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TUCoPS :: Unix :: General :: flash-1.htm

Olivier Debon's Flash Exploitable Buffer Overflow



Vulnerability

    Flash

Affected

    Olivier Debon's Flash

Description

    Neal K. found following.  There are two primary sources for Flash
    plugins:

        - Macromedia  provides  the  official  version.  They are  NOT
          affected by this latest defect.
        - Olivier Debon provides  an unofficial version that  has been
          ported to all operating systems not supported by  Macromedia
          (and  some  that  are  supported  by  Macromedia).   Systems
          affected  include:  Linux  (those  viewing Flash without the
          Macromedia  plugin),  FreeBSD,  HP-UX,  BeOS, Amiga, Solaris
          2.5-2.8.
        - The  port  to  Windows  CE  by  Conduit Technologies is  not
          affected.

    To determine which one you are using, use the URL  "about:plugins"
    under Netscape.   If you  see Olivier  Debon's name,  then you are
    vulnerable.  Even if you compiled it with the "NOSOUND" flag,  you
    are still vulnerable.

    Location of the defect:

        DefineSound

    The format of this tag:

        tag_14  length_of_tag  sound_id flags samples data

    Sound_id is  two bytes  giving the  sound object  a reference  ID.
    Flags is  one byte  that determine  things like  sampling rate and
    stereo.  "Samples"  are four bytes  telling the number  of samples
    in the recording (ID + Flags + Samples = 5 bytes).

    The remaining data  contains the actual  sound (Flags +  Samples +
    Data = length of tag).

    The defect: File "script.cc", in function "ParseDefineSound()".

        void CInputScript::ParseDefineSound()
        {
          Sound     *sound;
          U32 tagid = (U32) GetWord();
          long       nbSamples;
          long       flags;
          char      *buffer;
        
          sound = new Sound(tagid);
        
          flags = GetByte();
          sound->setSoundFlags(flags);
        
          addCharacter(sound);
        
          nbSamples = GetDWord();
          buffer = sound->setNbSamples(nbSamples);
        
          if (flags & soundIsADPCMCompressed) {
            Adpcm           *adpcm;
            adpcm = new Adpcm( &m_fileBuf[m_filePos] , flags & soundIsStereo );
            adpcm->Decompress((short *)buffer, nbSamples);
            delete adpcm;
          } else {
                  memcpy(buffer, &m_fileBuf[m_filePos], m_tagLen-5);
          }
        }

    The last memcpy/Decompress call  causes a write-overflow when  the
    number of  samples is  less than  the remaining  amount of data in
    the file.

    "buffer" is allocated in sound.cc:

        char *  Sound::setNbSamples(long n) {
                long size;
                nbSamples = n;
                size = nbSamples * (stereo ? 2 : 1) * sampleSize;
                samples = new char[ size ];
                memset((char *)samples,0, size);
                return samples;
        }

    The "sampleSize"  is either  1 or  2 (depends  on the flags used).
    The  size  of  "buffer"  is  allocated  to be "number of samples *
    sampleSize * 1 or 2 for stereo".  The memcpy in ParseDefineSound()
    copies all of the data into the allocated buffer.

    So the defect:

        - we can define nbSamples (number of samples).
        - we define it to be much less than the number of data bytes.

    Should be:  ID + Flags + Samples = length of tag - Data.

    Overflow when:  ID + Flags + Samples < length of tag - Data

    This is a write-overflow.  This is capable of running arbitrary
    code.

    There is example posted at:

        http://www.verinet.com/~nealk/Flash_and_Crash/

Solution

    Nothing yet.


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