Affected: Maven: org.yaml:snakeyaml

### Summary SnakeYaml's `Constructor` class, which inherits from `SafeConstructor`, allows any type be deserialized given the following line:

new Yaml(new Constructor(TestDataClass.class)).load(yamlContent);

Types do not have to match the types of properties in the target class. A `ConstructorException` is thrown, but only after a malicious payload is deserialized.

### Severity High, lack of type checks during deserialization allows remote code execution.

### Proof of Concept Execute `bash run.sh`. The PoC uses Constructor to deserialize a payload for RCE. RCE is demonstrated by using a payload which performs a http request to http://127.0.0.1:8000.

Example output of successful run of proof of concept:

``` $ bash run.sh

[+] Downloading snakeyaml if needed [+] Starting mock HTTP server on 127.0.0.1:8000 to demonstrate RCE nc: no process found [+] Compiling and running Proof of Concept, which a payload that sends a HTTP request to mock web server. [+] An exception is expected. Exception: Cannot create property=payload for JavaBean=Main$TestDataClass@3cbbc1e0 in 'string', line 1, column 1: payload: !!javax.script.ScriptEn ... ^ Can not set java.lang.String field Main$TestDataClass.payload to javax.script.ScriptEngineManager in 'string', line 1, column 10: payload: !!javax.script.ScriptEngineManag ... ^

at org.yaml.snakeyaml.constructor.Constructor$ConstructMapping.constructJavaBean2ndStep(Constructor.java:291) at org.yaml.snakeyaml.constru…