java - 如何防止 XML 注入(inject)，如 XML 炸弹和 XXE 攻击

Question

我正在开发一个安卓应用

android:minSdkVersion="14"

在这个需要解析 xml 的应用程序中。为此，我正在使用这样的 DOM 解析器

DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder dBuilder = null;
Document doc = null;
try {      
    dBuilder = dbFactory.newDocumentBuilder();
} catch (ParserConfigurationException e) {
    e.printStackTrace();
}

但是当检查代码的安全性时，我在线上遇到了两个安全问题

dBuilder = dbFactory.newDocumentBuilder();，它们是

1.XML Entity Expansion Injection (XML Bomb)

2.XML External Entity Injection (XXE attack)

经过一些研究，我添加了这一行dbFactory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);

但是现在执行这一行时出现异常

javax.xml.parsers.ParserConfigurationException: http://javax.xml.XMLConstants/feature/secure-processing

谁能帮帮我？

Answer 1

您是否尝试过 OWASP page 中的以下代码 fragment ？ ？

import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException; // catching unsupported features
...

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
try {
  // This is the PRIMARY defense. If DTDs (doctypes) are disallowed, almost all XML entity attacks are prevented
  // Xerces 2 only - http://xerces.apache.org/xerces2-j/features.html#disallow-doctype-decl
  String FEATURE = "http://apache.org/xml/features/disallow-doctype-decl";
  dbf.setFeature(FEATURE, true);

  // If you can't completely disable DTDs, then at least do the following:
  // Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-general-entities
  // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-general-entities
  FEATURE = "http://xml.org/sax/features/external-general-entities";
  dbf.setFeature(FEATURE, false);

  // Xerces 1 - http://xerces.apache.org/xerces-j/features.html#external-parameter-entities
  // Xerces 2 - http://xerces.apache.org/xerces2-j/features.html#external-parameter-entities
  FEATURE = "http://xml.org/sax/features/external-parameter-entities";
  dbf.setFeature(FEATURE, false);

  // and these as well, per Timothy Morgan's 2014 paper: "XML Schema, DTD, and Entity Attacks" (see reference below)
  dbf.setXIncludeAware(false);
  dbf.setExpandEntityReferences(false);

  // And, per Timothy Morgan: "If for some reason support for inline DOCTYPEs are a requirement, then 
  // ensure the entity settings are disabled (as shown above) and beware that SSRF attacks
  // (http://cwe.mitre.org/data/definitions/918.html) and denial 
  // of service attacks (such as billion laughs or decompression bombs via "jar:") are a risk."

  // remaining parser logic
  ...

    catch (ParserConfigurationException e) {
        // This should catch a failed setFeature feature
        logger.info("ParserConfigurationException was thrown. The feature '" +
                    FEATURE +
                    "' is probably not supported by your XML processor.");
        ...
    }
    catch (SAXException e) {
        // On Apache, this should be thrown when disallowing DOCTYPE
        logger.warning("A DOCTYPE was passed into the XML document");
        ...
    }
    catch (IOException e) {
        // XXE that points to a file that doesn't exist
        logger.error("IOException occurred, XXE may still possible: " + e.getMessage());
        ...
    }