Who influences my data stream?

Part of the code downloads the file from some URL and saves it in a local file. A piece of cake. What could be wrong here?

protected long download(ProgressMonitor montitor) throws Exception{
    long size = 0;
    DataInputStream dis = new DataInputStream(is);
    int read = 0;
    byte[] chunk = new byte[chunkSize];
    while( (read = dis.read(chunk)) != -1){
        os.write(chunk, 0, read);
        size += read;
        if(montitor != null)
            montitor.worked(read);
    }

    chunk = null;
    dis.close();
    os.flush();
    os.close();
    return size;
}

The reason I ask the question is that it works in 99.999% of cases and does not work properly if the computer that runs this code has antivirus software or some other protection software. I will blindly point my finger like this, because whenever I stop (or turn off) it, the code works fine again. The end result of such interference is that the MD5 of the downloaded file is not as expected, and a new saga begins.

, , , - "" , URL-, ? - ? ( Kasperksy Norton).

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