XML API DoS Attacks

Extensible Markup Language (XML) serves as a foundational technology, critical for the operation of countless modern web applications and Application Programming Interfaces (APIs). Its significant value lies in its inherent flexibility for structuring, storing, and transporting data across diverse systems, making it a cornerstone for data exchange. However, this very versatility, while a key strength, can also be exploited by malicious actors. These individuals can leverage XML’s capabilities to launch sophisticated Denial of Service (DoS) attacks. Such attacks are meticulously designed to overwhelm an application’s essential resources, including processing power, memory, and network bandwidth. The primary goal is to render the service entirely unavailable to legitimate users, thereby disrupting operations and potentially causing significant financial or reputational damage.

An XML API DoS attack is often asymmetric, meaning an attacker can trigger a massive resource drain on a server with a very small, seemingly harmless request. This guide will explain what XML API DoS attacks are, how they are executed, their potential business impact, and most importantly, how to defend against them.

When combined with a botnet, these DoS techniques can escalate into a highly effective application-layer attack. A botnet, comprising a network of compromised devices, can coordinate and amplify the assault, directing immense amounts of traffic toward the targeted application. This overwhelms not just the underlying infrastructure but also the application itself, exploiting its functionalities to exhaust resources further. Such a coordinated attack can bypass traditional network-layer defenses, making it exceedingly difficult to detect and mitigate. The attackers’ ability to distribute the attack across numerous devices significantly increases its scale and impact, posing a serious and complex threat to application security.

What Are XML API DoS Attacks?

Understanding XML APIs

XML APIs, or Extensible Markup Language Application Programming Interfaces, serve as a standardized mechanism for exchanging structured data between systems or applications. Widely used in enterprise environments, XML APIs utilize XML as their data format, enabling seamless communication across disparate platforms and technologies. These APIs ensure compatibility by defining clear protocols for data formatting, transmission, and interpretation, allowing systems to interact even with varying programming languages or architectures. Due to their flexibility and extensive adoption, XML APIs are integral for integrating applications, automating workflows, and streamlining complex processes. However, the complexity of XML-based interactions can also introduce vulnerabilities, making robust security measures essential to mitigate potential risks.

XML API Denial of Service (DoS) attacks are a category of cyberattacks that exploit vulnerabilities in how applications parse XML data. The goal is to make a service or system unavailable by exhausting its resources, such as memory (RAM) and processing power (CPU).

When an application receives XML data, it uses a component called an XML parser to read and process it. Attackers craft malicious XML payloads that, while often small and well-formed, cause the parser to enter a resource-intensive loop. The result is a system crash or freeze, effectively denying service to other users. These vulnerabilities can exist even in well-tested, standard XML processing libraries if they are not configured securely.

How Do XML API DoS Attacks Happen?

XML API DoS attacks leverage specific features of the XML standard, particularly Document Type Definitions (DTDs). A DTD defines the structure and legal elements of an XML document and allows for the use of “entities,” which are essentially variables or shortcuts. Attackers exploit this entity expansion mechanism to amplify a small request into a massive processing load.

The core principle is simple: a small request leads to disproportionately large resource consumption. An attacker sends a compact XML file that, once parsed, expands exponentially or quadratically, consuming gigabytes of memory and significant CPU time. Let’s explore the common methods.

Exponential Entity Expansion (Billion Laughs Attack)

The most well-known XML DoS attack is the “Billion Laughs” attack. It uses nested entities to create an exponential expansion.

Here’s a breakdown of how it works:

1. An initial entity (e.g., lol) is defined as a short string.
2. A second entity (e.g., lol1) is defined as ten instances of the first entity.
3. A third entity (e.g., lol2) is defined as ten instances of the second entity, and so on.
4. This nesting continues, with each level multiplying the previous one by ten.
5. Finally, the XML document references the highest-level entity just once.

When the parser attempts to resolve this single reference, it triggers a chain reaction. The final expansion can result in billions of strings (“laughs”), consuming all available memory and crashing the application.

