CVE-2015-6096 in .NET Framework
Summary
by MITRE
The XML DTD parser in Microsoft .NET Framework 2.0 SP2, 3.5, 3.5.1, 4, 4.5, 4.5.1, 4.5.2, and 4.6 allows remote attackers to read arbitrary files via an external entity declaration in conjunction with an entity reference, related to an XML External Entity (XXE) issue, aka ".NET Information Disclosure Vulnerability."
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Analysis
by VulDB Data Team • 06/27/2022
The vulnerability identified as CVE-2015-6096 represents a critical XML External Entity (XXE) flaw within Microsoft .NET Framework versions spanning from 2.0 SP2 through 4.6. This issue specifically affects the XML Document Type Definition (DTD) parser component that processes XML documents containing external entity declarations. The vulnerability stems from insufficient validation of external entity references during XML parsing operations, creating a pathway for malicious actors to exploit the framework's handling of XML data. The flaw enables attackers to construct specially crafted XML documents that reference external entities, potentially allowing unauthorized access to local file systems through server-side request forgery mechanisms. This vulnerability falls under the CWE-611 weakness category, specifically categorized as improper restriction of XML external entity reference, which directly aligns with the ATT&CK technique T1213.002 for Data from Information Repositories.
The technical exploitation of this vulnerability occurs when a .NET application processes XML input that contains external entity declarations pointing to local files or network resources. An attacker can construct an XML payload that includes a DTD with external entity references that resolve to sensitive files on the server hosting the vulnerable application. When the XML parser encounters these external entity references, it attempts to resolve them by reading the specified files, potentially exposing sensitive data such as configuration files, credential stores, or system information. The vulnerability is particularly dangerous because it operates at the parsing layer, meaning that applications relying on standard .NET XML processing methods are inherently susceptible to this attack vector without additional protective measures. The attack chain typically involves crafting malicious XML content that references external entities, sending this content to a vulnerable application, and then capturing the response that contains the read file contents.
The operational impact of CVE-2015-6096 extends beyond simple information disclosure, as it can serve as a stepping stone for more sophisticated attacks within compromised environments. Organizations running vulnerable .NET applications face potential exposure of sensitive business data, system configuration files, and potentially credential information stored in accessible files. The vulnerability affects a broad range of .NET Framework versions, making it particularly concerning for enterprise environments where legacy applications may not have been updated. Attackers can leverage this vulnerability to perform reconnaissance activities, gather intelligence about system configurations, and potentially identify other vulnerabilities within the network infrastructure. The impact is especially severe in environments where applications process untrusted XML input from external sources, as this vulnerability can be exploited through web services, file uploads, or API endpoints that accept XML data. This vulnerability directly relates to ATT&CK tactic TA0007 for Discovery and TA0006 for Credential Access, as it enables both reconnaissance and potential credential theft through file access.
Mitigation strategies for CVE-2015-6096 require a multi-layered approach that addresses both immediate protection and long-term architectural improvements. The most effective immediate solution involves applying Microsoft security patches and updates that specifically address this XXE vulnerability, ensuring all affected .NET Framework versions are properly updated. Organizations should implement XML parser configuration changes that disable external entity resolution and DTD processing entirely within their applications. This can be achieved through proper XML reader settings that restrict external entity declarations and prevent the resolution of external references. Additionally, input validation and sanitization mechanisms should be implemented at multiple layers to filter out potentially malicious XML content before it reaches the XML parser. Network-level controls such as web application firewalls and intrusion detection systems can provide additional protection by monitoring for suspicious XML patterns and blocking known attack signatures. Organizations should also consider implementing principle of least privilege access controls for XML processing components and regularly audit application code for XML input handling to identify and remediate potential XXE vulnerabilities in custom applications. The remediation process should include comprehensive testing to ensure that security measures do not inadvertently break legitimate application functionality while maintaining robust protection against external entity resolution attacks.