Detailed attack pattern

Draft

CAPEC-491: Quadratic Data Expansion

Also known as: XML Entity Expansion (XEE)

An adversary exploits macro-like substitution to cause a denial of service situation due to excessive memory being allocated to fully expand the data. The result of this denial of service could cause the application to freeze or crash. This involves defining a very large entity and using it multiple times in a single entity substitution. CAPEC-197 is a similar attack pattern, but it is easier to discover and defend against. This attack pattern does not perform multi-level substitution and therefore does not obviously appear to consume extensive resources.

Last updated June 1, 2026

Not rated

Typical severity

Per MITRE

Not rated

Likelihood of attack

Per MITRE

1

Related weaknesses

CWEs this targets

Detailed

Abstraction

MITRE classification

Overview

CAPEC-491 (Quadratic Data Expansion) is a detailed-level attack pattern catalogued by MITRE in the Common Attack Pattern Enumeration and Classification (CAPEC). It describes a recurring method attackers use to exploit software weaknesses.

How the attack works

The phases an attacker typically follows to carry out this attack.

Step 1
Explore
[Survey the target] An adversary determines the input data stream that is being processed by a data parser that supports using substituion on the victim's side.
- Use an automated tool to record all instances of URLs to process requests.
- Use a browser to manually explore the website and analyze how the application processes requests.
Step 2
Exploit
[Craft malicious payload] The adversary crafts malicious message containing nested quadratic expansion that completely uses up available server resource.
Step 3
Exploit
[Send the message] Send the malicious crafted message to the target URL.

What the attacker needs

Prerequisites

This type of attack requires a server that accepts serialization data which supports substitution and parses the data.

Consequences

What a successful CAPEC-491 attack can achieve.

Unreliable Execution, Resource Consumption

Affects: Availability

Denial of Service

How to mitigate it

Defenses that reduce the risk of CAPEC-491.

Design: Use libraries and templates that minimize unfiltered input. Use methods that limit entity expansion and throw exceptions on attempted entity expansion.
Implementation: For XML based data - disable altogether the use of inline DTD schemas when parsing XML objects. If a DTD must be used, normalize, filter and use an allowlist and parse with methods and routines that will detect entity expansion from untrusted sources.

Examples

In this example the attacker defines one large entity and refers to it many times. ]> &x;&x; ... [100K of them]... &x;&x; This results in a relatively small message of 100KBs that will expand to a message in the GB range.

Terminology & mappings

Alternate terms

XML Entity Expansion (XEE)

Frequently asked questions

Common questions about CAPEC-491.

What is CAPEC-491?

An adversary exploits macro-like substitution to cause a denial of service situation due to excessive memory being allocated to fully expand the data. The result of this denial of service could cause the application to freeze or crash. This involves defining a very large entity and using it multiple times in a single entity substitution. CAPEC-197 is a similar attack pattern, but it is easier to discover and defend against. This attack pattern does not perform multi-level substitution and therefore does not obviously appear to consume extensive resources.

How does a Quadratic Data Expansion attack work?

It typically unfolds over 3 phases. It begins with: [Survey the target] An adversary determines the input data stream that is being processed by a data parser that supports using substituion on the victim's side.

How do you prevent CAPEC-491?

Design: Use libraries and templates that minimize unfiltered input. Use methods that limit entity expansion and throw exceptions on attempted entity expansion.

What weaknesses does CAPEC-491 target?

CAPEC-491 exploits 1 CWE weakness, including CWE-770 (Allocation of Resources Without Limits or Throttling).

References

Attack-pattern data is sourced from the MITRE CAPEC catalog (v3.9). Weakness associations link to the corresponding CWE entries on RadicalNotion.AI.

Defend against CAPEC-491

Track the CVEs and weaknesses attackers exploit with this technique, with AI-written analysis and remediation guidance.

Start free See pricing

Overview

Overview

How the attack works

What the attacker needs

Prerequisites

Consequences

How to mitigate it

Examples

Related weaknesses

Related attack patterns

Parent

Related

Terminology & mappings

Alternate terms

Frequently asked questions

What is CAPEC-491?

How does a Quadratic Data Expansion attack work?

How do you prevent CAPEC-491?

What weaknesses does CAPEC-491 target?

References

Defend against CAPEC-491