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Advisory 14/2004: Linux 2.x smbfs multiple remote vulnerabilities
From: Stefan Esser <s.esser () e-matters de>
Date: Wed, 17 Nov 2004 18:40:59 +0100

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                           e-matters GmbH
                          www.e-matters.de

                      -= Security  Advisory =-



     Advisory: Linux 2.x smbfs multiple remote vulnerabilities
 Release Date: 2004/11/17
Last Modified: 2004/11/17
       Author: Stefan Esser [s.esser () e-matters de]

  Application: Linux 2.4 <= 2.4.27
               Linux 2.6 <= 2.6.9
     Severity: Several vulnerabilities within smbfs allow
               crashing the kernel or leaking kernel memory
               with the help of the smb server
         Risk: Moderately Critical
Vendor Status: Vendor has released a bugfixed version.
    Reference: http://security.e-matters.de/advisories/142004.html


Overview:

   Linux is a clone of the operating system Unix, written from scratch 
   by Linus Torvalds with assistance from a loosely-knit team of hackers
   across the Net. It aims towards POSIX and Single UNIX Specification 
   compliance.

   During an audit of the smb filesystem implementation within Linux
   several vulnerabilities were discovered ranging from out of bounds 
   read accesses to kernel level buffer overflows.

   To exploit any of these vulnerabilities an attacker needs control
   over the answers of the connected smb server. This could be achieved
   by man in the middle attacks or by taking over the smb server with
   f.e. the recently disclosed vulnerability in Samba 3.x
   
   While any of these vulnerabilities can be easily used as remote
   denial of service exploits against Linux systems, it is unclear if 
   it is possible for a skilled local or remote attacker to use any of 
   the possible bufferoverflows for arbitrary code execution in kernel 
   space.


Details:

   [ 01 - smb_proc_read(X) malicious data count overflow ]
   
   Affected Kernels: 2.4
   
   When receiving the answer to a read(X) request the Linux 2.4 kernel
   trusts the returned data count and copies exactly that amound of
   bytes into the output buffer. This means any call to the read
   syscall on a smb filesystem could result in an overflow withing
   kernel memory if the connected smb server returns more data than
   requested. While this is a trivial to exploit DOS vulnerability
   it is unclear if it can be used by a skilled attacker to execute
   arbitrary code.
   
   [ 02 - smb_proc_readX malicious data offset information leak ]
   
   Affected Kernels: 2.4
   
   When receiving the answer to a readX request the Linux 2.4 kernel
   does not properly bounds check the supplied data offset. The check
   in place can fail because of a signedness issue. This means that
   a local attacker can leak kernel memory simply by issuing the read
   syscall on a smb filesystem when the connected server returns a
   data offset from outside the packet. This can of course also lead
   to a kernel crash when unallocated memory is accessed.
   
   [ 03 - smb_receive_trans2 defragmentation overflow ]
   
   Affected Kernels: 2.4
   
   At the end of the TRANS2 defragmentation process the complete
   packet is moved to another place if a certain condition is true.
   In combination with [07] and the fact that the counters are not
   bounds checked befory coyping the data this can result in a 
   kernel memory overflow.
   
   [ 04 - smb_proc_readX_data malicious data offset DOS ]
   
   Affected Kernels: 2.6
   
   The server supplied data offset is decremented by the header size
   and then used as offset within the packet. While the supplied
   offset is checked against an upper bound it may have underflowed
   and therefore point outside the allocated memory. Any access to
   that memory could result in a crash.
   
   [ 05 - smb_receive_trans2 malicious parm/data offset info leak/DOS ]
   
   Affected Kernels: 2.4, 2.6
   
   Both versions of the kernel do not properly bounds check the 
   server supplied packet based offset of the parameters/data sent.
   This results in smbfs copying data from memory outside the received
   smb fragment into the receiving buffer. This can leak kernel memory
   to the calling function or result in a DOS because of accesses to
   unallocated memory.
   
   [ 06 - smb_recv_trans2 missing fragment information leak ]
   
   Affected Kernels: 2.4, 2.6
   
   The defragmentation process of TRANS2 SMB packets does not properly
   initialize the receiving buffer. An attacker may f.e. send several
   thousand times the first byte of a packet until the received data
   count reaches the expected total and so leakes the rest of the
   uninitialised receiving buffer to the calling function.

   [ 07 - smb_recv_trans2 fragment resending leads to invalid counters ]
   
   Affected Kernels: 2.4, 2.6
   
   The defragmentation termination condition is that atleast the
   expected parameter count and at least the expected data count is 
   reached. By using the fragment resending technique an attacker
   can increase one of those counters to an arbitrary high value.
   

Proof of Concept:

   e-matters is not going to release exploits for any of these
   vulnerabilities to the public.


Disclosure Timeline:

   25. September 2004 - Made initial contact with the Linux Developers
   27. September 2004 - Contacted vendor-sec about this issue
   22. October   2004 - Sent the 2nd round of smbfs vulnerabilities to
                        both parties
   27. October   2004 - Sent final patchset for 2.4 and 2.6 kernel
                        to the developers
   11. November  2004 - Linux 2.4.28-rc3 containing the final patchset
                        was made available by the developers
   17. November  2004 - Linux 2.4.28 released
   17. November  2004 - Public Disclosure


CVE Information:

   The Common Vulnerabilities and Exposures project (cve.mitre.org) has
   assigned the name CAN-2004-0883 to the issues 01-05 and the name
   CAN-2004-0949 to the issues 06, 07.


Recommendation:

   Anyone using smbfs with Linux should upgrade as soon as possible
   to the new kernels.


GPG-Key:

   http://security.e-matters.de/gpg_key.asc

   pub  1024D/3004C4BC 2004-05-17 e-matters GmbH - Securityteam
   Key fingerprint = 3FFB 7C86 7BE8 6981 D1DA  A71A 6F7D 572D 3004 C4BC


Copyright 2004 Stefan Esser. All rights reserved.

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