This vulnerability in Apache Solr 6.0.0 to 6.6.3, 7.0.0 to 7.3.0 relates to an XML external entity expansion (XXE) in Solr config files (solrconfig.xml, schema.xml, managed-schema). 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. Users are advised to upgrade to either Solr 6.6.4 or Solr 7.3.1 releases both of which address the vulnerability. Once upgrade is complete, no other steps are required. Those releases only allow external entities and Xincludes that refer to local files / zookeeper resources below the Solr instance directory (using Solrs ResourceLoader); usage of absolute URLs is denied. Keep in mind, that external entities and XInclude are explicitly supported to better structure config files in large installations. Before Solr 6 this was no problem, as config files were not accessible through the APIs.
The product processes an XML document that can contain XML entities with URIs that resolve to documents outside of the intended sphere of control, causing the product to embed incorrect documents into its output.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Solr | Apache | 6.0.0 (including) | 6.6.3 (including) |
| Solr | Apache | 7.0.0 (including) | 7.3.0 (including) |
XML documents optionally contain a Document Type Definition (DTD), which, among other features, enables the definition of XML entities. It is possible to define an entity by providing a substitution string in the form of a URI. The XML parser can access the contents of this URI and embed these contents back into the XML document for further processing. By submitting an XML file that defines an external entity with a file:// URI, an attacker can cause the processing application to read the contents of a local file. For example, a URI such as “file:///c:/winnt/win.ini” designates (in Windows) the file C:\Winnt\win.ini, or file:///etc/passwd designates the password file in Unix-based systems. Using URIs with other schemes such as http://, the attacker can force the application to make outgoing requests to servers that the attacker cannot reach directly, which can be used to bypass firewall restrictions or hide the source of attacks such as port scanning. Once the content of the URI is read, it is fed back into the application that is processing the XML. This application may echo back the data (e.g. in an error message), thereby exposing the file contents.