How to Identify and Mitigate XXE Vulnerabilities?

identify and mitigate xxe vulnerabilities

XXE Vulnerability: An Introduction

  • Perform Server-Side Request Forgery (SSRF) and compromise the underlying server/ other backend infrastructure.
  • Orchestrate Denial of Service (DoS) and cause downtime/ application crashes.
  • Engage in data corruption/ theft, malicious code execution, network attacks, and so on.

How Do XML External Entity Injection Attacks Work?

  • One of the main advantages of using XML is that it is extensible and the storage of all kinds of data is accepted. But its risks and vulnerabilities are exacerbated by this very advantage.
  • New entities and loosely defined in DOCTYPE declaration with a wide variety of values are accepted by XML.
  • These entities need not be defined in the document itself; these can be loaded from external sources (local files, URI, etc.).
  • The loosely defined external entities are often aimed at accessing remote/ local data via declared system identifiers.

So, how is an XXE attack orchestrated?

  • XXE vulnerability occurs when the potentially unsafe features are contained in XML specification and supported by the XML parsers.
  • Applications with these vulnerabilities are identified by attackers who send XML requests containing malicious payload within the Document Type Declaration (DTD).
  • The malicious external entities within the DTD are retrieved, validated, and resolved by the weak XML parser.
  • The attacker is, thus, enabled to gain access to confidential data/ resources and fulfill their motives.

How to Identify XML External Entity Vulnerabilities?

  • File retrieval: External entity, defined based on a well-known OS file, is used in the data obtained from the application’s response.
  • Blind XXE vulnerabilities: External entity is defined based on a URL to a system controlled by the tester/ developer and the interaction is monitored thereon.
  • XInclude attacks: Well-known OS file is attempted to be retrieved by testers leveraging XInclude attacks. Here, it is tested if user-supplied non-XML data is used within a server-side XML by the application.

How to Mitigate XXE Vulnerabilities?

  • In most cases, XXE attacks can easily be prevented by disabling features making the XML processor weak and the application vulnerable. By analyzing the XML parsing library of the application, features that can be misused can be identified and disabled.
  • DTD and XML external entity features must be disabled.
  • All XML processors and libraries used in the application must be patched and updated always.
  • Ensure that the user inputs are validated before being parsed.
  • File uploads, server-side user inputs, and URLs must be sanitized, validated, and whitelisted.
  • Web application security must be strengthened by onboarding a comprehensive, managed, and intelligent security solution like AppTrana.

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