalls with XML security rules, network segmentation, and monitoring for suspicious XML requests can provide additional layers of defense. The ATT&CK framework categorizes this vulnerability under the technique T1059.007 for XML external entity processing, and organizations should consider implementing defensive measures such as input validation, secure coding practices, and regular security assessments to prevent exploitation. Organizations should also conduct thorough security testing to identify similar vulnerabilities in other components of their XML processing pipelines and ensure that all third-party applications and libraries are updated to prevent similar XXE vulnerabilities from being exploited in their environments.