XML parsing vulnerable to XXE (SAXParser)

Bug Pattern: XXE_SAXPARSER

Attack

XML External Entity (XXE) attacks can occur when an XML parser supports XML entities while processing XML received from an untrusted source.

Risk 1: Expose local file content (XXE: XML eXternal Entity)

  1. <?xml version="1.0" encoding="ISO-8859-1"?>
  2. <!DOCTYPE foo [
  3.    <!ENTITY xxe SYSTEM "file:///etc/passwd" > ]>
  4. <foo>&xxe;</foo>

Risk 2: Denial of service (XEE: Xml Entity Expansion)

  1. <?xml version="1.0"?>
  2. <!DOCTYPE lolz [
  3.  <!ENTITY lol "lol">
  4.  <!ELEMENT lolz (#PCDATA)>
  5.  <!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
  6.  <!ENTITY lol2 "&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;">
  7.  <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
  8. [...]
  9.  <!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">
  10. ]>
  11. <lolz>&lol9;</lolz>

Solution

In order to avoid exposing dangerous feature of the XML parser, you can do the following change to the code.

Vulnerable Code:

  1. SAXParser parser = SAXParserFactory.newInstance().newSAXParser();
  3. parser.parse(inputStream, customHandler);

The following snippets show two available solutions. You can set one feature or both.

Solution using “Secure processing” mode:

This setting will protect you against Denial of Service attack and remote file access.

  1. SAXParserFactory spf = SAXParserFactory.newInstance();
  2. spf.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
  3. SAXParser parser = spf.newSAXParser();
  5. parser.parse(inputStream, customHandler);

Solution disabling DTD:

By disabling DTD, almost all XXE attacks will be prevented.

  1. SAXParserFactory spf = SAXParserFactory.newInstance();
  2. spf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
  3. SAXParser parser = spf.newSAXParser();
  5. parser.parse(inputStream, customHandler);

References
CWE-611: Improper Restriction of XML External Entity Reference (‘XXE’)
CERT: IDS10-J. Prevent XML external entity attacks
OWASP.org: XML External Entity (XXE) Processing
WS-Attacks.org: XML Entity Expansion
WS-Attacks.org: XML External Entity DOS
WS-Attacks.org: XML Entity Reference Attack
Identifying Xml eXternal Entity vulnerability (XXE)
Xerces complete features list