XML External Entity (XXE) Injection Affecting symfony/validator package, versions >=2, <2.0.17

Affected versions of symfony/validator are vulnerable to XML External Entity (XXE) Injection.

Symfony 2.0.11 carried a [similar] XXE security fix, however, on review of ZF2 I also noted a vulnerability to XML Entity Expansion (XEE) attacks whereby all extensions making use of libxml2 have no defense against XEE Quadratic Blowup Attacks. The vulnerability is a function of there being no current method of disabling custom entities in PHP (i.e. defined internal to the XML document without using external entities). In a QBA, a long entity can be defined and then referred to multiple times in document elements, creating a memory sink with which Denial Of Service attacks against a host's RAM can be mounted. The use of the LIBXML_NOENT or equivalent option in a dependent extension amplified the impact (it doesn't actually mean "No Entities"). In addition, libxml2's innate defense against the related Exponential or Billion Laugh's XEE attacks is active only so long as the LIBXML_PARSEHUGE is NOT set (it disables libxml2's hardcoded entity recursion limit).

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.

• Symfony Release Notes