./CVE-2020-0796.py servername

This script connects to the target host, and compresses the authentication request with a bad offset field set in the transformation header, causing the decompressor to buffer overflow and crash the target.

This contains a modification of the excellent smbprotocol with added support for SMB 3.1.1 compression/decompression (only LZNT1). Most of the additions are in smbprotocol/connection.py. A version of lznt1 is included, modified to support Python 3.

The compression transform header is in the SMB2CompressionTransformHeader class there. The function _compress is called to compress tree requests. This is where the offset field is set all high to trigger the crash.

    def _compress(self, b_data, session):
        header = SMB2CompressionTransformHeader()
        header['original_size'] = len(b_data)
        header['offset'] = 4294967295
        header['data'] = smbprotocol.lznt1.compress(b_data)

CVE-2020-0796 is a bug in Windows 10 1903/1909's new SMB3 compression capability. SMB protocol version 3.1.1 introduces the ability for a client or server to advertise compression cabilities, and to selectively compress SMB3 messages as beneficial. To accomplish this, when negotiating an SMB session, the client and server must both include a SMB2_COMPRESSION_CAPABILITIES as documented in MS-SMB2 2.2.3.1.3.

Once a session is negotiated with this capability, either the client or the server can selectively compress certain SMB messages. To do so, the entire SMB packet is compressed, and a transformed header is prepended, as documented in MS-SMB2 2.2.42. This header is a small (16 bytes) structure with a magic value, the uncompressed data size, the compression algorithm used, and an offset value.

CVE-2020-0796 is caused by a lack of bounds checking in that offset size, which is directly passed to several subroutines. Passing a large value in will cause a buffer overflow, and crash the kernel. With further work, this could be developed into a RCE exploit.