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asynchat.py: class async_chat: handle_read():
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 elif isinstance(terminator, int) or isinstance(terminator, long):
 # numeric terminator
 n = terminator
 if lb < n:
 self.collect_incoming_data (self.ac_in_buffer)
 self.ac_in_buffer = ''
 self.terminator = self.terminator - lb
 else:
 self.collect_incoming_data (self.ac_in_buffer[:n])
 self.ac_in_buffer = self.ac_in_buffer[n:]
 self.terminator = 0
 self.found_terminator()
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suppose, terminator is -10. "if lb < n" never match. So, "else" branch executed.
next, it will call "self.collect_incoming_data (self.ac_in_buffer[:n])", to push data to user. It should push some data from beginning of the buffer, intead of this, total buffer except last 10 characters pushed.
Moreover, "self.ac_in_buffer = self.ac_in_buffer[n:]" shoudl give tail of the buffer, ut instead of this, "self.ac_in_buffer" will contain part of the tail.
Such behaviour may break protocol parsing. In my case, malicious user pass 'Content-Length: -100' and totally break protocol parsing. Crafted values may gain memory leak.
In any way, author of this code does not thought about negative n in constructions [:n] or [n:].