SecurityError on Method#call? (might be DRb specific)

Moin.

I've recently having trouble with these kind of errors:

> SecurityError in  Member#list
>  
> Insecure operation `write' at level 4

I'm basically using .call on a DRb proxied method to invoke it. $SAFE is 
0 in both the server and the client script. The proxy object isn't 
tainted. (I'm untainting it and doing a .tainted?() check that always 
returns false even before I untaint it.) The server is running in a CGI 
environment, but I think that's not relevant.

The method comes from a Module defined in the source code in the usual way:

module Handlers
   extend self

   def breakpoint_handler(workspace, message)
     ...
   end

   def eval_handler(code)
     ...
   end

   ...
end

And I'm setting them like this:

service.eval_handler = Handlers.method(:eval_handler)

So what I'm asking is: Is there any other reason why a method would 
suddenly be executed in a $SAFE == 4 context or a way a method could get 
tainted when running with $SAFE == 0 at all?

I initially thought that this would not happen at all unless you 
manually set $SAFE, but it appears that this happens for tainted methods 
even in $SAFE == 0.

I can not reproduce the problem locally, but some users of the 
breakpoint library where this trouble can reproduce it reliably, so I'll 
be able to pass along further questions.

Thanks a lot for any information that might help with this problem. I'm 
unfortunately running out of ideas here...

Regards,
Florian Gross