XML External Entity (XXE) Injection

Affecting nokogiri gem, versions < 1.5.4

high severity

Overview

nokogiri is an HTML, XML, SAX, and Reader parser, with the ability to search documents via XPath or CSS3 selectors.

Affected versions of this Gem are vulnerable to XML External Entity (XXE) attacks. Nokogiri is affected by series of vulnerabilities in libxml2 and libxslt, which are libraries it depends on. When handling the expansion of XML external entities (XXE) in libxml2, you can specify documents to be read. This can be used by attackers to load specially crafted XML documents on an internal XML parsing service and may lead to unauthorized disclosure of potentially sensitive information.

Note: This vulnerability exists also in versions >= 1.5.4 only when opting into the DTDLOAD option and opting out of the NONET option. See information here

Details

XXE Injection is a type of attack against an application that parses XML input. XML is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. By default, many XML processors allow specification of an external entity, a URI that is dereferenced and evaluated during XML processing. When an XML document is being parsed, the parser can make a request and include the content at the specified URI inside of the XML document.

Attacks can include disclosing local files, which may contain sensitive data such as passwords or private user data, using file: schemes or relative paths in the system identifier.

For example, below is a sample XML document, containing an XML element- username.

<?xml version="1.0" encoding="ISO-8859-1"?>
   <username>John</username>
</xml>

An external XML entity - xxe, is defined using a system identifier and present within a DOCTYPE header. These entities can access local or remote content. For example the below code contains an external XML entity that would fetch the content of /etc/passwd and display it to the user rendered by username.

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

Other XXE Injection attacks can access local resources that may not stop returning data, possibly impacting application availability and leading to Denial of Service.

Remediation

There currently is no fix in libxml2 as of January 11th, 2017. Nokogiri will be waiting for a fix upstream to update.

Disclosure Timeline

  • January 11th, 2017 - Reported the issue to Mike Dalessio of Nokogiri Core.
  • January 11th, 2017 - Issue triaged and acknowledged by Mike Dalessio of Nokogiri Core.

References

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Credit
Snyk Security Research Team
CWE
CWE-611
Snyk ID
SNYK-RUBY-NOKOGIRI-20298
Disclosed
11 Jan, 2017
Published
16 Jan, 2017