Make maximum length of SpEL expressions in an `ApplicationContext` configurable

[bencehornak]

Affects: Spring Framework 5.2.24+

Background

Since #30325 (implemented in b73f5fc) the length of SpEL expressions is limited by default to 10000. As I understand it this is a mitigation against potential ReDoS exploits. However, in some cases this limitation is too low and prevents upgrading to recent Spring Framework versions.

While #30380 (implemented in aefcb9d) adds support for a custom maximumExpressionLength the feature is only accessible if one instantiates the SpelParserConfiguration class themselves.

In my case I would like to configure the SpelParserConfiguration created in the class StandardBeanExpressionResolver to accept my very long property by raising the maximumExpressionLength to a higher value than its default (10000).

spring-framework/spring-context/src/main/java/org/springframework/context/expression/StandardBeanExpressionResolver.java

Lines 91 to 105 in 0709797

	/**
	* Create a new {@code StandardBeanExpressionResolver} with default settings.
	*/
	public StandardBeanExpressionResolver() {
	this.expressionParser = new SpelExpressionParser();
	}
	/**
	* Create a new {@code StandardBeanExpressionResolver} with the given bean class loader,
	* using it as the basis for expression compilation.
	* @param beanClassLoader the factory's bean class loader
	*/
	public StandardBeanExpressionResolver(@Nullable ClassLoader beanClassLoader) {
	this.expressionParser = new SpelExpressionParser(new SpelParserConfiguration(null, beanClassLoader));
	}

Use case

I've got a huge map in my config:

myproperty={\
  a: {\
    x: { host: '10.1.1.1', port: 1234 },\
    y: { host: '10.1.1.1', port: 1234 },\
    z: { host: '10.1.1.1', port: 1234 }\
  },\
  b: {\
    x: { host: '10.1.1.1', port: 1234 },\
    y: { host: '10.1.1.1', port: 1234 },\
    z: { host: '10.1.1.1', port: 1234 }\
  },\
  c: {\
    x: { host: '10.1.1.1', port: 1234 },\
    y: { host: '10.1.1.1', port: 1234 },\
    z: { host: '10.1.1.1', port: 1234 }\
  },\
  # and so on, altogether 15000 characters
}

It is used by a property:

@Value("#{${myproperty}}")
private Map<String, Map<String,Map<String,String>>> myproperty;

If I try to start my application I get the following exception:

org.springframework.expression.spel.SpelEvaluationException: EL1079E: SpEL expression is too long, exceeding the threshold of '10,000' characters"}}

Proposal

Make the parameter maximumExpressionLength of SpelParserConfiguration configurable when it is instantiated in StandardBeanExpressionResolver.java (see the snippet above). Example (not sure what a conformant property name would be):

spring.standardBeanExpressionResolver.maximumExpressionLength=20000

[sbrannen]

I've discussed this with @jhoeller, and we've decided to make the maximum expression length configurable via a System/Spring property named spring.context.expression.maxLength.

The plan is to have StandardBeanExpressionResolver constructors honor this property when present.

[bencehornak]

Awesome, thanks!!