Spel security fix; fixes gh-917

docs/src/main/asciidoc/spring-cloud-sleuth.adoc

@@ -294,7 +294,9 @@ Precedence is:
 
 - try with the bean of `TagValueResolver` type and provided name
 - if one hasn't provided the bean name, try to evaluate an expression. We're searching for a `TagValueExpressionResolver` bean.
-The default implementation uses SPEL expression resolution.
+The default implementation uses SPEL expression resolution. If we do not find any expression to evaluate, return the `toString()`
+value of the parameter.
+**IMPORTANT** You can only reference properties from the SPEL expression. Method execution is not allowed due to security constraints.
 - if one hasn't provided any expression to evaluate just return a `toString()` value of the parameter
 
 ==== Custom extractor
@@ -950,4 +952,4 @@ class ReporterConfiguration {
 You can find the running examples deployed in the https://run.pivotal.io/[Pivotal Web Services]. Check them out in the following links:
 
 - http://docssleuth-zipkin-server.cfapps.io/[Zipkin for apps presented in the samples to the top]
-- http://docsbrewing-zipkin-server.cfapps.io/[Zipkin for Brewery on PWS], its https://github.com/spring-cloud-samples/brewery[Github Code]
+- http://docsbrewing-zipkin-server.cfapps.io/[Zipkin for Brewery on PWS], its https://github.com/spring-cloud-samples/brewery[Github Code]

pom.xml

@@ -13,7 +13,7 @@
 	<parent>
 		<groupId>org.springframework.cloud</groupId>
 		<artifactId>spring-cloud-build</artifactId>
-		<version>1.3.8.RELEASE</version>
+		<version>1.3.9.BUILD-SNAPSHOT</version>
 		<relativePath/>
 		<!-- lookup parent from repository -->
 	</parent>
@@ -241,7 +241,7 @@
 		<maven.compiler.testSource>1.8</maven.compiler.testSource>
 		<surefire.plugin.version>2.19.1</surefire.plugin.version>
 		<checkstyle.version>2.17</checkstyle.version>
-		<spring-cloud-build.version>1.3.8.RELEASE</spring-cloud-build.version>
+		<spring-cloud-build.version>1.3.9.BUILD-SNAPSHOT</spring-cloud-build.version>
 		<spring-cloud-commons.version>1.3.3.BUILD-SNAPSHOT</spring-cloud-commons.version>
 		<spring-cloud-stream.version>Ditmars.BUILD-SNAPSHOT</spring-cloud-stream.version>
 		<spring-cloud-netflix.version>1.4.4.BUILD-SNAPSHOT</spring-cloud-netflix.version>

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/annotation/SpelTagValueExpressionResolver.java

@@ -23,6 +23,7 @@ import org.apache.commons.logging.LogFactory;
 import org.springframework.expression.Expression;
 import org.springframework.expression.ExpressionParser;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
+import org.springframework.expression.spel.support.SimpleEvaluationContext;
 
 /**
  * Uses SPEL to evaluate the expression. If an exception is thrown will return
@@ -37,9 +38,12 @@ class SpelTagValueExpressionResolver implements TagValueExpressionResolver {
 	@Override
 	public String resolve(String expression, Object parameter) {
 		try {
+			SimpleEvaluationContext context = SimpleEvaluationContext
+					.forReadOnlyDataBinding()
+					.build();
 			ExpressionParser expressionParser = new SpelExpressionParser();
 			Expression expressionToEvaluate = expressionParser.parseExpression(expression);
-			return expressionToEvaluate.getValue(parameter, String.class);
+			return expressionToEvaluate.getValue(context, parameter, String.class);
 		} catch (Exception e) {
 			log.error("Exception occurred while tying to evaluate the SPEL expression [" + expression + "]", e);
 		}

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/annotation/SpelTagValueExpressionResolverTests.java

@@ -27,10 +27,16 @@ public class SpelTagValueExpressionResolverTests {
 	@Test
 	public void should_use_spel_to_resolve_a_value() throws Exception {
 		SpelTagValueExpressionResolver resolver = new SpelTagValueExpressionResolver();
+		MyObject myObject = new MyObject();
+		myObject.name = "hello";
 
-		String resolved = resolver.resolve("length() + 1", "foo");
+		String resolved = resolver.resolve("name + ' world'", myObject);
 
-		then(resolved).isEqualTo("4");
+		then(resolved).isEqualTo("hello world");
+	}
+
+	public static class MyObject {
+		public String name;
 	}
 
 	@Test