如何从XML外部攻击中保护javax.xml.transform.TransformerFactory

Question

我已就此主题进行了研究,但未找到相关的相关信息

我们是否需要采取任何安全措施来保护javax.xml.transform.Transformer免受XML外部实体攻击？

我做了以下,似乎扩大了dtd.

String fileData = "<!DOCTYPE acunetix [  <!ENTITY sampleVal SYSTEM \"file:///media/sample\">]><username>&sampleVal;</username>";
TransformerFactory transformerFactory = TransformerFactory.newInstance();
transformerFactory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
Transformer transformer = transformerFactory.newTransformer();
StringWriter buff = new StringWriter();
transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
transformer.transform(new StreamSource(new StringReader(fileData)), new StreamResult(buff));
System.out.println(buff.toString());

output包含文件中的值

<username>test</username>

Answer 1

你的代码似乎是正确的。当我运行这个稍微修改过的 JUnit 测试用例时：

@Test
public void test() throws TransformerException, URISyntaxException {
  File testFile = new File(getClass().getResource("test.txt").toURI());
  assertTrue(testFile.exists());
  String fileData = "<!DOCTYPE acunetix [  <!ENTITY foo SYSTEM \"file://" + 
                    testFile.toString() +
                    "\">]><xxe>&foo;</xxe>";
  TransformerFactory transformerFactory = TransformerFactory.newInstance();
  System.out.println(transformerFactory.getClass().getName());
  transformerFactory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
  Transformer transformer = transformerFactory.newTransformer();
  StringWriter buff = new StringWriter();
  transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
  transformer.transform(new StreamSource(new StringReader(fileData)), new StreamResult(buff));
  assertEquals("<xxe>&foo;</xxe>", buff.toString());
}

我得到以下输出：

com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl
[Fatal Error] :1:182: External Entity: Failed to read external document 'test.txt', because 'file' access is not allowed due to restriction set by the accessExternalDTD property.
ERROR:  'External Entity: Failed to read external document 'test.txt', because 'file' access is not allowed due to restriction set by the accessExternalDTD property.'

从setFeature JavaDocs：

所有实现都需要支持 XMLConstants.FEATURE_SECURE_PROCESSING 功能。当特征是：

• true：实现将限制 XML 处理以符合实现限制并以实现定义的安全方式运行。示例包括解析用户定义的样式表和函数。如果出于安全原因限制了 XML 处理，则会通过调用注册的 ErrorListener.fatalError(TransformerException exception) 来报告。请参阅 setErrorListener(ErrorListener 侦听器)。

如果我注释掉transformerFactory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);然后测试失败，因为实体已解析，该错误就会消失。

尝试向 TransformerFactory 和 Transformer 添加一个 ErrorListener：

transformerFactory.setErrorListener(new ErrorListener() {

  @Override
  public void warning(TransformerException exception) throws TransformerException {
    System.out.println("In Warning: " + exception.toString());
  }

  @Override
  public void error(TransformerException exception) throws TransformerException {
    System.out.println("In Error: " + exception.toString());
  }

  @Override
  public void fatalError(TransformerException exception) throws TransformerException {
    System.out.println("In Fatal: " + exception.toString());
  }
});

Transformer transformer = transformerFactory.newTransformer();
transformer.setErrorListener(transformerFactory.getErrorListener());

我现在看到以下新的控制台输出：

In Error: javax.xml.transform.TransformerException: External Entity: Failed to read external document 'test.txt', because 'file' access is not allowed due to restriction set by the accessExternalDTD property.

也许您的实现将其视为警告？否则，也许这是您正在使用的实现？看起来 JavaDoc 规范并不精确，所以一个实现可能会做一些与另一个不同的事情。我很想知道错误的实现！