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https://issues.apache.org/jira/browse/LOG4J2-1226?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15066533#comment-15066533
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Remko Popma commented on LOG4J2-1226:
-------------------------------------

Ah. I spoke too soon. ObjectArrayMessage and ObjectMessage have custom 
serialization logic.

Hmm. 

I see your point, but it is not clear that converting everything to strings is 
the answer. An argument could be made that it is the responsibility of whoever 
set up their system to pipe log events via the network to a remote receiver, to 
ensure that the receiver has the required classes in the classpath.

There may be scenarios where people _want_ to receive the object itself and not 
a String representation.

> Message instances are simply serialized. They mustn't.
> ------------------------------------------------------
>
>                 Key: LOG4J2-1226
>                 URL: https://issues.apache.org/jira/browse/LOG4J2-1226
>             Project: Log4j 2
>          Issue Type: Bug
>          Components: API
>    Affects Versions: 2.5
>            Reporter: Joern Huxhorn
>
> Right now, any Message instance used to call any log method are simply sent 
> as they are.
> Instead, the {{Throwable}} must be transformed into a {{ThrowableProxy}}. 
> Custom {{Message}} implementations must be transformed into one of log4j's 
> standard message implementations and care must be taken to convert the 
> {{Parameters}} {{Object[]}} into {{String[]}} before the message is 
> serialized.
> Otherwise, deserialization will fail if a custom {{Throwable}}, custom 
> {{Message}} or custom parameter is not contained in the classpath of the 
> application receiving the serialized {{LogEvent}}.
> I found those issues while implementing the circumvention for [Apache Commons 
> statement to widespread Java object de-serialisation 
> vulnerability|https://blogs.apache.org/foundation/entry/apache_commons_statement_to_widespread]
>  in [Lilith|http://lilithapp.com].



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