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Re: easy DoS in most RPC apps
From: okir () MONAD SWB DE (Olaf Kirch)
Date: Mon, 18 May 1998 17:45:07 +0200


--------
On Sun, 17 May 1998 15:48:55 EDT, Bill Paul wrote:
With these patches, you have 35 seconds to supply a valid record
containing an RPC message header and request, otherwise the session
is disconnected. If you enter garbage data, the connection is dropped
immediately.

Sun's RPC code has some more problems. If you send it a continuous
stream of zero bytes, it will loop forever because it interprets them
as a sequence of zero-length record fragments. It nicely gobbles the
empty record, notices that this hasn't been the last fragment (EOR bit
is 0 of course) and goes asking for more, etc ad inf.

Concerning the 35 second timeout Bill mentions above, this can also be
stretched out quite a bit if you transmit the RPC packet byte by byte,
each 30 seconds apart.

Given the way RPC was designed, I cannot think how to work around this
problem except by handling all RPC requests in a separate thread.

Finally, here's some stuff that I haven't checked so far, but which may
be equally interesting. The RPC code is cluttered with conversions
from unsigned long to int, and I have found at least one (quite important)
routine in the RPC server code that does something like this:

        int             len;

        get len from user request
        if (len > MAX_LEN)
                return FALSE;
        bcopy(buf, destination, (u_int) len);

where destination is on the stack...

Cheers
Olaf
--
Olaf Kirch         |  --- o --- Nous sommes du soleil we love when we play
okir () monad swb de  |    / | \   sol.dhoop.naytheet.ah kin.ir.samse.qurax
             For my PGP public key, finger okir () brewhq swb de 



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