Here is a common code example of a Billion Laughs attack payload:

<?xml version=”1.0″?>
<!DOCTYPE lolz [
<!ENTITY lol “lol”>
<!ENTITY lol1 “&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;”>
<!ENTITY lol2 “&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;”>
<!ENTITY lol3 “&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;”>
<!ENTITY lol4 “&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;”>
<!ENTITY lol5 “&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;”>
<!ENTITY lol6 “&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;”>
<!ENTITY lol7 “&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;”>
<!ENTITY lol8 “&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;”>
<!ENTITY lol9 “&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;”>
]>
<lolz>&lol9;</lolz>

Quadratic Blowup Attack

A quadratic blowup attack is a variation of XML entity expansion attacks that operates differently from the well-known Billion Laughs attack. Rather than relying on deeply nested entities to overwhelm the parser, this method defines a single, extremely large entity and then references it repeatedly throughout the document. While this approach is less efficient compared to the exponential growth caused by the Billion Laughs attack, it can be equally damaging in certain scenarios.

What makes the quadratic blowup attack notable is its ability to bypass simple countermeasures designed to detect and block excessively nested entities, which are the hallmark of the Billion Laughs attack. By exploiting a repeated large entity instead, it evades these basic detection mechanisms while still achieving its goal. When a parser attempts to process the repeated references to the oversized entity, it consumes a vast amount of memory, overwhelming the system. The end result is the same: the parser becomes overloaded and ultimately leads to a denial of service. This demonstrates the need for robust and comprehensive safeguards against various forms of XML entity expansion abuse.

Other Attack Vectors

• External Entity Attacks (XXE): In this type of attack, malicious actors define entities within the XML document that reference external resources using a Uniform Resource Identifier (URI). If the targeted external resource is exceptionally large or is configured never to return a response, the XML parser can become trapped in an infinite wait loop. This prolonged state of waiting consumes a significant amount of system resources, ultimately leading to resource exhaustion and a denial of service.
• Coercive Parsing: This attack vector involves sending XML documents with deeply nested elements but intentionally omitting their corresponding closing tags. As the parser attempts to process this malformed structure, it allocates more and more stack memory to keep track of all the open elements. This continuous allocation eventually exhausts the available stack memory, leading to a stack overflow and causing the application to crash. A notable example of this was a 2010 attack on the Firefox browser, which utilized 30,000 unclosed XML tags to successfully trigger the vulnerability.
• Decompression Bombs: This technique, also known as a “zip bomb,” targets applications that accept compressed XML streams, such as gzipped files. An attacker can craft and send a very small compressed file that, upon decompression by the parser, expands to an enormous size. This sudden and massive expansion of data quickly consumes all available server memory, leading to system overload and a denial of service.

Examples of XML API DoS Attacks in the Wild

While specific high-profile breaches directly attributed solely to XML DoS attacks are not always publicly detailed—often due to the complexity of attribution or companies’ reluctance to disclose such vulnerabilities—the underlying weaknesses are extensively documented. These vulnerabilities have consistently impacted and required remediation in major software platforms across various industries.

For instance, early implementations of the Simple Object Access Protocol (SOAP), which served as a widely adopted protocol for web services, were identified as vulnerable to such attacks. Although the SOAP specification explicitly prohibits the use of Document Type Definitions (DTDs), some processors failed to adhere strictly to this rule, inadvertently parsing them. This oversight created an entry point for attacks like the “Billion Laughs,” where a small, specially crafted XML entity could recursively expand to an unmanageable size, consuming system resources and causing a denial of service.

The “Attribute Blowup” attack represented another significant variant that capitalized on inefficient parsing mechanisms. This method exploited the way parsers handled an excessively large number of attributes within a single XML element, leading to a disproportionate consumption of processing power and memory. This particular vulnerability necessitated a fix in later versions of significant platforms, such as Microsoft’s .NET Framework (2.0 and subsequent releases), clearly demonstrating that even foundational software frameworks were not immune to these types of architectural flaws.

Moreover, the coercive parsing attack identified in Mozilla Firefox stands as a distinct, real-world illustration. This attack demonstrated how a malformed XML structure could be leveraged to force an application into a state of unresponsiveness or outright crash, directly resulting in a denial of service for the end-user attempting to interact with the software. These examples collectively underscore the pervasive nature and tangible impact of XML DoS vulnerabilities across diverse software environments.

How XML API DoS Attacks Impact Your Business

A successful XML API DoS attack can have severe consequences for a business. The most immediate and obvious impact is service unavailability.

