XML External Entity (XXE) Injection Affecting com.itextpdf:layout package, versions [,7.1.16)

Affected versions of this package are vulnerable to XML External Entity (XXE) Injection in the createParser method of PatternParser, which sets only http://apache.org/xml/features/nonvalidating/load-external-dtd to false and leaves disallow-doctype-decl, external-general-entities, external-parameter-entities, XMLConstants.FEATURE_SECURE_PROCESSING, and setXIncludeAware(false) unset. An attacker can read files readable by the JVM, reach internal network services including cloud metadata endpoints, and exhaust the heap through nested entity expansion by supplying a hyphenation pattern document whose internal DTD subset declares the entities, since blocking the external DTD fetch leaves an inline subset fully resolvable. This requires the application to accept a hyphenation pattern document or directory it does not control, through Hyphenator.getHyphenationTree, Hyphenator.setHyphenationDirectory, or HyphenationTree.loadPatterns, and whether file contents become visible depends on how the parsed data reaches output, logs, or error messages, though the expansion and out of band parameter entity paths need no such reflection.

Note: Version 7.1.13 installs an EntityResolver that returns an empty source, which neutralizes the file read, SSRF, and out of band loading paths that depend on resolving external entities, while leaving entity expansion intact. Version 7.1.16 replaces the reader with XmlProcessorCreator.createSafeXMLReader, which sets disallow-doctype-decl, both external entity features, load-external-dtd, and setXIncludeAware(false), closing the remainder including the expansion path.

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>
<?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>
<?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.

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