PostgreSQL XML Helpers

Context

This article delves into exploiting PostgreSQL's XML helpers for conducting error-based SQL injection attacks. The purpose is to educate on leveraging XML functions within PostgreSQL to extract sensitive information through error messages. The assumed knowledge requires familiarity with XML structures, SQL query execution, database error handling, and prior exposure to PostgreSQL error-based SQL injection techniques.

Theory

XML Parsing in PostgreSQL

PostgreSQL includes robust support for XML data types and functions, allowing databases to store and manipulate XML data effectively. These XML functions can be utilized to extract information from XML documents directly within SQL queries. However, if these functions are improperly handled, they may introduce SQL injection vulnerabilities, particularly within error-based attacks.

Error-Based SQL Injection Techniques

Error-based SQL injection is a method of injecting SQL queries that cause the database to return errors. These errors often reveal critical insights into the database structure or contents. The fundamental principle is that an attacker can craft payloads leading to errors, which are then analyzed to gain unauthorized insights into the database.

Exploiting XML Helpers for Information Disclosure

XML functions in PostgreSQL can be manipulated to trigger XML parsing errors deliberately. These errors, in turn, may disclose sensitive information inadvertently if not adequately sanitized. By inducing errors in XML parsing, attackers can expose vulnerabilities and retrieve protected information.

Understanding PostgreSQL Error Messages

PostgreSQL, like many databases, generates error messages when there are issues executing a SQL query. These error messages can be detailed and provide feedback, which attackers can leverage to extract information. Understanding the nature of these errors is critical for utilizing them effectively in an attack vector to glean sensitive data.

Practice

Exploiting XML Functions for Error-Based SQL Injection

To exploit PostgreSQL's XML functions for error-based SQL injection, consider the following techniques:

• Trigger XML Parsing Error: Use the xmlparse function with an external entity that PostgreSQL cannot resolve, leading to an error that might disclose file contents or structure.

  SELECT xmlparse(document '<!DOCTYPE root [<!ENTITY xxe SYSTEM "file:///etc/passwd">]><root>&xxe;</root>');
  

  This command attempts to parse malicious XML, potentially triggering an error that reveals system file contents.

• Use xmlexists for Logical Errors: This function checks whether a specific XML element exists, and a crafted query could cause logical errors that disclose content related to a non-existent element.

  SELECT xmlexists('//user[text()="admin"]' PASSING BY REF '<users><user>admin</user></users>');
  

  It searches for a ‘user’ element with the text ‘admin’ and if misconfigured, may trigger an error revealing internal database logic or content.

• Leverage xpath for Extraction Errors: The xpath function is used to extract data from XML, and by feeding it malformed documents, you can cause errors that might leak sensitive structural information.

  SELECT xpath('//user', xmlparse(document '<users><user>admin</user></users>'));
  

  This attempts to extract parsing information and can generate errors if the document is manipulated, thus revealing critical information.

Each technique aims to extract sensitive data from error messages that PostgreSQL might return improperly. Targeting these error messages can grant insight into protected areas of a database that an attacker should not normally access.

Tools

• psql
• sqlmap

These tools are essential for executing PostgreSQL commands and automating SQL injection techniques efficiently.