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https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=5007

           Summary: SNMP Dissector error when object length > 4 bytes
           Product: Wireshark
           Version: 1.2.9
          Platform: Other
        OS/Version: Windows XP
            Status: NEW
          Severity: Major
          Priority: Low
         Component: Wireshark
        AssignedTo: wireshark-bugs@xxxxxxxxxxxxx
        ReportedBy: p29731@xxxxxxxxx


Created an attachment (id=4915)
 --> (https://bugs.wireshark.org/bugzilla/attachment.cgi?id=4915)
example of a capture file 

Build Information:
Version 1.2.9 (SVN Rev 33171)

Copyright 1998-2010 Gerald Combs <gerald@xxxxxxxxxxxxx> and contributors.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Compiled with GTK+ 2.16.6, (32-bit) with GLib 2.22.4, with WinPcap (version
unknown), with libz 1.2.3, without POSIX capabilities, without libpcre, with
SMI
0.4.8, with c-ares 1.7.0, with Lua 5.1, with GnuTLS 2.8.5, with Gcrypt 1.4.5,
with MIT Kerberos, with GeoIP, with PortAudio V19-devel (built Jun  8 2010),
with AirPcap.

Running on Windows XP Service Pack 3, build 2600, with WinPcap version 4.1.1
(packet.dll version 4.1.0.1753), based on libpcap version 1.0 branch 1_0_rel0b
(20091008), GnuTLS 2.8.5, Gcrypt 1.4.5, without AirPcap.

Built using Microsoft Visual C++ 9.0 build 30729

--
When an SNMP packets contains an object with a length > 4 bytes and the
relevant SMI file is loaded, the dissector returns the following error and
aborts the parsing:
Dissector bug, protocol SNMP: proto.c:1047: failed assertion
"DISSECTOR_ASSERT_NOT_REACHED"

Unfortunately it is not very difficult to have values with a length greater
than 4 bytes. 
As example, the following requests would generate the error described before
--when the relevant SMI file is loaded--:
snmpset -v2c -d -c public 10.215.98.1 .1.3.6.1.4.1.111.1.1.0 u 4294967295
snmpset -v2c -d -c public 10.215.98.1 .1.3.6.1.4.1.111.1.1.0 u 2147483648

those of course even if are 32bit values are encoded in 5 bytes to respect the
BER encoding rules for unsigned int.

A SMI file could be as follow (this file must be loaded in the "SMI (MIB and
PIB) modules" to see the problem or the dissector would simply return "Missing
MIB"):

MY-MIB DEFINITIONS ::= BEGIN
IMPORTS
       MODULE-IDENTITY, Unsigned32, enterprises
               FROM SNMPv2-SMI;

myModule MODULE-IDENTITY
       LAST-UPDATED "201007120000Z"
       ORGANIZATION " "
       CONTACT-INFO 
                " "
       DESCRIPTION
               " "
       ::= { enterprises my(111) 1 }

myObject  OBJECT-TYPE
        SYNTAX      Unsigned32
        MAX-ACCESS  read-write
        STATUS      current
        DESCRIPTION
                "=   "
         ::= { myModule 1 }
END


The problem is visible (at least) with the following Window releases: 1.2.1 and
1.2.9. 


Frame 2 (91 bytes on wire, 91 bytes captured)
Ethernet II, Src: 24:01:9a (00:16:26:24:01:9a), Dst: 24:01:9a
(00:16:26:24:01:9a)
Internet Protocol, Src: 192.168.236.162 (192.168.236.162), Dst: 10.215.98.1
(10.215.98.1)
User Datagram Protocol, Src Port: 53719 (53719), Dst Port: snmp (161)
Simple Network Management Protocol
    version: v2c (1)
    community: public
    data: set-request (3)
        set-request
            request-id: 1210029970
            error-status: noError (0)
            error-index: 0
            variable-bindings: 1 item
                VarBind
                    Object Name: 1.3.6.1.4.1.111.1.1.0 (MY-MIB::myObject.0)
                        Scalar Instance Index: 0
[Dissector bug, protocol SNMP: proto.c:1047: failed assertion
"DISSECTOR_ASSERT_NOT_REACHED"]
    [Expert Info (Error/Malformed): proto.c:1047: failed assertion
"DISSECTOR_ASSERT_NOT_REACHED"]
        [Message: proto.c:1047: failed assertion
"DISSECTOR_ASSERT_NOT_REACHED"]
        [Severity level: Error]
        [Group: Malformed]

thanks
F.

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