• Service Disruption: When an API or application becomes unavailable due to an attack, legitimate customers, partners, and internal users are unable to access necessary services. This disruption can lead to direct lost revenue, frustrated users who may take their business elsewhere, and significant damage to your brand’s reputation as an unreliable service.
• Resource Depletion: These attacks are designed to consume substantial CPU and memory resources on your servers. In modern cloud environments where infrastructure scales automatically based on demand, this can trigger the provisioning of additional resources to handle the manufactured load, resulting in unexpectedly high operational bills without any corresponding increase in legitimate business activity.
• Compromised Availability and Integrity: Certain XML-based attacks, particularly those leveraging XML External Entity (XXE) vulnerabilities, can extend beyond simple service disruption. Attackers can use these techniques to exfiltrate sensitive data, perform server-side request forgery (SSRF) to attack other systems from your server, or conduct port scanning of your internal network to map out further targets. This escalates the incident from an availability issue to a severe breach of data confidentiality and system integrity.
• Financial Impact: The exploitation of XML-based vulnerabilities can lead to significant financial losses. Costs may include incident response efforts, regulatory fines for non-compliance with data protection laws, potential lawsuits from affected parties, and the loss of revenue due to downtime or service disruptions. These expenses can escalate quickly, particularly for organizations handling sensitive or highly regulated data.
• Damage to Business Reputation: A successful attack exploiting XML-based vulnerabilities can severely harm an organization’s reputation. Customers and stakeholders may lose trust in the organization’s ability to safeguard their data. Negative publicity and loss of credibility can result in reduced customer retention, challenges in acquiring new business, and long-term repercussions for the brand’s standing in the market.

Ultimately, the unavailability of critical APIs can halt business operations, impact customer trust, and result in direct financial losses.

Preventing XML API DoS Attacks

Protecting your applications from XML API DoS attacks requires a multi-layered approach focused on securely configuring your XML parsers and validating incoming data.

Use a Web Application Firewall

A Web Application Firewall (WAF) is an essential tool for protecting your system from malicious XML-based attacks. Configure the WAF to inspect and filter incoming XML traffic for known attack patterns, such as XML External Entity (XXE) attacks or oversized payloads designed to overwhelm your application. By leveraging a WAF, you add an additional layer of security that mitigates risks before the data even reaches your application backend. Additionally, keep the WAF regularly updated to ensure it can detect the latest threat vectors and provide comprehensive protection.

Disable DTD Processing

The most effective defense against entity expansion attacks like Billion Laughs is to disable DTD processing altogether. Since most modern applications use XSD (XML Schema Definition) for validation, DTDs are often not required for legitimate functionality. If your application does not need DTDs, turn them off in your XML parser configuration.

Limit Entity Expansion and Resource Allocation

If disabling DTDs is not an option, you must configure your parser to limit the resources it can consume.

• Limit Entity Expansion: Many modern XML parsers allow you to set a maximum number of character expansions from entities. For example, in .NET, the MaxCharactersFromEntities property in XmlReaderSettings can be set to a reasonable limit to thwart both exponential and quadratic blowup attacks.
• Limit Element Name Length and Attributes: Configure your parser to reject requests with excessively long element names or an unusually high number of attributes per element.

Disable External Entity Resolution

To prevent external entity attacks, configure your XML parser to not resolve external entities. This can often be done by setting the XmlResolver property to null or by using a custom resolver that restricts access to external resources, especially those on the local host.

Implement Request Timeouts

Set strict timeouts for requests. This can prevent attacks where the parser is made to wait indefinitely for a response from an external resource, which would otherwise tie up server threads and resources.

Validate Incoming XML

Always validate incoming XML requests.

