ADDITIONAL SYSTEM INFORMATION :
Running Windows 10 Pro 64-bit with Java 11.0.7.

A DESCRIPTION OF THE PROBLEM :
Line 193 of java.util.zip.GZIPOutputStream hard-codes the value of the OS flag in all written GZIP headers to 0. This appears to go against section 2.3.1.2 of RFC 1952 ("GZIP file format"), which specifies that a value of 255 for the OS flag should be provided as a default.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Deflate any String (such as "Hello World") using a GZIPOutputStream. Inspect the resulting byte array at index 9. This is the tenth and last bit of the GZIP header, which precedes the compressed blocks,


---------- BEGIN SOURCE ----------
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.zip.GZIPOutputStream;

public class Main {

    public static void main(String[] args) throws IOException {
        byte[] compressed = compress("Hello World");
        System.out.println("Tenth bit (OS flag) = " + compressed[9]);
    }

    private static byte[] compress(String s) throws IOException {
        ByteArrayOutputStream bytes = new ByteArrayOutputStream();

        try (GZIPOutputStream gzip = new GZIPOutputStream(bytes)) {
            gzip.write(s.getBytes());
        }

        return bytes.toByteArray();
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Manually set the tenth byte in the compressed byte array to 255.

FREQUENCY : always