A privilege escalation vulnerability exists in the clfs.sys driver which comes installed by default on
Windows 10 21H2, Windows 11 21H2 and Windows Server 20348 operating systems.
The clfs.sys driver contains a function CreateLogFile that is used to create
open and edit '*.blf' (base log format) files. Inside a .blf file there are multiple blocks of data which
contain checksums to verify the integrity of the .blf file and to ensure the file looks and acts like a
.blf file. However, these files can be edited with CreateFileA or with fopen and then modified with
WriteFile or fwrite respectively in order to change the contents of the file and update their checksums accordingly.
This exploit makes use to two different kinds of specially crafted .blf files that are edited using the technique
mentioned above. There are multiple spray .blf files. The spray .blf files are specially crafted to initiate an out of
bounds read which reads from a contiguous block of memory. The block of memory it reads from contains a read-write pipe
that points to the address of the second type of .blf file - the trigger .blf file. The trigger .blf file is specially
crafted read the SYSTEM token and write it in the process of the exploit to achieve the local privilege escalation.
The exploits creates a controlled memory space by first looping over the CreatePipe function to
to create thousands of read-write pipes (which take up 0x90 bytes of memory). It then releases a certain number of
pipes from memory and calls CreateLogFile to open the pre-existing spray .blf files which when being opened fill the
0x90 byte gaps created by the deallocation of the pipes in memory, creating the controlled memory space.
This is a very brief and high overview description of what the exploit is actually doing. For a more detailed and in
depth analysis please refer to the following [reference](https://github.com/fortra/CVE-2023-28252).
- Ricardo Narvaja