CVE-2018-8026 in Solr
Summary
by MITRE
This vulnerability in Apache Solr 6.0.0 to 6.6.4 and 7.0.0 to 7.3.1 relates to an XML external entity expansion (XXE) in Solr config files (currency.xml, enumsConfig.xml referred from schema.xml, TIKA parsecontext config file). In addition, Xinclude functionality provided in these config files is also affected in a similar way. The vulnerability can be used as XXE using file/ftp/http protocols in order to read arbitrary local files from the Solr server or the internal network. The manipulated files can be uploaded as configsets using Solr's API, allowing to exploit that vulnerability.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 02/25/2020
Apache Solr versions 6.0.0 through 6.6.4 and 7.0.0 through 7.3.1 contain a critical XML external entity expansion vulnerability that stems from improper input validation in configuration file processing. This vulnerability exists within the currency.xml, enumsConfig.xml, and TIKA parsecontext configuration files that are referenced from schema.xml, creating a pathway for attackers to exploit XML parsing mechanisms. The flaw manifests when Solr processes these configuration files, failing to properly sanitize external entity references and XInclude directives that allow for arbitrary file access through file, ftp, or http protocols. The vulnerability is particularly dangerous because it leverages Solr's legitimate configuration upload functionality, where malicious configsets can be uploaded via Solr's API and subsequently trigger the XXE exploitation. This creates a direct attack vector where an unauthenticated remote attacker can potentially read arbitrary files from the Solr server filesystem or access internal network resources that are normally protected from external access. The technical implementation of this vulnerability aligns with CWE-611 (Improper Restriction of XML External Entity Reference) and represents a classic XXE attack pattern that has been documented extensively in cybersecurity literature. The impact extends beyond simple file reading capabilities as it can potentially lead to information disclosure, internal network reconnaissance, and in combination with other vulnerabilities, could facilitate further compromise of the affected system. Attackers can craft malicious configuration files that contain external entity declarations pointing to sensitive local files such as system configuration files, credential stores, or internal resource files that are accessible to the Solr process. The XInclude functionality within these config files compounds the risk by providing additional attack surfaces where remote entities can be included and processed without proper validation. This vulnerability directly maps to ATT&CK technique T1083 (File and Directory Discovery) and T1046 (Network Service Scanning) as it enables attackers to enumerate system resources and potentially discover internal network services. The exploitation requires minimal privileges since the vulnerability is accessible through Solr's standard API endpoints, making it particularly attractive to attackers seeking to establish a foothold within network environments. Organizations running affected Solr versions should immediately implement mitigations including disabling external entity processing in XML parsers, restricting configuration file uploads, and implementing proper input validation for all configuration files. The vulnerability demonstrates how legacy XML processing mechanisms in enterprise search platforms can create significant security risks when proper sanitization controls are not implemented. This issue highlights the importance of secure configuration management and the potential for seemingly benign configuration file processing to become attack vectors when external entity references are not properly controlled. The vulnerability's persistence in multiple versions of Solr indicates a systemic issue in XML processing security controls that required immediate remediation through version upgrades and configuration hardening measures.
The exploitation of this vulnerability through Solr's API upload mechanism creates a particularly concerning attack scenario where attackers can bypass traditional network segmentation controls. When configuration sets are uploaded and processed, the XML parsers execute external entity references without sufficient validation, allowing attackers to craft malicious configuration files that can access files on the server. The use of file, ftp, and http protocols provides multiple attack vectors, with the file protocol being particularly dangerous as it can access local filesystem resources directly without requiring network connectivity to external servers. This vulnerability essentially transforms Solr's legitimate configuration management capabilities into a potential attack surface for lateral movement and information gathering within the network environment. The TIKA parsecontext configuration file adds another dimension to the attack surface as it allows for additional XML processing that can be manipulated to achieve the same objectives. The vulnerability's classification under CWE-611 emphasizes the core security flaw in XML processing, where external entity references are not properly restricted or validated. From an operational standpoint, this vulnerability can lead to significant data breaches as attackers can potentially access sensitive configuration files, credential files, or other system resources that are typically protected by filesystem permissions. The impact is particularly severe in enterprise environments where Solr is often used to index sensitive business data, making the potential for data exposure extremely high. Organizations should implement network segmentation controls, restrict API access to trusted sources, and ensure that all Solr installations are updated to versions that have addressed this vulnerability through proper XML parser configuration and external entity restriction mechanisms. The vulnerability serves as a reminder of the critical importance of secure XML processing in enterprise applications and the potential for configuration management features to become security risks when proper input validation controls are not implemented.