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Many Java enterprise frameworks provide configuration settings intended to be used as a defense against arbitrary file upload. Unfortunately, most of them fail to provide adequate protection. Mitigation of this vulnerability involves checking file size, content type, and file contents, among other metadata attributes.
Noncompliant Code Example
This noncompliant code example shows XML code from the upload action of a Struts 2 application. The interceptor code is responsible for allowing file uploads.
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The value of the parameter type maximumSize ensures that a particular Action cannot receive a very large file. The allowedTypes parameter defines the type of files that are accepted. However, this approach fails to ensure that the uploaded file conforms to the security requirements because interceptor checks can be trivially bypassed. If an attacker were to use a proxy tool to change the content type in the raw HTTP request in transit, the framework would fail to prevent the file's upload. Consequently, an attacker could upload a malicious file that has a .exe extension, for example.
Compliant Solution
The file upload must succeed only when the content type matches the actual content of the file. For example, a file with an image header must contain only an image and must not contain executable code. This compliant solution uses the Apache Tika library [Apache 2013] to detect and extract metadata and structured text content from documents using existing parser libraries. The checkMetaData() method must be called before invoking code in execute() that is responsible for uploading the file.
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The AutoDetectParser selects the best available parser on the basis of the content type of the file to be parsed.
Applicability
An arbitrary file upload vulnerability could result in privilege escalation and the execution of arbitrary code.
Bibliography
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