• Ensure Well-Formed XML: The initial step in robust XML validation involves verifying that all incoming requests are syntactically correct and adhere to the fundamental rules of XML structure. Any request that is not a well-formed XML document should be rejected immediately, typically with a “400 Bad Request” HTTP status code, to prevent processing malformed or potentially malicious data.
• Validate Against a Schema: Beyond basic well-formedness, it is crucial to validate the structural integrity and content of incoming XML data against a predefined XML Schema Definition (XSD). This ensures that the XML not only conforms to basic syntax but also matches your expected data model and business logic. Validation against an XSD helps in identifying and rejecting documents that, while syntactically correct, do not align with the application’s required structure and data types.
• Implement XML Content Restrictions: To prevent resource exhaustion and ensure stable processing, establish and enforce strict limits on various aspects of XML content. Key restrictions include:
• • • Maximum Element Depth: Define a maximum permissible nesting level for XML elements. This safeguard prevents excessively deep structures, which can lead to stack overflow errors during parsing and hinder application stability.
    • Element and Attribute Limits: Implement controls on the total number of elements within a document, the maximum number of child elements allowed per parent element, and the maximum number of attributes permitted per element. These limits prevent the construction of overly complex or resource-intensive XML documents.
    • Name and Value Lengths: Enforce reasonable maximum lengths for element names, attribute names, and attribute values. This measure helps to prevent the use of verbose or intentionally inflated inputs that could consume excessive memory or processing time.
    • Character Data Length: Limit the maximum length of character data contained within each XML element. This restriction prevents large text blocks from consuming disproportionate resources.
    • File Size Constraints: Set both maximum and minimum allowable file sizes for incoming XML requests. This ensures that only appropriately sized payloads are processed, preventing both excessively large “XML bombs” and unusually small, potentially malformed requests.
    • Entity Expansion Control: To counter various XML bomb attacks that exploit entity expansion, limit the total number of entity expansions and define a maximum depth for nested entity expansions. This control prevents the parser from consuming excessive resources by recursively expanding entities.
    • Namespace Constraints: Restrict the number of namespace declarations within an XML document and impose limits on the length of their Uniform Resource Identifiers (URIs). This helps manage complexity and prevent resource abuse related to namespace processing.
• Block Malicious Constructs: Proactively prevent known attack vectors by configuring your XML parser to reject specific dangerous constructs:
• • • Blocking Processing Instructions: Reject any special processing instructions (e.g., <?php … ?>) that could be used to embed executable code or alter server behavior.
    • Disabling DTDs: Explicitly disable Document Type Definitions (DTDs) entirely. DTDs are a common vector for external entity attacks (XXE) and XML bombs, and disabling them significantly enhances security.
    • Blocking External Entities: Ensure that your parser explicitly prevents all references to external entities. This includes both external general entities and external parameter entities, which are frequently exploited to access local files or initiate network requests.
• Configure SOAP Array Checks: For applications utilizing SOAP, implement specific validation checks tailored to SOAP array structures:
• • • Maximum SOAP Array Size: Set a total size limit for all SOAP arrays contained within a single request. This prevents large array payloads from overwhelming server resources.
    • Maximum SOAP Array Rank: Define the maximum number of dimensions (rank) allowed for any single SOAP array. This prevents the creation of extremely high-dimensional arrays that could lead to complex and resource-intensive processing.

Fortify Your Defenses

XML remains a powerful tool for data interchange, but its inherent complexities can be turned against you. XML API DoS attacks represent a significant threat to the availability and stability of any application that processes XML data.

By understanding the mechanics of these attacks—from exponential entity expansion to coercive parsing—you can take proactive steps to secure your systems. Disabling DTDs, limiting resource allocation, and validating all incoming data are not just best practices; they are essential security measures. By implementing these defenses, you can protect your business from disruptions and ensure that your services remain resilient and available to your users.

How DigiCert Can Help

DigiCert UltraWAF is a robust web application firewall designed to safeguard modern applications against a wide range of cyber threats, including XML API denial-of-service (DoS) attacks and other application-layer vulnerabilities. Leveraging advanced threat detection mechanisms, DigiCert UltraWAF identifies malicious patterns such as excessive resource consumption, malformed requests, and suspicious payloads—hallmarks of XML-based exploits. By analyzing application traffic in real time, it can effectively block attacks before they disrupt operations or compromise data integrity. With features such as customizable policies, automated updates to address emerging threats, and seamless integration into existing infrastructures, DigiCert UltraWAF provides a comprehensive solution for maintaining the security, availability, and performance of your critical services.

For more information about how DigiCert UltraWAF can protect your organization from emerging threats and ensure the reliability of your critical services, contact us today. Our team of security experts is ready to assist you in implementing a solution tailored to your unique needs.