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 Date disclosed: 10/13/2022

Denial of Service (DoS)

A flaw was found in the snakeyaml package due to a stack-overflow in parsing YAML files. By persuading a victim to open a specially-crafted file, a remote attacker could cause the application to crash.

A flaw was found in the snakeyaml package due to a stack-overflow in parsing YAML files. By persuading a victim to open a specially-crafted file, a remote attacker could cause the application to crash, resulting in a denial of service.

A flaw was found in the snakeyaml package due to a stack-overflow in parsing YAML files. By persuading a victim to open a specially-crafted file, a remote attacker could cause the application to crash.

A flaw was found in the snakeyaml package due to a stack-overflow in parsing YAML files. By persuading a victim to open a specially-crafted file, a remote attacker could cause the application to crash.

Stack Overflow in Snakeyaml

Last updated 28 March 2025