[NT] SecureW2 Weak Random Seeding for PMS

From: SecuriTeam (support_at_securiteam.com)
Date: 09/27/05

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Date: 27 Sep 2005 15:20:08 +0200

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      SecureW2 Weak Random Seeding for PMS
    ------------------------------------------------------------------------

    SUMMARY

     <http://www.securew2.com/uk/> SecureW2 - "powerful open source EAP-TTLS
    Client for Windows."

    SecureW2 uses weak random number seeding when it generates the PMS (Pre
    Master Secret).

    DETAILS

    Vulnerable Systems:
     * SecureW2 versions 3.0 and prior

    SecureW2 uses weak random numbers when generating the pre-master-secret.

    Vulnerable code:
    The code quoted below can be found in
    "./Components/Common/release3/version 0/source/CommonTLS.c":
    //
    // Name: TLSGenPMS
    // Description: Generate the 48 random bytes for the PMS (Pre Master
    Secret)
    // Author: Tom Rixom
    // Created: 17 December 2002
    //
    DWORD
    TLSGenPMS( IN OUT BYTE pbPMS[TLS_PMS_SIZE] )
    {
            int i = 0;
            SYSTEMTIME SystemTime;
            DWORD dwRet;

            dwRet = NO_ERROR;

            AA_TRACE( ( TEXT( "TLSGenPMS" ) ) );

            pbPMS[0] = 0x03;
            pbPMS[1] = 0x01;

            //
            // Time (DWORD)
            //
            GetLocalTime( &SystemTime );

            srand( ( unsigned int ) SystemTime.wMilliseconds );

            //srand( ( unsigned )time( NULL ) );

            //
            // Random bytes
            //
            for( i=2; i < TLS_PMS_SIZE; i++ )
                    pbPMS[i] = ( BYTE ) ( rand() % 255 );

            AA_TRACE( ( TEXT( "TLSGenPMS::random bytes: %s" ), AA_ByteToHex(
    pbPMS, TLS_PMS_SIZE ) ) );

            return dwRet;
    }

    It appear to be using the weak srand/rand functions seeded by the
    milliseconds field from the system clock. That doesn't provide you with
    48 bytes of strong randomness, you are lucky to get even a few bytes.

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:jas@extundo.com> Simon
    Josefsson.

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